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THE INFINITE VARIETY: THE BEGINNING OF LIFE
The world is rich in animals and plants, some of which still remain to be
discovered. A small area of the Tropical Forests of South America will still yield
insects that have never been described, the difficulty is finding a specialist whose
is able to classify them. The understanding of such diversity would have been
almost impossible, if it had not been for Charles Darwin and his trip around the
world. For example Darwin described the adaptations of the Giant Tortoises
(Testudo elephantopus) that occur on the Galapagos Islands in the South Pacific.
Tortoises occurring on the well-watered islands, with short, cropped vegetation
had gently curved front edges to their shell, whereas those individuals occurring
on more arid islands had to stretch their necks to reach branches of cactus and
other vegetation. Consequently, these later individuals had longer necks and a
high peak to the front edges of their shells, which enabled them to stretch their
heads almost vertically. Observations such as these were the foundations for the
theory of evolution, which suggests that species were not fixed for ever but
changed with time and thereby contribute to the immense diversity of life.
Darwin's argument for the evolution of different necks in these tortoises was as
follows:- all individuals of the same species are not identical. In a single clutch of
eggs there will be some hatchlings, which, because of their genetic constitution,
will develop longer necks than others. In times of drought such individuals will be
able to reach leaves higher off the ground than their siblings and therefore will
survive. The brothers and sisters in the clutch who possessed shorter necks
would be unable to stretch and reach food and therefore would starve to death.
Since this time natural selection has been debated and tested, refined, quantified
and elaborated. Later discoveries about genetics, molecular biology, population
dynamics and behaviour have developed the theory of natural selection still
further. It remains the key to our understanding of the natural world and it
enables us to recognize that life has a long and continuous history during which
organisms, both plants and animals, have changed, generation by generation, as
they colonized all parts of the world.
Occasionally some animals after dying may be covered in mud, where their
bones can be preserved. Dead plant material may also accumulate and is turned
to peat, in time peat is compressed and turned to coal. Great pressure from
overlying sediments and mineral-rich solutions that circulate through them cause
chemical changes in the calcium phosphate of the bones. Eventually they are
turned to stone giving an accurate representation of the original bones. This
process is called fossilization. The most suitable sites for fossilization are in seas
and lakes were sedimentary deposits like sandstone and limestone are slowly
accumulated. Fossils are exposed when such deposits erode away. Fossils can
often be dated with the discovery of radioactivity in the surrounding rocks. Some
chemicals in rocks decay with time producing radioactivity, for example
potassium turns to argon, uranium to lead and rubidium to strontium. The
amount of change from one chemical to the other depends on the amount of
elapsed time. Consequently the proportion of the second element to the first can
be used to calculate the time when the rocks were first laid down around the
fossil.
When rocks occur as undisturbed layers, we find that the lowest layers will be the
oldest and topmost layers will be the youngest. Frequently rivers cut incisions
into the earths's surface and expose such layers. The grand canyon in the
U.S.A. is the deepest cleft on the earth's surface. The upper rocks of this
canyon are about 200 million years old and contain traces of reptiles, impressions
of fern leaves and wings of insects. Halfway down the canyon you find limestone
of about 400 million years old which contains the remains of primitive armoured
fish. Further down the canyon there are no traces of vertebrates. Three-quarters
way down there are no apparent traces of life. Close to the bottom of the canyon
the rocks are more than 2000 million years old.
Rocks as old as those of the bottom of the Grand Canyon have been found to
contain a fine-grain flint-like substance called chert. Contained in this chert are
simple organisms some of which resemble filaments of algae and others
resemble bacteria. These are the earliest known organism and are referred to as
cyanophytes or blue-greens. These organisms are able to extract hydrogen from
water and thereby produce oxygen which is essential for other organisms to
survive. The chemical agent responsible for this process is called chlorophyll and
process is called photosynthesis, and occurs in true algae and higher plants.
Even before these blue-greens existed organic molecules must have evolved.
The original atmosphere of the earth was very thin and contained hydrogen,
carbon-monoxide, ammonia and methane, but no oxygen. This chemical
mixture, together with ultra-violet radiation and frequent electrical discharges
causing lightening was simulated in an experiment in the 1950s. After a week's
treatment complex molecules were found to have formed in the mixture, including
sugars, nucleic acids and amino acids, which are the building blocks for proteins.
It is likely that molecules such as these were formed in the seas of the earth at
the very beginning of its history. With time these substances probably increased
and interacted with each other to form more complex molecules. Eventually one
substance essential to life as we know it appeared. This substance was called
deoxyribonucleic acid or DNA. This molecule can act as a blueprint for the
manufacture of amino acids and has the capacity to replicate itself. Such
properties occur in all life as we know it including the simplest forms such as
bacteria. DNA's ability to replicate itself is due to its double helix structure.
During cell division, the DNA molecule splits longitudinally, and each side acts as
template to which simpler molecules become attached until each half has once
more become a double helix. The simpler molecules from which DNA is mainly
built are of four kinds, but they are grouped in trios and arranged in particular and
significant orders. These sequences of amino acids on the immensely long DNA
molecule specifies how various amino acids are arranged in a protein, and how
much protein is to be synthesized. A length of DNA bearing the information for
an unbroken sequence of manufacture is called a gene. Occasionally, the DNA
copying process goes wrong. A mistake may be made at a single point on the
length of the DNA and a particular molecule may become temporarily dislocated
and be re-inserted in the wrong place. The copy is then imperfect and the protein
that it synthesizes will be different. Such mistakes are sources of variation from
which natural selection can produce evolutionary change. We know from microfossils that distinct forms of bacteria-like blue-greens had evolved as long ago as
3000 million years.
The arrival of blue-greens dictated the rest of the development of life. The
oxygen they produced accumulated and created the atmosphere as we know it
today. Atmospheric oxygen and ozone forms the screen which filters ultra-violet
rays which provided the original energy to synthesize the first amino-acids and
sugars. From primitive blue-greens the first single-celled organisms evolved.
Such organisms are called protista. Each celled organism is more complex than
any bacteria and includes a DNA filled nucleus and elongated bodies called
mitochondria which provide energy from burning oxygen. Some of these
unicellular organisms have tail or flagellum which resemble the filamentous
bacterium called a spirochaetae. These unicellular organisms may also contain
chloroplasts (packets of chlorophyll which like blue-greens use energy from
sunlight to assemble complex molecules as food for the cell). Consequently
each of these tiny organisms appear to be a committee of simpler organisms. It
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is even possible that the first cells engulfed and incorporated bacteria and bluegreens to form a communal life. Cells of this complexity first appeared about
1200 million years ago.
These protistans like bacteria can reproduce by binary fission, but since their
internal organization is more complex, the division process is more complex and
includes the division of the separate structures within the cell. The division of
mitochondria and chloroplast (each with their own DNA) may be independent of
division of the main cell. There are, however, other means of reproduction which
involves the exchange of genetic material when two individual cells conjugate.
Some protistans contain two complete sets of genes which after exchange of
genetic material divide to make new cells with only one set of genes. These cells
are of two types, a large and comparatively immobile one and a smaller active
one that possesses a flagellum and are referred to as egg and sperm cells.
When the two types unite in a new amalgamated cell the genes are once again in
two sets but with new combinations of genes that occur from two parent sources.
This sexual reproduction increases the possibilities for genetic variation and an
accelerated rates of evolution.
There are thousands of species of protistans, some possessing cilia or flagellum,
whereas others use pseudopodium for locomotion. Some protistans secrete
shells of silica or lime, whereas others have combined individual cells to produce
a colony (eg Volvox). The constituent cells of Volvox, however, are co-ordinated,
for all the flagellum around the sphere beat in an organized way and give
direction to locomotion.
Increased co-ordination between colonial cells appeared with the evolution of the
sponges (Porifera). Sponges may be formless lumps on the sea floor reaching
two metres in size. Their surfaces are covered with tiny pores through which
water is drawn into the body by flagella and then expelled through larger vents.
The sponges feed by filtering particles from this stream of water passing through
its body. Some sponges produce a soft flexible silica-based substance which
supports the whole organism, whereas other sponges secrete lime or silica to
create a hard "skeleton" for support. Despite the elaborate skeletons that some
sponges are able to produce they cannot be considered as an integrated
multicellular animals since they have no nervous systems nor muscle fibres.
The simplest organisms to possess such structures are the coelenterates which
are represented by the jellyfish, sea anemones and their relatives. These
animals have two cellular layers, each layer one-cell thick. The outer layer of
cells is the ectoderm whereas the inner layer is the endoderm. The individual
cells of the ectoderm are specialized for various functions such as protection,
secretion, defence and cell replacement whereas the endoderm is specialized for
digestion, absorption and assimilation of food. The stinging cells (nematocysts)
of the ectoderm are highly specialized and contain coiled threads inside. When
food or an enemy comes near, the cell discharges the thread which is armed with
spines like a miniature harpoon and often loaded with poison. These cells are
often concentrated at the ends of tentacles. Coelenterates reproduce by releasing
eggs and sperm into the sea. The fertilized egg first develops into a free
swimming creature that is quite different from its parents. It eventually settles
down at the bottom of the sea and develops into a tiny flower-like organism called
a polyp which filter-feed with the aid of tiny-beating cilia. Eventually, the polyps
bud in a different way and produce miniature medusae which detach themselves
and once again become free-swimming. True jellyfish spend most of their time
as free-floating medusae with only the minimum period fixed to the rocks as
solitary polyps, whereas sea anemones do the reverse with most of their life
spent attached to rock as solitary polyps. Yet other coelenterates exist as
colonies of polyps which have given-up a sessile life and have become free-
floating eg Portuguese Man O'War (Physalia).
Although Coelenterates are relatively simple organisms and appeared fairly early
in the history of life, fossil evidence for them was only recently found (1940's) in
the Flinders Range, southern Australia in rock strata that has been dated at about
650 million years. Not all coelenterates are soft-bodied, and some produce
skeletons of limestone in a similar way to the sea sponges and are better known
as corals. These animals secrete their skeletons from their base. Each polyp is
connected with its neighbours by strands that extend laterally. As the colony
develops new polyps form, leaving a limestone skeleton that is riddled with tiny
cells were polyps once existed. Live polyps are restricted to a thin surface layer.
The size of these colonial polyps are enormous and create entire coral islands
such as the great barrier reef running parallel to the east coast of Australia. This
coral reef extends for over a sixteen hundred kilometres and is the greatest
animal construction prior to man's artifacts.
ASSIGNMENTS
IN YOUR OWN WORDS WRITE A ONE TO TWO PAGE ESSAY ON THE
FOLLOWING TOPICS
Discuss the structure and significance of DNA to life as we know it.
Describe the process of fossilization and its significance in the
interpretation of evolutionary events.
Describe how cells have become specialized to perform different functions
in a multicellular organism.
BUILDING BODIES: INVERTEBRATES OF THE OCEANS
Living in association with the Great Barrier Reef is a multitude of higher animals
which include shelled animals of the phylum Mollusca (clams, cowries, mussels
and sea-snails), radially symmetrical creatures of the phylum Echinodermata and
includes sea urchins and starfish, elongated animals with segmented bodies
occurring in the phylums Annelida and Arthropoda which includes bristle worms,
shrimps and crabs as well as the vertebrates (phylum Chordata) which includes
cartilaginous and bony fishes and marine mammals such as dolphins and seals.
To trace the invertebrate lines we must also look for fossils where animals were
deposited continuously and the fossil remains to have survived in a relatively
undistorted condition such as has occurred in the Atlas Mountains of Morocco.
From this fossil record and from other sites scattered around the world there
appears a clear dichotomy in the history of earth where fossils are found and they
are not found. This period of transition corresponds with about 600 million years
and records the first annuals which are characterized by the presence of shells.
It is conceivable that before this period the animals were soft bodied and did not
fossilize. It has also been suggested that seas were not at the right temperature
and or chemical composition to favour deposition of lime from which most marine
shells and skeletons are constructed.
Simpler animals than those first found in the fossil records still inhabit the earth
and its oceans and their ancestors may have represented the predecessors for
the shelled invertebrates that are found in the fossil records. These soft-bodied
animals belong to the phylum Platyhelminthes. The most basic of these animals
is the flatworm, a flat-leaf shaped worm which like jellyfish have a single opening
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to their gut through which food is ingested and waste is ejected. Their bodies
have differentiated into three layers, the ectoderm, mesoderm and endoderm.
Cells with a different structure and function have aggregated to form a primitive
system (eg nervous system which consists of a network of nerve fibres).
Nevertheless, they have no breathing system with oxygen diffusing directly
through the skin. Their undersides are covered with cilia which, by beating,
permits them to glide over surfaces. Their front end has a mouth on the undersurface and a few light sensitive spots above. There are some 3000 species
varying in size from microscopic to 600 mm, and although most are marine some
species have managed to inhabit humid terrestrial environments and move on a
bed of mucus. Many species in this phylum have become parasitic and live on
the surface and inside bodies of other animals including man. Some of these
parasitic forms such as liver flukes still resemble a basic flatworm form whereas
others such as the tape worm have a highly modified morphology with hooks on
their heads and an ability to detach egg-bearing sections of their posterior body
parts.
It is hypothesized that the period between 600 and 1000 million years
considerable erosion of the continents was producing great expanses of mud and
sand adjacent to the continental shelf. This environment may have contained
abundant quantities of organic material. However, in order to give protection and
concealment in this environment burrowing would be a pre-requisite, and more
tubular body plan would become necessary. It is possible that under such
conditions the segmented worms evolved. Some of these animals became
active burrowers who tunnelled through mud in search of food, whereas others
lay half-buried, with their mouthparts filtering food above the sediment. Some of
these animals lived in secreted protective tubes, whereas others evolved two flat,
protective shells which represented the first Brachiopods descendants which
exist belong to the genus lingula. Brachiopods had great variations in their
design, including heavy lime shells, and large tentacles contained inside,
whereas others developed a hole at the hinge end of one of the valves through
which a stalk emerged and fastened the animal onto the ground.
Other kinds of annelids also developed in which the animal did not attach itself to
the sea floor but continued to crawl and secreted a small conical tent under which
it could escape from predators and probably represented the prototype for the
Mollusc group, with a primitive representative being Neopilina. Today there are
at least 60 000 different species of mollusc. Anatomically these animals usually
possess a foot which may be used for locomotion, a shell, a mantle composed of
thin sheets of body tissue that covers the internal organs, and an internal cavity
that coats the central part of the body, in which most species have gills which
extract oxygen from water.
The shell is secreted by the upper surface of the mantle, with limpets producing
shell at equal rates along the edge of the mantle, in other animals the front end of
the mantle secretes at a faster level than the rear end and produces a flat spiral.
The maximum secretion may be to one side and develops twisted or turretedshaped shells, or in the case of cowries the secretion is concentrated along the
sides of the mantle creating a shell resembles a clenched fist. Molluscs may
have either single shells (limpets), two shells or bivalves (mussels) or a number
of shell plates (chitons). In some molluscs the shell has become reduced and
totally internal (cuttlefish) whereas in others it is total absent (octopuses).
Molluscs have a variety of different feeding mechanisms. The bivalve molluscs
can filter-feed fine particles form the water. Some of the single-shelled molluscs
(limpets) possess a ribbon-shaped tongue or radula, covered with rasping teeth,
which enables the animal to scrape algae from the rock. Whelks have a radula
on a stalk that can extend beyond the shell and be used to bore into the shells of
other molluscs. Through these holes that they have bored they poke the tip of
the radula and suck out the flesh of the victim. The cone-shells also have a
stalked radula which is modified into type of harpoon with which they secure their
prey before injecting it with poison. In still more active carnivores the heavy shell
is reduced in size and may even be lost as has occurred in the sea-slugs which
have an upper surface covered with tentacles. One species of sea-slug actively
hunts jelly fish and ingests these animals stinging cells which it then concentrates
in the tentacles and uses them for protection.
An early group of molluscs retained the protection of a shell yet were still able to
maintain a high degree of mobility. This was achieved through the development
of a gas-filled floatation tanks. The prototype forms had a flat-coiled shell with an
end walled-off to form a gas chamber. As the animal grew it added buoyancy
with the development of new chambers. Such animals survive today and are
known as nautiluses. A tube runs from the body chamber of the nautilus to the
floatation tanks in the shell. The nautilus is an active carnivore eating animals
such as crabs and moves in a form of jet-propulsion where water is squirted
through a siphon. In this animal the original muscular foot is divided into long
grasping tentacles with which it secures its prey. The mouthparts are modified to
form a horny beak with which the nautilus is able to crack shells of other animals.
Variations on the float chamber theme gave rise to the enormously successful
group of animals called the ammonites whose circular shells were up to 2 meter
in size.
One of these group of molluscs took the same path as the sea slugs and
disposed of its shell entirely (octopuses and squids) whereas relict of the
ancestral shell persist as the cuttlebone found in the cuttlefishe. One species of
octopus (Argonauta) secretes a paper-thin replica of the nautilus shell, the
chambers of which are used to lays its eggs.
Both squids and octopuses have reduced the number of tentacles (10 and 8
respectively), but squids have become more mobile with the development of
undulating lateral fins. The brains and eyes of these animals is the most
advanced of any invertebrate, eyes greater than 400 mm in size have been
recorded for squid. Squids, in particular can reach immense sizes with one
individual 21 m long (found in New Zealand in 1933).
Another group of animals that had diverged from early stage and also reached
immense sizes are the crinoids or sea lilies which belong to the phylum
Echinodermata. These animals have an architecture plan that is based on a fivefold symmetry and possess large lime plates that occur just below the skin. Fossil
crinoids were up to 20 m long, although their present day counterparts are
considerably reduced in both size and species diversity. The bodies of all
members work on a unique hydrostatic principle. The hydrostatic skeleton is
closed fluid-filled system that terminates as a series of blind tubes called tubefeet. Each tube feet ends in a sucker. Changing the local pressure within the
tube feet allows to be extended and contracted. Extensions and contractions of
these tube feet occur as waves down the length of the arms (or ray) and this
allows the animal to move itself and to move particulate matter down the arm.
The water from this system circulates separately from that in the body cavity. It is
drawn through a pore into a canal surrounding the mouth and circulated
throughout the body into the myriads of tube feet. When suspended particles of
food touches an arm, the tube feet fasten on to it and pass it from one to another
until it reaches the groove that runs down the upper surface of the arm to the
central mouth. Although stalked, sessile sea-lilies were the most abundant
crinoids in the fossil records, the most common form today is the stalkless feather
stars.
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Five-fold symmetry and hydrostatically operated tube feet also occur in the
starfish and the brittle stars, however their body plan has become inverted and
the mouth is on the undersides. Yet in another group of echinoderms the fivefold symmetry is less conspicuous and the body plan is elongated with a mouth
and anus at the two ends. At the mouth the tube feet have become modified into
tentacles which filter fine food particles. The five-fold symmetry and hydrostatic
mechanisms did not develop further and the group is generally considered to be
an evolutionary cul-de-sac.
The third major line in the evolution of invertebrates was the development of the
segmented bodies (Arthropoda) which evolved at a very early stage and are
contemporary with the jellyfish fossil patterns found in Flinders, Australia. This
group of animals shares one important feature with the molluscs, and that is a
spherical larvae possessing a belt of cilia, whereas the echinoderm larvae have a
twisted morphology with winding bands of cilia. This suggests that molluscs and
arthropods evolved from flatworms (Platyhelminthes), with the echinoderms
having an independent evolutionary line.
Segmentation may have increased the efficiency for burrowing in mud. A line of
separate limbs that are repeated down the length of the body seems to have
been the most primitive form. Each segment is equipped with its own set of
organs - on either side, leg-like projections sometimes accompanied by bristles
and feathery appendages through which oxygen could be absorbed, and within
the body wall, a pair of tubes opening to the exterior from which waste is
secreted. A gut, a large blood vessel and a nerve cord run through all segments
from the anterior to the posterior end of the organism and co-ordinates the
segmentation. a great variety of these segmented animals have been almost
perfectly fossilized in the Burgees shale of the Rocky Mountains in British
Columbia, Canada.
exoskeleton varies considerably from the paper-thin exoskeleton of the almost
microscopic water flea (Daphnia) to the carapace of giant Japanese spider crab
(Macrocheira kaempferi) which measures 3 m from claw to claw. In the
crustaceans the paired legs have become modified for a variety of purposes. At
the anterior end they have become modified into pincers or claws, those in the
middle are paddles, or walking legs or tweezers. Some have feather branches
acting as gills through which oxygen can be absorbed. All limbs are jointed,
tubular and operate by way of muscles. Like the primitive trilobites for
crustaceans to grow they need to dispose of their calcareous carapace. As time
approaches for moulting the animal absorbs as much calcium carbonate from the
carapace into the blood stream, and begins to secrete a new soft wrinkled skin
under the carapace. The outgrown armour splits and the crustacean swells its
body by absorbing water, and wrinkled new skin stretches and hardens into a
new carapace.
This exoskeleton may work to advantage for animals to colonize land if a
mechanism of breathing in air as opposed to water can be secured. By
developing almost closed air chambers lined with folds of moist skin crustaceans
are able to absorb oxygen from air. In this way sand shrimps, beach hoppers
and wood lice have been able to colonize land that retains a moist environment.
The most spectacular of land dwelling crustacean is the big robber crab Birgus
which exploits coconuts.
Other descendent of the invertebrates have left the sea for a terrestrial life style
the first of which were probably derived from segmented marine worms, but more
recently included the familiar snails and slugs. These changes started about 400
million years ago and gave rise to the most numerous and diverse of land
animals; the insects.
ASSIGNMENTS
An early segmented animal was the trilobite. These animals had a bony armour
composed of lime and a horny substance called chitin. The armour was not
expandable and therefore shed periodically. Many of these shed exoskeletons
have been preserved as fossils. Where the entire animal is preserved you can
observe the jointed legs that are attached to each segment of the body, the
feathery gill next to each leg, two feelers at the front of the head, the gut running
the length of the body, and even muscle fibres along the back which enabled the
animal to roll itself into a ball. Comparatively high resolution eyes composed of
mosaics of separate cells and a crystalline calcite lens. The very thick lens of
some trilobites may have reflected their colonization of deeper water where light
is considerably reduced. However, the optimal properties of the calcite lens
operating in water would not have permitted a fine focus. This shortcoming was
compensated by the evolution of the two-part lens with a waved surface at the
junction of the two lens elements. Although they radiated throughout the oceans,
only one descendent of this group survives today, the horse-shoe crab (Limulus).
This animal is larger than its ancestral trilobites, and segmentation of its armour
have fused to form a large domed shield. These animals generally live at great
depths but each spring they migrate towards the coast and during full moon and
high tides they drag themselves onto the beach where they copulate. Today the
similarities between the horse-shoe crabs and the trilobites are only evident in the
larval stage where segmentation of the armour plates are clearly discernable in
the horse-shoe crab larvae.
Another group of armoured animals also evolved from the original segmented
worms the crustaceans which exist today in the form of some 35 000 species.
They may prowl around rocks and reefs as crabs, shrimps, prawns, lobsters and
crayfish, they may become sessile such as barnacles, or congregate and swim in
vast shoals such as krill. The size of the crustacean and the form of the
IN YOUR OWN WORDS WRITE A ONE TO TWO PAGE ESSAY ON THE
FOLLOWING TOPICS
Discuss the variations in shell structure that have occurred in the phylum
Mollusca.
Describe the water vascular system that characterizes animals that occur
in the phylum Echinodermata.
Describe the diversity of segmented marine invertebrates that have
evolved.
THE FIRST FORESTS
The first land available for colonization was inhospitable due to the considerable
amounts of volcanic action. Consequently as volcanoes erupted on land, life in
the oceans multiplied with a diversity of species with different structures and
adaptations, but the land remained unconquered. Marine algae may have
secured an existence on the littoral zones of the ocean in the same way they do
today. Around 420 million years ago the first waxy layers developed in plants to
prevent desiccation, but this did not totally free such plants from an aquatic
environment since they required an aquatic medium for reproduction. Algae
reproduce through both asexual division and sexual methods. Sexual
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reproduction involves the production of sex cells which require locomotion in
water for the fusion of the cells to take place. Such a problem still exists for
primitive plants living today such as the liverworts and mosses. Such plants
practice sexual and asexual reproduction in their alternate generations. The
familiar green moss is the generation which produces the sex cells. Each large
egg cell remains attached to the stem at the top of the moss plant, while the
smaller microscopic sperm cells are released into water and thrash their way to
fertilize the egg cell. The egg cell develops while still attached to its parent plant
and produces the next asexual generation which is composed of a thin stem with,
at its tip, a hollow capsule in which a large number of spores are produced. In a
dry atmosphere the capsule splits releasing airborne spores. If the spores land in
a suitable site they develop into new moss plants.
Moss plants have no structural strength and rely on close packing to achieve only
modest heights. Their tissues are soft and permeable and they can only exist
and reproduce under moist environments. Such plants probably represented the
earliest colonization of the terrestrial environment, although no fossil evidence for
this has been discovered.
The earliest fossilized land plants (400 million years ago) were simple leafless
branching strand filaments found in rocks and cherts of the United Kingdom. Like
mosses no root tissue had differentiated, however, long thick-walled cells
enabling water to be conducted along stems had differentiated and represented a
major advance which gave plants structural strength to grow bigger. Such plants,
together with primitive mosses and liverworts created the first vegetation which
permitted animals to colonize from the sea onto the land.
The first land-dwelling animals were segmented and probably represented the
ancestors of the millipedes you encounter today and reached spectacular sizes
(up to 2m in length). The exoskeleton inherited from aquatic ancestors needed
only minor modification, but the external gills were unsuitable and in its place a
network of breathing tubes (tracheae) evolved. Each tube has an exterior
opening on the side of the exoskeleton, and the network of tracheae provides
each cell with a supply of oxygen.
Reproduction on land required changes since their aquatic ancestors relied on
water to transport the sperm cell to the egg cell. In millipedes the reproductive
cells are located close to the base of the second pair of legs. The male and
female animals meet and intertwine, the male reaches forward with his seventh
leg and collects his sperm and transfers it to the sexual pouch of the female.
Such copulation was laborious but safe, but was not suitable for the predatory
animals that evolved then but still survive today as centipedes, scorpions and
spiders. These three groups of animals have all undergone a reduction in
segmentation and all may indulge in cannibalism. As a consequence of this
scorpions armed with large poison glands and spiders have evolved ritualized
courtship patterns prior to copulation.
During this early period of evolution in the segmented animals, plant were also
evolving, with the development of rooting systems which were absent in the
mosses. Rooting systems permitted water sources below the ground to be
utilized. Consequently root development permitted plants to survive in less moist
environments. Three groups of plants, all of which have living descendants
evolved root structures (club mosses Lycopodium, horsetails Sphenophyta and
ferns Pterophyta) and possessed within their stems strong woody vessels for the
transport of water absorbed by the roots. Such adaptations provided the
structural rigidity for some of these plants to grow big (up to 30 m) and created
the first true forests.
The development of forests would have necessitated changes in habitat (from the
ground to arboreal) for some animals. Evolving at this time were the first
vertebrate animals which had four legs, a backbone and moist skins and were
also carnivorous on the invertebrates. Among the invertebrates bristletails and
springtails evolved and remain one of the most numerous of invertebrates with
the most familiar being the silverfish. Silverfishes have clear but even more
reduced segmentation consisting of a conspicuous head supporting compound
eyes and antennae, a thorax bearing three pairs of jointed legs (a result of three
segments being fused) and segmented abdomen which has lost its limbs but
possesses three filaments at the extreme end. These animals breath much as
millipedes do with a tracheae system, they copulate like scorpions do with the
female walking over packets of sperm and taking them up into the genital pouch.
The characteristics of six legs and a body divided into three parts became
numerically the most successful group of animals: the insects. Although
ancestral insects probably climbed about the vegetation, one important ingredient
for their success was the development of wings and the ability to fly. How wings
evolved is unknown but it may have reflected attempts on insects to increase
surface area and become more efficient at warming up their bodies so that they
can become active (thermoregulation). Winged insects appeared some three
hundred million years ago with animals resembling dragonflies. In the absence of
early competition, early dragonflies radiated with some species developing
enormous sizes (eg wingspan of 700 mm). Dragonflies have two pairs of wings
with a simple up and down movement, and consequently cannot be folded back.
Today's dragonflies have large compound eyes and catch smaller insects in
flight, but are able to hunt only during the day. Consequently today's carnivorous
dragonflies must have been preceded by herbivorous animals or carnivorous
forms that prey on non-flying insects. Modern dragonflies probably evolved from
primitive omnivorous or herbivorous insect forms such as cockroaches,
grasshoppers, locusts or crickets.
The development of flight in insects was to have a major consequence on the
evolution of plants. Early plants including tree forms existed in two alternating
forms, a sexual and an asexual generation. Becoming tall would have no effect
on the transport of spores and may even enhance their wind-dispersal, however,
the distribution of sex cells which, hitherto, was achieved by the male cells
swimming through a droplet of water and reaching a female cell. This demanded
that the sexual generation was small and grew close to the ground, a situation
that is found today in ferns, club mosses and horsetails. The spores of such
plants develop into a filmy plant called a thallus which produces sex cells on the
undersurface where there is permanent moisture. After fertilization of the female
egg cells the thallus develops into the tall spore-bearing plants.
A thallus life cycle stage induces considerable vulnerability, since it is small and
possesses little or no protection against herbivory or desiccation. A less
vulnerable sexual stage appeared about 350 million years ago with the evolution
of plants like the cycads which exist today. Cycads superficially resemble ferns,
with some species having spores of the archaic form which are distributed by
wind. In other species some spores become large and remain attached to the
parent plant where they develop into a conical-shaped structure containing egg
cells (that is functionally equivalent to a thallus). When a wind-blown spore, now
called a pollen lands on these egg bearing cones, no filmy thallus develops, but a
pollen tube which burrows its way into the female cone occurs. The large sperm
cell is transported down to the bottom of the pollen tube, where it enters a small
drop of fluid secreted by the surrounding tissues of the cone, there it swims to the
egg cell and fuses with it and thereby completing the fertilization process.
6
Similar morphological changes resulted in the evolution of the conifer group
(pines, larches, cedars and firs). These plants, unlike cycads produce pollen and
egg-bearing cones on the same plant individual, however, fertilization and the
development of the seed takes longer, but the seeds are equipped with a rich
supply of food and a hard, water-proof coat that permits the seed to remain
dormant until conditions are right for germination and the establishment of the
seedlings. Conifers are successful, even today, with one-third of global forests
being composed of them. Both the biggest and most long-lived individual
organism in the world are conifers (the redwoods and the bristle-cones
respectively).
Conifers are also able to repel insect damage with a gummy substance called
resin. Insects are often caught in the resin which has proved to be a good
fossilizing medium called amber. The first amber containing flying insects
appeared 100 million years ago and includes representatives of all major insect
groups known today. Each group has developed its own characteristic way of
flying. Dragonflies have two pairs of wings which flap up and down
synchronously, bees and wasps have linked the fore and hind wings together
with hooks, butterflies have overlapped the wings, hawkmoths have reduced the
hind wings considerably in size and latched them onto long narrow fore-wings
with a curved bristle, beetles have the front pair modified into thick covers which
protect the rear flying wings, and flies use only the front pair of wings for flight
with the hind wings reduced to tiny knobs.
Although insects were the first animals to invade the air, they nevertheless fell
prey to their arachnid adversaries, the spiders, who evolved the ability to spin
webs between branches and thereby trap and consume flying insects.
Plants also responded to the flying skills of insects by using such mobility for the
distribution of the male reproductive cells (pollen). Unlike spores in the lower
plants, pollen needs to reach the female cell for the development of more adult
plants. Wind-dispersal of pollen which is typical in the pines (Gymnosperms),
requires vast quantities of pollen for even moderate pollination success.
Alternatively if insects could be used to carry pollen to the female cells by using a
small incentive (e.g. food), much less pollen would be required to achieve similar
levels of pollination success. Such incentives for insect pollination evolved with
the earliest of the flowering plants; the magnolias which appeared about one
hundred million years ago. In these plants the egg cells are clustered in the
centre, each protected by a green coat with a receptive spike on the top called a
stigma with which it receives pollen and is necessary for fertilization. Grouped
around the egg cells with their stigmas are stamens which produce the pollen. In
order to bring these organs to the notice of insects, the whole structure is
surrounded by brightly coloured modified leaves called petals.
pollen transportation. The services of flies were enlisted with the evolution of
flowers that mimicked the scent of rotting flesh, the usual food of such animals.
Some stepelia plants have taken this deception further by producing brown,
wrinkled petals covered with hairs which resemble the decaying skin of a dead
animal. To complete the illusion, the plant generates heat to mimic the warmth
generated by decomposition of flesh. Flies not only visit and transport the pollen
of the stepelia plants, but they even lay their eggs in the flower as if it were
carrion.
Possibly the most bizarre imitations are those occurring in orchids which attract
insects through sexual impersonation. One orchid species produces a flower that
closely resembles the form of a female wasp including eyes, antennae and wings
and an odour (pheromone) that is emitted by the female wasp during the mating
period. Male wasps are deceived into copulating with the flower and so doing get
covered with pollen before carrying on to the next bogus female wasp which will
receive and deposit more pollen.
Sometimes plant and insect become totally independent on each other. Yucca
plants which produce rosettes of cream flowers attract a small moth with a
specially curved proboscis that enables it to gather pollen from the yucca
stamens. It moulds the pollen into a ball and the carries it off to another yucca
flower. First it goes to the bottom of the flower, pierces the base of the ovary with
its ovipositor and lays several eggs on some of the ovules that lie within. Then it
climbs back up to the stigma rising from the ovary and rams the pollen ball into
the top. The plant has now been fertilized and in due course, ovules in the base
of the chamber develop into seeds. Those that carry the moth's eggs will grow
particularly large and be eaten by the developing caterpillars. Those ovaries
without caterpillars will not be eaten and permit the yucca to propagate itself.
ASSIGNMENTS
IN YOUR OWN WORDS WRITE A ONE TO TWO PAGE ESSAY ON THE
FOLLOWING TOPICS
Discuss the form of adaptations required by the first invertebrate animals
which made the transition from life in the sea to life on land.
Describe how the first plants and animals evolved and became dependant
on each other.
Describe the diversity of flying insect life that has evolved.
Beetles had already learnt to feed on the pollen of cycads, and were one of the
first to transfer their attentions to the early flowers like those of the magnolias and
waterlilies. As they moved from one to another flower, beetles collected meals of
pollen and paid for them by becoming covered in excess pollen which they
involuntarily delivered to the next flower they visited. One danger of having both
eggs and pollen in the same structure is that the plant may pollinate itself,
however, this is overcome by egg and pollen cells being mature at different times.
THE SWARMING HORDES
Other flowers developed alternative bribes to pollen this being nectar, a
completely specialized adaptation to recruit even more potential pollination
agents which included bees, flies, butterflies and moths. Even brighter signals
were used to draw attention to the nectar being offered and attractive scented
chemicals evolved as additional means of soliciting the services of insects for
It is estimated that there are three-times as many insects as all other species of
animal put together. Too date more than 700 000 species have been described,
7
probably only a fraction of those still waiting to be discovered and described.
Insects have invaded all aspects of terrestrial life. There is no known species of
plant that is not attacked by an insect species. Insects may still remove up to
three-quarters of crops grown by people in Africa.
This success, diversity and variation is all achieved with a tripartite body plan
consisting of a head bearing a mouth, mouthparts (modified jointed appendages)
and most of the sense organs; a thorax filled with muscles which operate three
pairs of legs, and usually one or two pairs of wings and an abdomen which
contains the organs for digestion and reproduction. All three sections are
enclosed within an external skeleton made principally of chitin, a substance that
is chemically similar to cellulose but has both flexibility and permeability. Insects
may cover this chitin with sclerotin to make it hard so as to create armour (e.g.
beetles) and produce mouthparts sharp and tough enough to gnaw wood and cut
metals. It is the responsiveness of this chitinous exoskeleton to evolutionary
change that has permitted insects to diversify. Leg morphology is easily modified
to propel an animal for more than two-hundred times its own length, or to create
broad oars to row across the water or thin hair tipped stilts to stride across the
surface of water. Many limbs may carry special tools moulded from chitin such
as pouches to hold pollen, combs to clean compound eyes, spikes to act as
grappling irons and notches to create sounds.
This exoskeleton still restricts growth and needs to be shed periodically, and a
new shell created to replace it. Primitive insect forms like bristle tails and
springtails do not change their shape significantly with successive changes of the
exoskeleton, but this does permit them to increase their size. The early wingedinsect forms (cockroaches, cicadas, crickets and dragonflies) similarly moult
without significant changes to the body shape with the exception of acquiring
wings in the final moult (although damsel flies take two moults to perfect their
wing structure. Even when insects adopt significantly different environments for
their early and later lives, their body structure is recognizably similar.
The more advanced insects, undergo structural changes that make it impossible
to link the larvae with the adult forms without observing the changes for oneself.
In this way maggots change to flies, grubs to beetles, caterpillars to butterflies.
Since the earlier form is not required to breed, it has no sex organs and does not
need to attract a mate, it needs no wings to fly, since it has probably been placed
in environment that is near optimal for its development. Such larvae consume
great quantities of food and therefore need efficient jaws and digestive systems.
Since these larvae have no exoskeleton, the locomotion is generally slow and
they have little protection against predators. This is of little consequence to grubs
and maggots which live inside the tissues of plants and animals, but caterpillars
which feed in the open frequently use camouflage techniques to resemble a twig,
a bit of leaf or a bird dropping. Other defences may exist including squirting
formic acid, having an unpleasant taste, or covering the body with unpalatable or
even poisonous hairs. Some animals possessing chemical defences advertise
this with a conspicuous coloration which warns potential predators of this fact.
Other species with no such defences mimic the colours of those species
possessing chemical defences and thereby avoid predation. The larval stage of
some insects may last a considerable length of time, with grubs of beetles boring
through wood for up to seven years before developing into adult forms.
Only the larvae of insects possess silk glands which have been used to construct
communal tents, to extrude life-lines guiding them over plants and getting them
from one twig to another. These silk glands are also used to construct a cocoon
in which further development takes place (e.g. moths).
Caterpillar larvae undergo one final development before becoming adults
(metamorphosis). The larvae sheds its skin and develops a hard shell around
itself and is now called a pupa. The pupa has spiracles for breathing, and its tip
may twitch sporadically. When the larva first developed from the egg cells it was
segregated into two groups. Some of these cells divided after a few hours but
remained generalized in form, whereas other cells continued to build the
caterpillar body. After the larva hatch these cells enlarge with no further cell
division. Within the pupa the original giant cells of the caterpillar are used to feed
cell division of those other group of cells which are re-organizing the new body of
the butterfly.
The butterfly exits from its pupa head-first and immediately pumps blood into the
network of veins, and the limp wings begin to take their shape. Now the blood is
withdrawn from the veins of the wing and the veins harden to create rigid struts,
at which point the wings are ready for their maiden flight. All further growth has
ceased, and they use food collected when they were larvae and stored as body
tissue. Some species like Mayflies do not even have mouthparts. In this adult
stage their primary function is to find a mate. However, unlike the larvae,
butterflies have large compound eyes, that are sensitive to most wavelengths.
The colours and patterns on their wings are created by tiny scales which have
pigments and microscopic structures that split light, reflecting back a narrower
range of wavelengths. These colourful wing patterns may be useful for species
recognition and mating.
Other insects use sound to summon prospective mates (e.g. cicadas, crickets
and grasshoppers). Sound in Grasshoppers is produced by sawing the notched
edge of their hindlimb against the strengthened vein of the wing. Cicadas have
an abdomen which contains two chambers, the inner wall of each chamber is stiff
and when it moves in or out it makes a click. In the abdomen behind there is a
large muscle which can pull the wall back 600 times a second and the noise
created is amplified in the abdomen using a hollow vibrating plate and two hollow
rectangular resonators. Sound is received from eardrums on either side of the
thorax in cicadas, but grasshoppers use a membrane situated between two deep
slits along their first pair of thighs. With each species having a unique sound,
they can recognize and attract appropriate mates of the same species. Moths
use a third sense, smell to attract mates. Females produce chemical compounds
called pheromones which male moths are able to detect with their large, feathery
antennae.
Using sight, sound and smell adult insects attract their mates and copulation can
take place. The female then lays her fertilized eggs in an environment suitable
for her larvae to exploit. Butterflies seek suitable plants for the young caterpillars,
beetles lay eggs in pellets of buried dung, flies deposit eggs in carrion, wasps
catch and paralyse spiders and lay their eggs on them so that the young larvae
can feed on the spiders. Ichneumon wasps use a beetle grub to lay her eggs,
with the hatched larvae eating the grub alive.
The only apparent limitation to insect forms appears to be size, the largest moth
is 300 mm in wingspan, the heaviest beetle is 100 g in mass. This reflects
insects reliance on tracheae and spiracles, without an effective pumping system
to force the air down. Some insects do use contractions of the abdomen to
improve circulation and have tracheae that swells into thin-walled balloons which
can be depressed and expanded.
Insects have, however, transcended even these limits in size, by creating highly
social community living, an example of which is the termite hill. The termites that
inhabit these colonies in effect all belong to the same family and were derived
8
from the same parents. The body plan of these animals is so modified that they
are incapable of an independent life, the workers are blind and sterile, the
soldiers are armed with jaws so large that they cannot forage and have to be fed
by workers. At the centre of the colony is the queen who is encaserated within
earthwalls and has an abdomen that is distended to 120 mm and produces eggs
at a rate of 30 000 per day. She is fed by workers and her eggs collected for
incubation elsewhere in the termitaria. The only other sexually active male is the
wasp-sized king who stays by the queen and is also fed by the workers. An
effective communication co-ordinates these individuals and is generally induced
by chemicals, although soldier termites sound an alarm by beating their large
hard heads on the passage walls. Other chemical hormones (also called
pheromones) in effect circulate instructions and dictate both actions and the
development of the colony. All members of the colony exchange food and saliva
with each other by way of the workers who also gather the excrement in order to
reprocess it for food to obtain the maximum nutrition from it. The queen
produces a pheromone, which is collected by workers and circulated through the
colony. Although the queen termite produces both sexes, the queen's
pheromones inhibit development maintaining them as sterile, wingless and blind
(=workers). How soldier termites are produced is unknown (either specialized
eggs or preferential treatment of larvae). Soldier termites have their own unique
pheromone which is circulated through the colony and reaches the queen who
probably regulates their numbers. At certain times, however, the queen does not
suppress larval development and sexually mature winged termites of both sexes
are produced and leave the colony by way of splits in the termitaria and take-off
ramps. With the commencement of the first rains the flying termites pour out.
Following dispersal and pairing the wings fall off and the male and female
termites excavate a new nest. These become the royal pair for a new termite
colony. Within the small royal cell they copulate and produce the first larvae
which have to be feed by the parents until they are able to forage independently
and continue with the construction of the new nest and founding of a new colony.
Termites construct fortresses that may contain several tons of mud and contain
several million inhabitants. Ventilation and temperature control are therefore
critical for survival of these communities. Around the margins of these termitaria
are tall, thin-walled chimneys. As the sun warms the walls of these chimneys air
becomes hotter than the air inside of these nests, the air in the chimneys
therefore rises and with it draws air from the termitaria. Since the chimney walls
are thin and porous, oxygen from the outside diffuses in. This air rises to the top
of the nest and re-oxygenates the colony. In very hot weather the workers
descend in tunnels that go deep into the ground water, and carries back a crop
full of water that wets the wall and lowers the temperature through evaporation.
Wasps and bees also have a colonial lifestyle comparable to termites. Wasps
show transitions in degrees of colonialism. Some hunting wasps are entirely
solitary, with a female wasp constructing a nest of mud in which she lays her
eggs and stores a provision of parasitized wasps. In other species the female
wasps remain by the nest and brings daily food to the larvae. In other wasps the
females construct nests next to one another, some of the nests are abandoned
and wasps may join other wasps in constructing theirs. Eventually one female
wasp assumes dominance and lays eggs in the amalgamated nests with other
wasps building more cells to house larvae and collect food.
The evolution of community living is also elaborate in bees. A single queen bee
is also a specialist egg-layer, that is supported by worker bees. The community
is also bound by a system of chemical messages (pheromones) but they also use
a dance behavioural pattern to communicate to each other. When a worker bee
returns from a new nectar laden flower a dance behavioural sequence is initiated.
If the source is nearby, the bee performs a simple round dance, alternatively
circling in clockwise and counterclockwise directions. The other bees are excited
by the dancing scout and follow it outside, and they find the food by orientating to
chemical signals present on the scouts body. If the food source is more than 80
m from the hive, the scout expresses this in its dance with a distance and
direction of the source. A waggle dance traces two semi-circles with a straight
run between them. The food's distance is described by sounds and wagging
movements executed during the straight run. The further away the food lies the
longer the sounds last and the more slowly the dancing bee waggles its
abdomen. The angle of the straight run describes the direction of the food source
in relation to the sun. A run straight up the hive wall denotes a location directly
towards the sun. When food exists at an angle to the left or right of the sun, the
bee runs at the same angle to the left or right of the vertical. Even on cloudy
days these dances are effective, because bees detect the sun's location by the
analysis of polarized light. The interpretation of these behavioural patterns have
been debated, since inexperienced workers do not seem to be as efficient at
foraging for pollen whereas experienced workers are almost always successful.
The returning scout bee is usually covered with pollen and some researchers feel
that the bees respond to olfactory signals rather than the interpretation of
behavioural patterns.
The most complex and highly evolved forms of colonialism in the insect world are
those created where the organisms (wasps, bees and ants) live within plants,
stimulating the tissue of their host to provide them with custom-built homes, by
growing special galls, hollow stems or thorns with swollen bases. The leafcutting ants of South America build vast underground nests and have expeditions
via long tunnels. They may remove entire trees (leaves, roots and stems)
converting the material to pulp in their chambers which forms a compost for
cultivating edible fungi.
Most ants, unlike termites and leaf-cutting ants are carnivorous. Such ants may
prey on termites, devouring the workers and larvae. Yet other ant species make
slaves of other ant species, by raiding a nest and collecting the pupae and
rearing them to be slaves. Yet other carnivorous ants do not make nests, but
march in great masses. Such an army of ants may forage on animals caught in
its wake for several weeks. When the larvae produce pheromones they are
circulated within the army and keep it on the move, when the larvae pupate, no
pheromones are produced and the army clusters around roots of a tree.
Individuals clinging to each other create a living nest of tunnels and chambers.
The queen starts producing eggs which hatch into larva, while soldier ants
emerge from their pupae. The next generation of larvae produce pheromones
which stimulate the army to move-off.
ASSIGNMENTS
IN YOUR OWN WORDS WRITE A ONE TO TWO PAGE ESSAY ON THE
FOLLOWING TOPICS
Describe the forms of social life that occur in insects.
Describe the signals used by insects to attract a mate for sexual
reproduction.
Many insects have long larval stages and have larvae that differ
significantly in morphology from the adult forms. Discuss this statement
giving suitable examples.
9
the lamprey clamps itself on to fish which it parasitizes.
THE CONQUEST OF THE WATER AND THE BIRTH OF THE
VERTEBRATES
Although animals without back-bones (invertebrates) are more abundant
numerically and more diverse in species variety, they have never been able to
reach the sizes that animals possessing a backbone can (vertebrates). All
animals with a back-bone and some with a stiffened cartilage rod called a
notochord belong to the phylum Chordata. One of the most primitive members of
this group are the Tunicates or sea-squirts. Although the sessile adult phase
bears a superficial resemblance to a sea-anemone (Coelenterata), the rest of the
vertebrate fauna was derived from such a simple organism. Evidences for this
ancestry is in the tunicate's larval stage which resembles a tadpole and has the
following features which are shared with all other vertebrates:
1. Perforations in the wall of the pharynx, or pouches that suggest ancestral
perforations.
2. A nerve cord dorsal to the gut that is tubular and reflects its embryonic
development from a tough piece of ectoderm that became roofed over.
3. A stiff rod called a notochord that supports the nerve cord from below.
The larva is short and the animal attaches itself to a rock and loses its tail and
becomes a sedentary filter-feeder.
The next most advanced animal in the evolutionary tree is the lancelet or
amphioxus, which is more fish-like in appearance but also has a stiff rod or
notochord. This animal is 50 mm in length has a well-developed segmented
muscular system that allows it to bury itself quickly in the sand. This animal has
no clearly defined head region, only a light-sensitive spot at the anterior, no heart
only a few pulsating arteries, no fins or limbs but only a slight dilation at the hind
end. The strong muscles rhythmically contract against the notochord and the
animal is propelled forward in a series of waves. These lancelets and the larval
tunicates therefore resembled each other and considerable argument arose as to
which form was the most direct ancestor for the rest of the vertebrates. The
embryonic development of many animals often reflects their phylogeny or
ancestry. Consequently, larval termites resemble bristletails and larval
horseshoe crabs resemble the segmented trilobites. It was therefore argued that
the lancelet was the ancestor to the tunicates, However, fossil evidence in the
Burgess shales (550 million years ago) included a finned or backboned
swimming animal similar to the living lancelet called a Pikaia and was the
predecessor to a group of fish-like animals that were jawless (apart form modified
parasitic forms) and consequently could only feed on micro-organisms and small
animals. These animals belong to the class Agnatha.
Another larva provides evidence for the next step in the vertebrate evolution.
This is the Lamprey, class agnatha (= without jaw) which have larvae that are
also jawless, blind and without fins except for a fringe around the tail and very
similar to lancelets. These larvae were once thought to be adult creatures called
ammocoetes. The adult lamprey is very fish-like except being jawless. It
possess the beginnings of a backbone in the form of cartilaginous elements.
They also have a clearly defined head, with two small eyes, a single nostril
leading to a blind sac, and on either side of the neck a row of gill slits. The mouth
is a circular disk and possess a tongue with sharp spines. It is with this disk that
Within the agnathan group were other small fish-like animals called ostracoderms
and possessed heavy armour-plating which may have originated from deposition
of salts derived from their food. This marks the first presence of bone, the
material that was destined to influence much of the evolution of the vertebrates.
These early bony plates may have provided protection against the large (2 m)
sea scorpions that co-existed at the same time. Heavy deposition of salts in the
head region have permitted remarkable fossilisation of these animals where the
structure of the brain and nerve and blood vessels can be identified. In addition a
balancing mechanism composed of two arching tubes at right angles to the
vertical plane has been recognized. The liquid within these tubes, moved over
the sensitive inside surfaces enabled these animals to be aware of their posture
in the water. These animals dominated the freshwater streams 500 million years
ago and the largest representatives reached 600 mm in length. The single
median fins down the midline of their back provided stability in locomotion, but
only the group Cephalaspidomorpha had paired lateral appendages that may
have had a similar function to the lateral fins of true fishes. All these animals had
gills located in pouches
An important development in one group of protofish was the development of bony
rods stiffening the pillars of flesh between the gill slits. The first pair of which
hinged forward and were supported with muscle tissue, and produced the first
jaws. The evolution of jaws permitted fish and their descendants to utilize larger
and harder food, and thus enabled them to become adapted to many new and
diversified ways of living. This advance was of sufficient importance so that fish
and tetrapods (four-legged animals) are together called gnathostomes (= jaw +
mouth). Some of the bony scales in the skin which covered these animals
enlarged and became the first teeth. Lateral flaps of skin evolved into the first
true fins and their swimming skills improved. These animals were called
Placoderms and may have pioneered the gas bladder for vertical movement in
water and eventually evolved into lungs. The most impressive of the placoderms
was the Arthrodira which reached 9 m and possessed large jaws equipped with
serrated teeth.
One of these animals (Acanthodii) were acquiring an internal bony skeleton and
included the beginnings of a vertebral column running longitudinally through the
body and encompassing the primitive notochord. These were the probable
ancestor to the bony fish we know today and possessed a streamlined body,
large lateral eyes and wide mouths with numerous teeth. Their heads are bony
and their small scales are thick and hard, but unlike the placoderms they did not
have armour. The numerous lateral fins of these animals are unique in that each
has a thin membrane supported at its leading edge by a long stout spine.
At this time a pronounced split appeared in the fish dynasty, with one line of
animals losing all their bone and developing cartilage, a softer more elastic and
lighter material. The descendants of this are the fish belonging to the class
Chondrichthyes and represented by sharks (orders Galeomorpha and
Squalomorpha), rays (order Batoidea) and chimeras (order Chimaerida).
Although this lightened them they would still need to continue to swim or they
would sink. Swimming is still accompanied by a powerful thrash of the tail and
pectoral fins which prevent them from diving nose down. Since the pectoral fin is
stiff these have less mobility than the pectoral fins of the bony fish. Some of
these fish rested by sinking to the sea-floor, and one group has adopted such a
position on a semi-permanent basis (rays and skates). As a consequence they
have become greatly flattened with pectoral fins expanded into undulating lateral
triangles which they use for locomotion and the muscle in the tail is almost
completely lost (although it may bear a poisonous spine at the end). Rays and
10
skates are not as fast swimming as sharks, but this is of less importance since
they feed on molluscs and crustaceans.
Sharks have mouths on their undersides and water passes through the mouth
and over the gills and out through the slits. With bottom-dwelling mantas and
skates this would cause mud to get into the gills, so instead, they have two
openings or spiracles on the upper surface of the head that take in water and
lead it straight to the gills. It is then expelled on the underside through the gills.
One kind of ray, the manta has reverted from bottom-dwelling to surface dwelling,
using the large lateral extensions to remain afloat.
The other group of fish which retain bone in its skeleton, also had to overcome
weight problems in the water. Early fish with heavy bone-based scales,
colonized shallow lagoons and swamps which had warm, poorly oxygenated
water. The bichir (Polypterus)(order Polypteriformes), a heavy scaled fish
occurring in Africa indicates how these early fish overcame such problems.
These animals rise regularly to the surface and take a gulp of air which goes into
a pouch leading off the top part of the gut. A concentration of capillaries in the
walls of the pouch absorb the gaseous oxygen. These air-filled pouches which
were the first lungs also provided buoyancy and the ability to float without using
the tail and eventually evolved into swimbladders. With the ability to absorb gas
from the blood systems there was no need to collect air from the surface and the
connecting tube to the throat became no more than a solid thread. The diffusion
of gases into and the expelling of air out of the swimbladder would permit a
precise means of vertical control in the water. The pectoral fins would provide
refinement to this control. However, swimming skills were improved still further
with increased tapering of the twin-bladed symmetrical tail that is driven by banks
of muscles on either side of the backbone. Streamlining was enhanced with
reduction of heavy scales into smaller tightly fitting ones that overlap like tiles of a
roof and are covered by slippery mucous, and pectoral and pelvic fins being able
to fold back into depressions in the lateral sides of the fish. The respiration using
gills was further refined with the development of a movable, bony operculum
which by inducing negative pressure forces water over the gills and improves
respiration.
The diversity of morphological forms is testimony to the success of the group.
One group, the flying fish (order Atheriniformes) leap out of the water and glide
hundreds of metres in the air using the elongated pectoral fins. This may be an
anti-predator tactic. Garfish (order Lepisosteiformes) have pectoral fins that have
become filmy skulls rotating slowly bach and forth which permits them to hover in
water. Dragonfish (order Pegasiformes) have lateral fins modified into defensive
mechanisms with each ray barbed with poison.
The swimbladder has released fish from weight problems, and therefore, some
like the box-fish (family Ostraciontidae) and sea-horse (order Gasterosteiformes)
have regained armour.
Down the flanks and around the head of fish runs a series of pores, connected by
a canal running just below the surface. This is called a lateral line and enables
the fish to detect differences of pressure in water. As a fish swims, it creates a
pressure wave ahead of it, when this wave meets another surface the fish can
detect pressure changes created by this surface. It is this ability that permits
them to detect other fish and to polarize themselves into swimming in shoals.
Vulnerability to predators is thought to be reduced by shoaling. Fish also have an
acute sense of smell and detect minute changes in the chemical composition of
water. This sense of smell may guide fish to food. Fish also detect sound with
the addition of a third canal (in a horizontal plane and below the sac) which
supplements the two semicircular canals that are found on either side of the skull
of the lamprey. All three canals and the sac have very sensitive linings and
contain small calcium particles which move and vibrate. Sound waves, which
travel better in water, penetrate the semicircular canals without the need for
passages which are required by terrestrial animals.
The eyespot of the lamprey is primitive compared with the bony fishes. The eye
of the bony fish and higher vertebrates is a closed chamber with a transparent
window and a lens in front and a photosensitive lining at the back (retina). The
photosensitive lining contains two kinds of cells, rods for distinguishing light and
dark and cones which are sensitive to colour. Sharks and rays lack cones and
are unable to perceive colour; this may reflect the lack of highly coloured
examples within the group. Bony fish have both types of cells in their retina, and
are also characterized by vivid colours and striking patterns. The Butterfly fish
(Family Chaetodontidae) showing particularly diverse colours and patterns which
permits species recognition. Colour is also an important asset in male fish during
spawning. Such displays serve to chase other male fishes away, and to attract
female fish. Pigment granules diffuse within the skin as the fish become excited
and fights other rivals or to stimulate a female fish to lay her eggs.
Eyes of fish have become adapted in various ways to vision below and above
water. The archer fish (Toxotes jaculator) squirts fluid at an insect above the
water and knocks them into the water where they can be eaten. This required
compensation since light bends as it passes from water to air due to differences
in density. Anableps has a horizontal division across its pupils which effectively
gives it four eyes, the two lower halves for underwater use and the two upper
halves for above water. Since fish can occur at great depths (below 750 m)
where there is no light, they may posses modified cells producing luminescent
chemicals which are activated rhythmically and may represent some form of
communication to the rest of the shoal. The whiskery angler fish Antennarius
scaber (order Lophiiformes) has a modified dorsal fin spine with an elongated
thread at the end of which are cells producing luminescence. This is used to
entice other fish to explore the light and be consumed.
Water that is covered with floating mats of vegetation is also turbid, and in such
an environment some fish have generated electricity from modified muscles in
their flanks. Electrical signals are transmitted almost continuously creating flow
patterns of current in the immediate vicinity. Any object encountered disrupts
these flow patterns and the fish perceives these changes through receptor pores
located over the body. The electric eel of South America Electrophorus
electricus, although not a true eel, has additional body tissues that produces a
massive shock of waves with which it kills or stuns prey items.
From the jawless armour-laden prototype fish have evolved some 30 000
different forms to occupy seas, lakes and rivers of the world.
ASSIGNMENTS
IN YOUR OWN WORDS WRITE A ONE TO TWO PAGE ESSAY ON THE
FOLLOWING TOPICS
Discuss the development and modifications of the eye that have occured in
the fish. Also discuss why sharks and skates are generally dull-coloured,
whereas many bony fish have bright colours.
Describe the evolutionary transitions from the earliest chordate (e.g.
amphioxus) to the most advanced bony fish.
Describe the morphological differences that exist between the
11
cartilaginous and bony fish.
THE INVASION OF THE LAND
The first fish may have crawled onto land during Devonian times (350 million
years ago) and probably did so, in response to drying swamps. This required
that two problems must be overcome, how to move without the support of water
and how to obtain oxygen from air rather than water. The mud skipper (eg
Periophthalmus sobrinus which occurs in our mangrove swamps) suggest
adaptations that ancestral fish may have developed in their quest to colonize
land. Each pair of front flippers has a fleshy base supported internally by bones
and is able to be used to lever the animal forward. Another animal which showed
the beginnings of limbs is the coelacanth Latimeria chalumnae (order
Crossopterygii), the living fossil that was thought to have been extinct for 70
million years.
In order to breathe in air the mudskipper retains water in its mouth which swills
over the limning of it. The African Lungfish Protopterus (Order Dipnoi) can
burrow into mud, curls itself into a ball and secretes mucous which creates a
parchment-like case around the hole it has encased itself in and avoid
desiccation during the dry season. The lungfish has a pouch opening from the
gut (similar to the primitive bichir fish) which functions as a lung and extracts air
from the tube it created when burrowing through the mud. By flexing its throat
muscles the fish draws air into its pouch which is supplied by numerous blood
vessels which absorb gaseous oxygen. With the termination of the dry season
the fish returns to an aquatic existence and breathes with its gills, but like the
Bichir may take gulps of air if a lack of oxygen develops.
Although all of these animals have been regarded as possible ancestors of the
first tetrapods which colonized the land, their skull morphology is unlike the first
fossil tetrapods which were amphibians. Neither Coelacanth nor Lungfish have a
passage linking the nostrils with the roof of the mouth, a characteristic feature of
all land vertebrates. However, another lobe-fin fish called Eusthenopteron, which
only exists today as fossils, possessed such a passage and well developed
lobes. Careful examination of the fins of these fossils revealed that the base of
the lobe was supported by one stout bone close to the body, two bones joined to
it and at the terminal end a group of small bones; an arrangement found in the
limbs of land vertebrates. A link between lobe-fin fish and amphibians has been
found in the fossilized Ichthyostega found in Greenland in 1938. The swamps
through which such animals waded was thick with horsetails and club moss trees
which became fossilized as coal and also contained the first fossils of the
terrestrial vertebrate (tetrapods) which belong to the class Amphibia. These
animals had evolved only 50 million years after the first bony fish and reached
greatest expansion some 100 million years later in the Upper Carboniferous
period. Some of these early forms grew to four metres in size and possessed
jaws spiked with cone-like teeth. Today relatively few amphibians have survived,
but they are nevertheless distributed in tropical and temperate areas of the world
and in a variety of habitats. The modern amphibians differ considerably from
their large ancestors. The living amphibian that most resemble early forms are
the salamanders and the newts which collectively are called Caudata ("tailed
ones"). The largest member of this group comes from Japan and has a body
length of 1,7 m (Megalobatrachus).
In general amphibians are only partly successful in their colonization of land,
since their limbs are short and they need to flex their body laterally in order to
take reasonable strides. Amphibian skin is permeable and in a dry atmosphere
would quickly dehydrate, they even do not have the mechanisms to drink water.
A moist skin is also required to supplement respiration, since the lungs are
comparatively simple and not totally adequate for its needs. These limitations
restrict amphibians to moist environments. For reproduction amphibians are also
almost entirely dependent on water since, like fish, their eggs have no waterproof covering and their larvae (tadpoles) are quite fish-like. These larvae initially
have no legs swim using a long tail and breathes using external feathery gills.
The two life phases of some species of Caudata (entirely aquatic and semiterrestrial) have been used to exploit a greater variety of habitats. A Mexican
salamander (Ambystoma mexicanum) regularly changes from an aquatic form to
a land form. If there is a particularly wet season and/or the lake does not shrink
greatly the larval stages are maintained and the larvae may become as big, or
bigger than the land-living forms. A lack of iodine in the water may have
prevented metamorphosis. Another species of Caudata has reverted
permanently to an aquatic life. It always breeds in a larval condition and its
external gills develop into branching bushes on either side of the neck. Using a
thyroid extract it can be induced to lose its external gills, develop lungs, and turn
into an animal that resembles a burrowing salamander that lives in Florida.
However, another species called the mud-puppy Necturus maculosus has
reverted irrevocably to water-living and has external feathery gills and very
reduced lungs. The Greater Siren (Siren lacertina) is more elongated, has lost its
back legs altogether and also breathes using gills. The Three-toed Amphiuma
(Amphiuma means tridactylum), from southern USA is extremely elongated with
tiny legs that have no function and is known locally as a Congo eel. This
tendency to retain larvae characteristics in adult forms is called paedomorphosis.
The abandonment of lungs and limbs, the cornerstone adaptations that permitted
colonization of the land is not entirely restricted to aquatic amphibians, but even
occurs in animals that live almost entirely on land. Such animals breathe through
their skin and the moist membranes lining their mouths. The elongated body
forms permit maximum surface area, but they nevertheless remain only a few
centimetres in size and are restricted to very moist environments.
Another group of Amphibians called the caecilians are also limbless, but are
adapted to a burrowing existence and almost resemble earthworms. Their
anatomy is so different from the salamanders that they are classified in the order
Gymnophiona. They have several primitive features such as the retention of
small scales in the skin and a very short tail. The solidly build skull, the
elongated body comprising as many as 270 vertebrae, no internal girdles for
supporting limbs and blindness (compensated by having extendable, sensory
tentacles) are all adaptations to a burrowing existence. However, they are
carnivorous and have mouths with a large gape.
There are about 300 species in the order Caudata and 160 in Gymnophiona, but
the most numerous group of amphibians belongs to the order Anura (tail-less
ones) with about 2600 species. The Anurans include frogs which are generally
characterized by smooth, moist skins and inhabit moister environments, and
toads which have a drier, warty skin and often occur in drier environments.
Unlike the members of Gymnophiona, this group has shortened the body and
have even fused vertebrae together and have developed their hind legs
enormously to become accomplished jumpers. The Goliath Frog (Gigantorana)
can achieve 3m and the tree-living frog Rhacophorus reinwardti can achieve
fifteen metres by gliding. To do this they increased the size of toes and with it the
web of skin that unites them to form a parachute on each leg.
Jumping represents a major way of escaping predators. Since amphibians are
12
generally soft bodied, they are sought after as food items by larger predators,
however, many rely on having a cryptic coloration of green camouflaged with
blotches of brown and grey. The common European Toad (Bufo bufo) inflates its
body and stands on its toes to appear as large as possible and thereby
discourage any potential predator. More active defence occurs in the fire-bellied
toad whose mucous which keeps the skin moist is also extremely bitter tasting.
The poison arrow frogs (Family Dendrobatidae) include some species whose
mucous is lethal to mammals and local people used its poison to tip their arrows.
Such defences are of little value if their attacker dies after they themselves have
been eaten, and therefore are often accompanied by bright warning colours and
patterns (aposematic coloration).
Amphibians are all carnivorous, and indeed some are quite formidable such as
the horned toads (Family Leptodactylidae) which can easily prey on a nestling
bird or a small vertebrate such as a mouse. Although these frogs have a large
mouth serviced with sharp teeth, their purpose is for defence and to grip the prey
item, but does not help with breaking the prey item into smaller parts for
digestion. Most amphibians are smaller and restrict their prey items to
invertebrates. For this purpose an extendible tongue attached to the front of the
mouth has evolved. The tongue is sticky at its tip and when it is flipped out it
adheres to the small prey item which is bodily brought back into the mouth. The
tongue helps with swallowing since it produces mucous which lubricates the food
before passing it into the gut.
Amphibian eyes are fundamentally similar to their fish ancestors, however, they
do require a membrane that can be drawn across the eyeballs to keep the
surface clean. However, the mechanisms that fish use to perceive sound using
resonances generated in their swimbladder will not work in air, and consequently
ear drums have evolved to detect sound waves. With their increasing ability to
detect sound waves frogs have also developed the ability to produce sound using
the huge swelling of their throats to amplify the sound produced by air blown from
their lungs over simple vocal cords. Such sounds are unique to individual frog
species and are used during courtship (a prelude to mating) and to recognize
frogs of the same species.
Mating for most amphibians still takes place in water, with the males grasping the
females and fertilization taking place externally, with the sperm cells swimming to
the egg cells. Large numbers of eggs are produced to offset high mortalities of
eggs and tadpoles. Other frog species have a different strategy whereby
comparatively few eggs are laid, but considerable parental investment protects
them from predators. Some tropical pond-dwelling frogs (e.g. Dendrobates
pumilio and Osteopilus brunneus) find safety for their tadpoles by depositing eggs
in centres of plants such as bromeliads which create small reservoirs of water in
forests where the rainfall is high. These sites are safe from aquatic predators. In
Dendrobates pumilio males and females divide parenting duties; males guard the
eggs until they hatch (10 to 12 days) thereafter the females assumes care for the
young. The females begin by transporting each newborn tadpole to a bromeliad,
at the base of which a small pool of water has collected. Although protected from
desiccation and predation, the tadpole has no food supply and is entirely
dependant on its mother for nutrition until it metamorphoses into a froglet which
takes six to eight weeks. In Brazil, another small frog builds its own ponds at the
margins of forest pools, constructing a crater ringed with low mud (100 mm in
height). The eggs are laid and the tadpoles stay in their exclusive water
residence until the rain raises the level of the main pool and floods the crater
created by the parent frogs.
When the first amphibians appeared, the terrestrial environment would have been
a much safer site for the development of their offspring than an aquatic
environment which has many predators (especially fish). As a consequence
anurans evolved mechanisms to exploit the terrestrial environment for the
breeding of its young. The midwife toad Alytes obstetricans (Family
Discoglossidae) of Europe lives in holes close to water and mates on land. After
fertilization the long strands of eggs are twisted around the hind leg of the male
toad. The male carries them around until the tadpoles are ready to hatch and
then takes them to water. The South American Centrolene frogs defend calling
sites which are leaves overhanging streams. Such sites are where the eggs are
laid, and parent frogs attend the eggs until they hatch and the tadpoles fall into
the water below. In Africa some species of frog (e.g. Chiromantis) breed on
branches of trees above ponds. The female excretes a liquid which is beaten
into a ball of froth by the male frog. The eggs are then laid and the outside
surfaces of the froth harden into a crust which retains moisture. The female frogs
may bring up additional moisture to the nest. The eggs hatch and tadpoles
develop within the hardened froth. The tadpoles are released when the lower
part of the froth ball liquifies and they fall into the water below. Frogs producing
foam nests occurs in five anuran families (Rhacophoridae, Hyperoliidae,
Myobatrachidae, Hylidae and Leptodactylidae). In some tropical American frog
species (Eleutherodactylus) a considerable yolk is provided in each egg which
makes it possible for all stages of larval development to take place within the
eggs and fully developed froglets emerge directly from them, this is term direct
development and occurs in nine families of frogs.
Many frogs invest considerable parental effort. The toad Pipa carvoelhi have a
normal anuran copulation in water, however, only a few eggs are fertilized and
the male frog using his webbed hind feet gathers the eggs and spreads them
onto the female's back. This process is repeated until about a hundred eggs are
gathered. The skin below begins to swell and embed the eggs and a membrane
develops over the top of the eggs and covers them. After 14 days the female's
back is rippling with the movements of hatched tadpoles. After 24 days the
young break holes and are released from their mother. The frog species
Gastrotheca have brood patched on their backs where fertilized eggs develop
into tadpoles. When the tadpoles are ready to be released the female finds a
shallow pond to sit in and deposit her young. In Gastrotheca ovifera more yolk is
provided with the eggs, and the tadpoles remain until they are froglets before
leaving the pouch. Egg brooding is usually done by the female frogs, although in
the Australian hip-pocket frog Assa darlingtoni it is the male who broods them.
The stage at hatching for egg-brooding frogs, also called marsupial frogs, is
determined by the amount of yolk in each egg, which in turn reflects the number
of eggs produced per clutch. Species in which the eggs hatch as small tadpoles
produce 100 or more ova, each ca. 2 mm in diameter. Where froglets emerge
directly, only about six ova are produced, each ca. 10 mm in diameter.
A West African frog Nectophrynoides has taken parental effort even further.
These frogs have internal fertilization with the fertilized eggs retained in the
oviduct. Tadpoles develop, complete with mouths and external gills and they
feed within the oviduct on tiny white flakes excreted from its walls. After nine
months development which is co-ordinated with the arrival of the first rains, the
female gives birth, by bracing her body against the ground with her forelegs and
then inflating her lungs to full capacity which in turn swells the abdomen and
squeezes the young out by pneumatic pressure. The tiny frog Rhinoderma found
in southern Chile deposits her eggs on moist ground, the males sit in groups
around the eggs and guards them. When developing eggs move within the
gelatinous coats, the males take the eggs into their large vocal sacs where they
continue developing until they are fully-formed froglets. Phyllobates
subpunctatus, one of the South American poison dart frogs (Family
Dendrobatidae), also lays the eggs on moist ground in close proximity to a
guarding male frog. When the tadpoles hatch they wriggle themselves onto the
13
male's back, where his copious quantities of mucous keeps them attached and
prevents them from drying out. These tadpoles have no gills, but obtain oxygen
by absorbing it through the skin of their body and from the surface areas of their
greatly enlarged tails.
However, the most bizarre form of parental care is the Australian frogs
Rheobatrachus silus and Rheobatrachus vitellinus. In these species the female
frogs swallows the eggs after fertilization and broods them in her stomach for six
weeks. Such a breeding system presents interesting problems, such as how the
eggs and hatched tadpoles escape being digested within the mother's stomach?
The nurturing females appear to cease feeding during the breeding period. The
production of hydrochloric acid and pepsin are halted in the stomach by a
hormone-like substance prostaglandin E2 which is secreted by the egg capsules
and then by the tadpoles. With this shutting down of normal stomach activities,
the stomach's digestive functions are transformed into that of a protective
gestational sac. The eggs, which range from 21 to 26, are relatively large, ca. 5
mm and rich in yolk. Consequently, the tadpoles do not need an external source
of nutrition but feed exclusively on yolk throughout their six-week development
period. During birth the female's oesophagus dilates in a manner analogous to
the vaginal canal of mammals, and the young froglets are propelled from her
mouth. Within a few days after expulsion of the young, the stomach begins to
function again as a digestive organ, and the frog resumes feeding. Unfortunately
neither of these two species has been found recently, and it is sadly concluded
that these interesting frogs are now extinct.
From these patterns it is clear that size and complexity of parental investment
reflects clutch size. Another trend is that development time appears to reflect
climatic conditions. Frogs within tropical regions develop relatively rapidly,
sometimes spending only two or three weeks in the tadpole stage, whereas those
living in cool temperate climates develop much more slowly. One such species is
the spotted frog, Rana pretiosa, which lives in the cold streams that cascade
down the Rocky Mountains. Because the cold water in which the frogs live slows
their metabolism, more than one year is needed to produce fully yolked eggs, and
the females lay eggs only once every two or three years. Tadpoles also
metamorphose more slowly in cooler areas. For example bull frogs in northern
USA (Rana catesbeiana) typically spend two years in the tadpole stage and
another species Ascaphus truei needs three years to reach adulthood.
In arid habitats, development is limited not by temperature but by moisture. One
example are the rainfrogs Breviceps (Family Microhylidae) which lives in arid
regions of Africa. These animals only emerge above ground during heavy
downpours. Although much of the biology of this elusive group of frogs is
unknown, it appears that they form pairs during the breeding season. Adults
emerge from their underground burrows and absorb rainwater through their skins,
thus replenishing their body fluids. In particular the bladder is filled with water.
The male is far smaller than the female and is unable to clasp the female in order
to copulate with her. Instead the male glues himself to the female's back. With
the male riding on her back, the female burrows into the ground and proceeds to
lay eggs that are then fertilized by the attached male partner. Periodically the
female wets the eggs from her extended bladder, keeping them moist until the
froglets hatch. This breeding process takes place on only one or two nights per
year, when there is a sufficiently heavy downpour. Once fertilization occurs,
growth proceeds rapidly. The Spadefoot toads (Scaphiopus) in southwestern
deserts of the USA, have tadpoles that develop into frogs in less than two weeks.
Such rapid development is necessary in a habitat where the water will only last
for a few weeks.
The zenith of amphibian's adaptations to minimize their dependence on water
under arid conditions is the water-holding frog, Cyclorana which inhabits the
central desert regions of Australia. During the brief and infrequent periods of rain
these frogs feed on the flush of insects, they mate and lay their eggs in tepid
shallow pools of water, the eggs hatch and tadpoles rapidly develop into froglets.
As the rain soaks away the frogs and froglets absorb as much water as possible
and bury themselves deep into the sand where they secrete a membrane around
themselves to prevent moisture loss. They remain in this condition until the first
significant rains, which could be in several years time.
ASSIGNMENTS
IN YOUR OWN WORDS WRITE A ONE TO TWO PAGE ESSAY ON THE
FOLLOWING TOPICS
Discuss the adaptations required to make the transition from an aquatic to
a terrestrial life using the amphibian group as an example. What
limitations to a terrestrial life do amphibians exhibit?
Why have some amphibians after evolving limbs then lose them to become
limbless? Support your answer with both terrestrial and aquatic examples.
The anurans have evolved a variety of reproductive strategies to reduce
predation of eggs and tadpoles and to exploit arid regions. Discuss such
adaptions with special reference to the amount of parental investments
existing between different anuran species.
A WATER-TIGHT SKIN AND THE SHELLED EGG
The reptiles evolved from an early ancestral group of amphibians (Subclass
Labyrinthodontia) which have been extinct for 175 million years. Terrestrial
Labyrinthodontia had strong limbs, robust bodies. The first animal with a dry skin
was probably Seymouria which lived in the Permian (230 million years ago) is
thought to be the link between the amphibians and the reptiles and was probably
the first animal to have a hard-shelled egg that entirely freed its reproduction from
water or extremely moist habitats. All amphibians require a moist environment to
survive and reproduce, but the reptiles can occupy a dry environment due to their
water-tight skin and the shelled eggs.
A modern example of how reptiles manage to survive in hot dry conditions can be
found in the marine iguanas (Amblyrhyncus cristatus) which are able to survive
on barren larva fields on the tropical arid islands of the Galapagos Islands.
These animals bask in the sun which helps raise their body temperatures without
the risk of desiccation. Physiological processes of animal's body, like other
chemical reactions, are affected by temperature. Up to about 40oC the higher the
temperature the quicker the physiological processes and the more energy they
produce and the more active the animal can be. Neither reptiles nor amphibians
generate their heat internally like we do, but they draw it directly from the
environment usually in the form of solar radiation. The daily activity cycle of
these marine iguanas maintains the body at the most efficient temperatures. At
dawn, when ambient temperatures are lowest, they climb to ridges and expose
themselves broadside to the rising sun. As temperatures rise, the risk of
14
overheating increases, the iguanas respond by lifting their bodies off the ground
and positioning themselves so that air currents can pass below them. They can
also pack themselves into the few shady places that exist (such as rock
crevices). The sea surrounding the islands is influenced by the cold Humboldt
Current, and is only entered to feed on green alga at the hottest time of day
(noon). During foraging their bodies would cool-off rapidly and they will need to
conserve as much heat as possible. These animals therefore constrict the
arteries near the surface of their bodies so that blood circulates only in the centre
of the body. Nevertheless body temperatures will drop up to 10oC before they
have to return to land. On land they stretch-out their bodies and absorb warmth
from the black larva surfaces. With the setting sun they again congregate on the
ridges with their bodies broadside to the last of the solar radiation for the day.
These behavioral sequences maintain the body temperatures close to 37oC,
although it varies considerable more than in endothermic animals (eg our
bodies). Animals like iguanas are ectothermic since their body temperatures tend
to fluctuate. Endothermy has advantages since it permits greater independence
of the prevalent temperatures (eg can maintain activities at night and in cold
regions), but is energetically expensive. About 80% of daily calories is invested
in maintaining body temperatures constant in endothermic animals. In contrast
an ectothermic animal uses only 10% of the energy that an equivalent
endothermic animal would use. As a consequence they survive in desert
conditions were endothermic animals would have more difficulty surviving.
The ability to breed under dry conditions is achieved by a gland located in the
lower part of the oviduct and secretes a parchment-like shell which prevents
desiccation of the shell. However, the shell still needs to be supplied with
sufficient yolk to support the development of the embryo and the shell needs to
be porous to enable oxygen to diffuse through. Clearly fertilization of eggs needs
to be internal (male reptiles therefore evolved a penis) and to be completed
before the shell is deposited. The Tuatara Sphenodon punctatus (Order
Rhynchocephalia) an ancient lizard that occurs on New Zealand has no penis
and males and females press their genital openings close together in order to
achieve internal fertilization in a way similar to amphibians. These lizards have
another amphibian feature that is an ability to be active down to 7oC, a much
lower temperature than for any other reptile. Fossilized bones of these creature
have be dated to 200 million years ago and may represent one of the most basic
four-legged (tetrapod), tough skinned, egg-laying ectotherm that was a
predecessor to the great dinosaurs that conquered all parts of the earth (except
the polar region). The diversity of dinosaurs also included forms that returned to
the sea (ichthyosaurs and plesiosaurs).
The amphibians and earliest reptiles that evolved from them are often referred to
as cotylosaurs, and the stem reptiles themselves are called captorhinomorphs.
Less than 100 million years after their first appearance, the captorhinomorphs
had already divided into three major divisions (Subclasses) based on the skull
structure.
One lineage referred to as the Anapsids has turtles and tortoises (Order
Chelonia) as living representatives. The anapsids are characterized by a solid
skull roof with no temporal openings in the skull (viz. area behind the orbits of the
eyes).
A second lineage referred to as the Diapsids produced the most diverse and
spectacular radiation of animals. Diapsids skulls primitively possess upper and
lower temporal openings behind the orbit of each eye. Living representatives of
this group include snakes and lizards (Order Squamata) and the Tuatara (Order
Spheodontida). Extinct forms within this group included the marine reptiles
(ichyosaurs and plesiosaurs) which are sometimes referred to as Euryapsids.
However, the largest group within this lineage are the Archosaurs (ruling reptiles)
most of which are now extinct except crocodiles and alligators (Order
Crocodylia). Extinct Archosaurs included the famous dinosaurs represented by
two orders; Saurischia (lizard-hipped dinosaurs with a triradiate pelvis) and
Ornithischia (bird-hipped dinosaurs with a tetraradiate pelvis), the flying
pterosaurs (Order Pterosauria) and thecodonts the ancestral stock o f all
archosaurs and birds. Thecodonts were relatively small and often bipedal
reptiles that had a resemblance to the first crocodiles.
The third lineage refers to the Synapsids, which possess skulls with a single
(lower) temporal opening behind the orbit of each eye. These were the first group
of reptiles to colonize land during the Permian period and are referred to as the
mammal-like reptiles. Within the synapsids two orders have been identified. The
primitive Order Pelycosauria was characterized by animals which developed
elongated spines from the vertebrae and are commonly referred to as sailbacks.
The most spectacular example was Dimetrodon with vertebrae projecting more
than a metre above the back at their highest point. These vertebral spines
supported a web of skin and probably served as a temperature-regulating device
that added a great area of skin surface for warming up and cooling off. The other
group of synapsids are classified in the Order Therapsida. The therapsids
developed into animals that resembled dog-faced tanks, for their limbs extended
beneath their bodies, rather than to the sides, they may have had fur, and
exhibited specializations of bone and teeth structure. These mammal-like
reptiles suffered at least six distinct mass extinctions during the last eight million
years of the Permian. The survivors of each extinction appeared to be more
warm-blooded, to have more specialized jaws and teeth and to possess a more
efficient respiratory system. Although this line ultimately lead to the evolution of
the mammals, they came to dominate only fairly recently during the Tertiary
period (starting some 65 million years ago).
The Triassic period produced new forms of reptiles (archeosaurs), the
ichthyosaurs, crocodiles and the flying pterosaurs and the first of the dinosaur
line, which were small active animals about the size of a pheasant, many of
which were bipedal and had probably evolved high metabolic rates. Some may
even have been covered with down and later feathers, an evolutionary line that
ultimately evolved into birds. These dinosaurs remained in the shadow of the
dominant group which were the mammal-like reptiles. Towards the end of the
Triassic at about 220 million years ago, a mass extinction of the mammal-like
reptiles may have facilitated the radiation of the other reptile group (archeosaurs)
during the next million years (Jurassic). The oldest true dinosaur Eoraptor has
been dated at 230 million years ago. This animal was a primitive, small (ca. 1
metre), carnivorous dinosaur. Like the more recent and better known dinosaur
Tyrannosaurus rex, Eoraptor belonged to the saurischian group of dinosaurs
(lizard-hipped). Eoraptor is considered primitive because it has an exceptionally
simple jaw, and probably evolved shortly after saurischians and ornithischians
diverged. Only 10 million years after Eoraptor the entire dinosuar group had
already diverged, whereas the other reptile groups, such as the crocodiles and
mammal like reptiles were declining rapidly.
The richest deposits of dinosaur fossils have been found in the midwestern states
of North America. Although recent excavations to Mongolia suggest that this
region will provide the greatest number of new fossil dinosaur species. Some of
these dinosaurs were no bigger than a chicken called Compsognathus, whereas
others represent the largest land animals that have existed on the earth such as
Apatosaurus which measured 25 m long and weighed at least 30 tonnes. A fossil
dinosaur, Seismosaurus, unearthed in 1986 appeared to have been 43 metres
long and weighed about 100 tonnes. Another dinosaur called Ultrasaurus, may
have been heavier still with an estimated mass of 150 tonnes.
15
The simple peg-like teeth of these animals meant that food, particularly plant
material such as the tough leaves of the cycads that existed at that time, had to
be broken down in the stomach. Mammals have specialized teeth that breakdown and grind food to a considerable extent before entering the stomach for
further processing. Consequently herbivorous dinosaurs probably needed large
guts and even used stones (gastroliths) to process their food and this may have
been the reason for them becoming so large. Carnivorous dinosaurs, like
Tyrannosaurus, would also need to be large to prey on these mega-herbivores.
Many Apatosaurus fossil bones have teeth marks which correspond to the fit of
carnivorous dinosaur's jaws such as Allosaurus. Some scientists have reinterpreted such findings and have suggested that these apparently carnivores
were more likely to have fed on the large carcasses of the mega-herbivores.
The large size may also reflect temperature control. The bigger the body the
more heat it retains and the more constant the temperature will remain for the
animal. Evidence for warm-bloodedness is that the chest cavities are large
enough to hold huge hearts, like birds do today. The dinosaurs were known to
migrate, and both their northerly and southerly limits to these migration routes
would have not been possible for a cold blooded animal. The bone histology of
dinosaurs (particularly the more advanced thecodonts) suggest that they may
have regulated their temperatures the way birds and mammals do today.
Specialized structures such as the parallel rows of plates on Stegosaurus have
been interpreted as additional temperature-control mechanisms. These plates,
although made of bone, are spongy and probably carried many blood vessels
which could either dissipate excess heat or absorb heat from the environment.
Anatomical analyses of many dinosaurs suggested that they were active, fastmoving animals, and therefore probably possessing endothermic metabolisms.
Finally the ratio of predator-prey ratios of fossilized dinosaurs do not correspond
to the expected ratios assuming them to be ectothermic but does more closely
resemble those of endothermic mammals. It is recognized that endothermy may
take several forms and that some dinosaurs may have fell short of fully fledged
endothermy. It has even been speculated that Tyrannosaurus rex underwent
three vastly different growth stages and may have been equipped with a variable
metabolism. A 2 metre juvenile would have been very active, capable of
scampering around like some groundbirds do today. By contrast, mid-sized
individuals, averaging 3.5 to 4.5 metres were probably less agile, and may have
traveled in packs. A fully grown 12 metre adult weighing 8 tons would not have
been agile, and may have reverted to a solitary life-style scavenging on
carcasses. Further, all, but a few highly specialized endotherms have some kind
of heat insulation in the form of hair, fat or feathers. Without it, the demands on
energy are so extravagant, that it is difficult for such an animal to survive.
However, the only fossil impressions of a dinosaur skin discovered suggests that
their hides were not furry or leathery, but scaly and covered with bony bumps. It
has even been suggested that the large herbivorous dinosaurs (sauropods)
would have required hundreds of kilos of vegetation a day to sustain their
enormous bulk and that they had a unique endothermic metabolism fueled by the
heat given off by non-stop digestion.
The dinosaurs had several extinction phases, with the gigantic dinosaurs, being
replaced by smaller, low browsing, beaked dinosaurs at the end of the Jurassic
and early Cretaceous. Again, another extinction occurred and marked the late
Cretaceous period. These dinosaur extinctions may have been related to the
radiation of angiosperm plants (viz plants possessing flowers) which attracted
animals to disseminate their pollen and seeds. A new generation of low browsing
dinosaurs may have promoted the spread of these plants. Overgrazing by
dinosaurs may have threatened many low-growing plants with extinction, except
for the angiosperms which possessed reproductive superiority. The late
Cretaceous period witnessed the Hadrosaurs or duckbill dinosaurs (Anatosaurus,
Lambeosaurus, Corythosaurus and Parasaurolophus) occupying swamps and
forests and large herds of Triceratops and their relatives on the grass plains
together with Tyrannosaurus rex.
The discovery of fossilized egg-filled dinosaur nests belonging to the Hadrosaur
Maiasaura gives new light on the life-styles of dinosaurs. Grouped nests were
found in a single layer of sediment, implying that they were all built in the same
year. These nests were spaced at an average of 7 metres apart:- about the size
of an adult Maiasaura. Some bird species lay their eggs close enough together
for maximum mutual protection, yet far enough apart so that they can move
easily past their neighbours. Tiny eggshell fragments within the nests suggested
that baby dinosuars remained in the nests to be cared for and fed by their mother.
Had the Maiasaur simply hatched and wandered off to fend for themselves, the
shells would be broken in a few large pieces rather than smashed into fragments.
It is now accepted that these hadrosaurs nurtured and protected their young,
probably feeding them by mouth like young birds until they were strong enough to
leave the nests.
The amazing aspect of these mesozoic reptiles were their exploitation of not only
the terestrial surface but their conquest of the air by pterosaurs and their
recolonization of the aquatic environment by Ichthyosaurus and Plesiosaurus.
The ichthyosaurs were completely adapted to a marine life, like mammal such as
dolphins are today. Fossil evidence suggested that egg-laying on land had been
abandoned, and that the young were born alive and at sea. The body sahpe was
completely reconverted to that of a fish; the neck telescoped to give a fusiform
body shape, the limbs shortened into small steering devices. LOcomotion was
performed, fish-like, by undulations of the trunk and tail; a fishlike fin was
developed on the back (but like that of dolphins, it lacked the skeletal support
found in dorsal fins of fishes), but the tail became a powerful swimming organ, in
appearance like that of a shark. In this last regard, however, there is a notable
structural difference; for whereas in a shark the end of the backbone tilts into the
upper lobe of the tail fin, that of the ichthyosuar turns sharply down at the back,
with the fin expanding above it. Most ichthyosaurs were presumably fish-eaters,
but some feed on ammonites. The plesiosaurs were less extreme in their
adaptations and probably were able to wadddle up on to a beach for egg-laying
rather like marine turtles do today. They possessed a long neck or long snout or
both; the body was short, broad, and relatively flat. Reversion to a truely fishlike
means of locomotion was impossible, for the trunk was inflexible and the tail
short; instead the limbs were developed into powerful oarlike structures, with
which the creatures "rowed" its way through the sea.
The pterosaurs were probably the first flying vertebrates, and evolved from an
early line of thecodonts. Although pterosaurs were not ancestral to birds they did
share some traits that indicate similarities in anatomy and physiology such as
hollow bones. In addition, both bird and pterosaur skulls have relatively larger
cerebellar and optic lobe capacities than the skulls of modern reptiles. Many of
the earlier pterosaurs were small animals not even as large as crows. However,
pterosaurs of the late Jurassic and Cretaceous periods grew to be the largest
ever flying animals. Pteranodon had a 7 metre wingspan and a weight of ca. 17
kg, and the discovery of fossilized fish within their fossilized ribs, indicated that
they must have flown great distances over water. What is difficult to explain is
how they kept from crumpling their wings if they splashed into the water after
prey, and even more difficult to understand how such large animals regained
altitude. One suggestion is that they scooped up fish pelican-fashion and soared
on ocean breezes. Even so, the lack of a stabilizing tail and the position of the
wings behind the centre of gravity made them aerodynamically unstable. The
rudderlike head may have provided some lateral stability, but other pterosaurs
16
such as the largest Quetzalcoatlus (which had a wingspan of 16 metres and a
weight of 65 kg) were even more unbalanced and lacked such stabilizing devices.
Flight in Quetzalcoatlus has been compared to shooting an arrow backwards,
even so this large beast must have had some means of contolling its flight since
it evidently feed on the carcasses of other dinosaurs.
The pterosaur wing was supported from an enormously extended fourth digit
(finger) on the front limb. From this the wing was extended, in somewhat batlike
fashion, a great wing membrane. Manipulation of a wing of this sort would
appear to have been an awkward matter, and flight was originally considered to
be mostly achieved by soaring rather than flapping. Further since there are no
intermediate fingers extending into the wing memebrane, it was originally thought
to have been very fragile. The hind legs of pterosaurs, in stark contrast to most
birds, were feeble structures, and it is difficult to see how these creatures could
have stood up, let alone get a running take-off as birds do today.
Some recent findings have required some radical changes to our thinking on the
pterosaurs. Some Soviet scientists have reported that one of the smaller
pterosaurs (Sordes pilosus) had fur like mammals; implying that they were
endothermic. Recent analysis of pterosaur Sandactylus (5 metre wingspan) the
skin of the pterosaur wing was quite thick, with epidermal, dermal and muscle
fibre layers, and therefore not just a membrane. Within the upper dermal layer
were blood vessels. This antomy and arrangement of blood vessels is similar to
that of a bats wing which uses its blood vessels to cool itself while flying. If the
pterosaur needed to cool down, the flying must have involved energy
expenditure, and therefore be active (flapping) rather than gliding flight. The lack
of stiffeness in the pterosaur wing is difficult to interpret if they flapped their
wings. It is, however, hypothesized that Sandactylus kept its wings at a constant
tension by moving its hind legs, which were also attached to the wing. The
implications of these findings is that pterosaurs had more control over their flight
than scientists had previously thought, and that their flight was not limited to
passive gliding. These pterosaurs were obviously fascinating animals which
dominated the skies for 100 million years, unfortunately they left no descendants
for us to study.
Although the fossils of dinosaurs during the entire mesozoic era suggest a high
diversity of organisms adapted to a variety of habitats, the reason for their final
wholesale extinction some 65 million years ago is not completely resolved.
However, this extinction does correlate with a thin band of iridium-enriched clay
that marks the boundary between Cretaceous and Tertiary periods (nicknamed
the K-T boundary). Because iridium is rare on earth, but common in meteorites,
it was proposed that the earth was hit by an asteroid 10 km in size. More
recently proof of such a meteorite has been found in the Gulf of Mexico (off the
continental shelf of the Yucatan Peninsula). This impact site has formed the
Chicxulub crater. To have formed this crater the meteorite would have needed to
be at least 10 km in diameter. The impact of such a meteorite would have
caused massive impact earthquakes, perhaps hundreds of times greater than the
largest measured earthquake. Massive tsunami waves (tidal waves) would have
radiated out.
When such a meteorite struck the earth, dust would have blanketed the globe,
darkness would have occurred for one to three months and land temperatures
would have plummeted. Since the meteorite very likely hit the sea, the water
vapour could have created a greenhouse effect, making the short-term climate
exceptionally hot, although in the long-term the temperature declined. Hot nitric
acid would have rained out of the atmosphere and threatened many organisms
with death, particularly those possessing shells. Recent evidence of large
amounts of soot in the K-T sediments suggest that large-scale fires accompanied
such a catastrophe (as much as 90% of the world's forests may have burned).
Such events would have had a profound effect on the ecosystems of the world.
One theory suggests that mammals, which were on the brink of a great radiation
during the Cenozoic, may have been predators of dinosaur eggs, or in some
other way outcompeted the dinosaurs for resources. At this time mammals were
only represented by shrew-like creatures, a few centimetres in size. Numerous,
but tiny cone-shaped teeth from these mammals were found together with the
gigantic fossilized bones of the great dinosaurs.
In the fossil records of the Montana Badlands there is a black marker of coal and
some excellently preserved fossilized tree stumps. Below this marker was the
last of the cycad and tree fern forest, but the tree stumps represent the coniferous
redwoods (Sequoia). These later plants prefer a much cooler climate than the
cycads and tree ferns. Although a large body can retain heat more efficiently, if it
becomes cooled, it becomes increasingly more difficult to gain heat. In contrast
very small animals can find micro-habitats that reduce exposure to unfavourable
conditions and can more quickly warm their bodies up during favourable
conditions. Aquatic animals also have a greater buffer against temperature since
water maintains heat more efficiently than land. Consequently the three main
types of reptiles that endured the late Cretaceous extinction were lizards,
tortoises and turtles and crocodiles, all either small-sized or aquatic animals.
Crocodiles (Order Crocodilia) are the largest living reptiles and possibly the most
advanced, having a nearly complete four-chambered heart. The nostrils are at
the end of the snout and the eyes protrude from the head so that these animals
can float near the surface of water with only these parts exposed above the
water. It was possibly these features that allowed them to survive the sudden
global cooling that almost definitely occurred at the end of the Cretaceous period.
Under hot conditions crocodiles open their mouths and air passes over the soft
skin on the inside of the mouth and cools the animal down. The crocodile eyes
are unusual in that the photo- pigments receptive to light are different in the upper
and lower hemispheres of each retina. The upper retinal hemisphere which looks
down into the water has a photopigment similar to that of freshwater animals
(porphyropsin), whereas the ventral retinal hemisphere has the pigment of
terrestrial animals (rhodopsin). The skin is thick and covered with horny
epidermal scales and dorsal bony plates (osteoderms) which may extend to the
ventral surface and are like those in turtle shells.
The social lives of crocodiles is complex. Male Nile crocodiles establish and
defend breeding territories adjacent to the water and courtship occurs in the
water. As the females approach; the males roar with such intensity that their
flanks vibrate throwing up clouds of spray from the water, and their jaws clap
furiously. Mating lasts for a few minutes with the male clasping the female. Their
jaws and tails become intertwined during copulation. The female excavates a
hole in the bank close to the waterline and lays about forty eggs in several
batches. She ensures that the eggs are buried so that temperature remains
relatively constant to within 3oC. Saltwater crocodiles build mounds of vegetation
as a nest and sprays urine to cool it if it becomes to hot. The alligators occurring
in the New World piles up rotting vegetation into a nest which is regularly turned
over in order to provide the eggs with appropriate temperature and moisture
conditions. Just before hatching the female Nile Crocodile waits and when she
hears the pipping calls of the hatching babies she will scrap the earth away and
will pick her young up and put them into a pouch at the bottom of her mouth and
will transfer them to the water. The male will escort these baby crocodiles to a
nursery area where they will remain for the next few months with the parents
closely guarding them. The Crocodiles and its allies invest considerable parental
care in the rearing of its young after their hatching. Many dinosaurs were also
17
thought to invest in considerable parental care, since they built fairly elaborate
nests out of mud which would have retained the young dinosaurs until they were
large enough to climb over the perimeter of the nest edge.
Tortoises, terrapins and turtles belong to the Order Chelonia and have an
ancestry that is even older than the crocodiles. The strengthened bony plates
occurring in crocodiles (ossicles) have in tortoises become modified to form a
continuous dorsal carapace and a ventral plastron. This represents the most
effective armour developed by any vertebrate group, and this pattern has
changed very little since it first evolved. The turtles reverted to an aquatic life
style where the heavy shell was less of an impediment to locomotion. However,
the shelled egg, an essential adaptation to terrestrial life, did become an
impediment since the membrane beneath the shell by which the embryo breathes
through the shell pores functions by gaseous exchange and cannot work in water.
Consequently turtles come on to beaches to lay their eggs in a terrestrial
environment. However, when the young turtles hatch they have a perilous
journey from where they hatched (above spring high tide) to the sea, and many
succumb to predation.
The third group of reptilian survivors are the lizards (Order Squamata) and are
very much more numerous (3000 species) and have many more modifications
arising from their ancestral stock than either of the other surviving reptile groups.
Snakes are essentially highly specialized lizards that have elongated bodies
through increasing the number of vertebrae and have lost their limbs and even
have a reduction of the left lung.
Lizards belong to the suborder Sauria and includes geckos (Family Gekkonidae),
iguanas (Iguanidae), chameleons (Chamaeleonidae), skinks (Scincidae), worm
lizards (Amphisbaeridae) and monitor lizards (Varanidae). They have all
enhanced their water-tight integument with the development of scales, which
have become highly modified. The Australian shingleback skink (Trachysaurus)
has stout polished scales, the Gila monster (Neloderma) has round pink and
black ones (and has additional protection by being venomous) and the horned
lizards occurring in arid areas have enlarged them into spiny appendages which
are scored with fine grooves which allow dew to condense on them and be
collected in the mouth. Spines in the chameleons have also become horned with
one to four occurring in the head region. The scales on the underside of the toes
of geckoes have become highly modified with numerous microscopic hairs
(lamellae) which enable them to climb smooth surfaces (including glass) with
relative ease by each hair engaging on the smallest irregularity of the surface.
Many lizard families have members with reduced limbs that may even be lost
altogether and parallels the amphibian groups Gymnophiona and Caudata.
Skinks show a progression of limb reduction. The snake lizards of South Africa
(Family Pygopodidae), even within their single genus, have some species with a
complete complement of functional legs each with five toes; another species
possesses very small limbs, with only two fully developed toes on each foot and
a third species has hind legs with a single toe and no externally visible front
limbs.
A hundred million years ago limb reduction occurred among ancient lizards and
resulted in the evolution of snakes (Suborder Serpentes) of which about 2300
species live today. They differ from lizards in the following respects: (1) the right
and left halves of the lower jaw are not firmly united, instead they are connected
by an elastic ligament; (2) there is no pectoral girdle; (3) a urinary bladder is
absent; (4) the braincase is closed anteriorly; (5) the eyelids are fused over the
eyes but a transparent window exists which allows the snake to see; and (6) no
external ear openings exist.
These adaptations and loss of structures suggest that the snake's ancestors had
previously adopted a burrowing existence, and their surface dwelling is
secondary. The loss of legs for locomotion on the surface has been overcome
with the development of flank muscles that flex in alternate bands so that their
body is drawn up in a series of S-shaped curves. As the contractions travel in
waves down the body the flanks are pressed against obstacles on the ground
such as stones and the snake is able to push itself forward. When snakes hunt
they are able to creep up on their prey without oscillating its body. The scales on
the underside are shaped like narrow rectangles running across the width of the
body and overlapping one another with their free edges to the rear. The snake is
able to hitch these scales up and forward in groups by contracting its belly
muscles. The back edges catch the ground and as the contractions pass
downwards in waves, the snake advances smoothly and silently with no lateral
movement.
Snakes are predators with prey being seized with their mouths. In boas and
pythons they swiftly coil themselves around the body of the prey and suffocate it.
With the backward pointing teeth the snake engages onto the prey and the
snake draws it into the mouth by using the loosely connected lower jaw. Other
snakes deliver venom via specially modified teeth to kill the prey before ingesting
it. In back-fanged snakes a poison gland lies above the teeth and the venom
trickles down a groove in the tooth. The snake therefore has to drive its fangs
deep into the prey before it is able to deliver its venom. Other snakes have their
fangs placed in the front of the upper jaw and have an enclosed canal through
which the venom is delivered. Cobras (Naja) and mambas (Dendroaspis) have
short immobile fangs which inject the venom, whereas vipers have long fangs
which are kept hinged back and are rotated forward when it attacks it prey. Still
other snakes spit poison into the eyes of it prey.
Possibly the most advanced snakes are the pit vipers (Family Cotalidae) which
include the rattle snakes (Crotalus) of the southwestern regions of the United
States. These animal invest heavily in parental care and like some amphibians
retain their eggs inside their body. The shell is reduced to a thin membrane so
that the embryos, as they lie inside the oviduct, not only feed on their yolk but
draw sustenance from their mother's blood diffusing from the walls of the oviduct
pressed against them. Such a system for nourishing of its young is functionally
analogous to the placenta used by mammals. The mother snake will also
safeguard her young after they have hatched, warning intruders with sound of the
vibrating rattle at the end of the tail. Each time a rattle snake sheds its skin a
special, hollow scale remains and accumulates at the end of their tail. Up to
twenty scales may accumulate.
Rattle snakes are nocturnal hunters and use a pit located between the nostril and
the eye to detect infra-red radiation. The detection of heat given by a small
mammal is also directional, and therefore it is able to attack its prey even in pitch
darkness. Being ectothermic, food requirements are, however, small and
therefore less time is spent foraging than the equivalent sized endothermic
mammal. This ensures their success even in the most inhospitably dry regions of
the world.
ASSIGNMENTS
IN YOUR OWN WORDS WRITE A ONE TO TWO PAGE ESSAY ON THE
FOLLOWING TOPICS
18
Discuss the origins, morphology and lifestyle of animals belonging to the
suborder Serpentes.
Discuss the adaptive radiation of reptiles living in the Mesozoic period.
conspicuous claws on the digits of the wings with which it is able to climb away
from possible predators. These claws are usually considered to be a secondary
development, however, their resemblance to the claws of the Archaeopteryx is
remarkable. When the Hoatzin chicks grow up they lose these claws, but the
adult birds are nevertheless poor fliers.
Review the evidence we have that dinosaurs were warm-blooded animals.
Describe the adaptions that pterosaurs required in order to fly. In what
ways is the pterosaur wing similar and different to the wings of birds and
bats.
What adaptations allowed reptiles to better colonize the terrestrial
environments than their amphibian counterparts?
LORDS OF THE AIR
Many characteristics of birds show close resemblance to those of reptiles and in
particular the early bipedal reptiles before they evolved into the great dinosaurs.
In the early Triassic (225-200 million years ago) small pseudosuchians such as
Saltoposuchus showed the essential characteristics of birds including bipedalism.
There are no fossils detailing the change from the ectothermic bipedal reptiles
into endothermic flying birds except for five fossil specimens of the upper
Jurassic (about 150 million years ago) found in the lithographic slates of
Solnhofen, Bavaria. These Archaeopteryx lithographica probably achieved some
degree of gliding, and are certainly the earliest known animal to possess
feathers. Anatomically these animals are much less specialized than the modern
birds but does represent the earliest animal classified as a member of Aves and
is in its own subclass Archaeornithes. All other birds were extinct or living belong
to the subclass Neornithes. In 1860 the first fossilized feather was found, and a
year later the first Archaeopteryx was found. The whole body axis was
elongated, the dorsal vertebrae were not fixed and only five were fused to form
the sacrum. There was a long tail, with feathers arranged in parallel rows along
its sides. The fore-limbs ended in three clawed digits, with separate metacarpals
and carpals. This limb was used as a wing since feathers were attached to the
ulna and hand, but the wing was small and the shape rounded. The pelvic girdle
and hind-limb resembled that of the archosaurs. In the skull there were sharp
teeth in both jaws, and the eyes and brain were considerably smaller than
modern birds. The bones were not hollowed and since the sternum bone (keel)
was not well developed, it could not have had muscles that could achieved
flapping flight. It has been suggested that it used its feathers which probably
originally evolved as some form of insulation, as a kind of net to trap insects while
running fast across land. Alternatively it was suggested that it was arboreal and
the feathers which were originally derived from reptilian scales, enabled
Archaeopteryx to glide short distances much as gliding lizards do today (e.g.
Draco volans). Thus the two theories that flight evolved 'from ground up' and
'from trees down' have been proposed. The descendants of Archaeopteryx and
other ancient birds underwent a dramatic adaptive radiation during the
Cretaceous period when both aquatic and terrestrial habitats were invaded.
Hesperornis was a loon-like diver that possessed teeth, and had already lost its
power of flight since the wings had become functionless and is the only other bird
species known to have teeth.
That Archaeopteryx almost definitely used its claws on the front wings to climb is
clearly paralleled by the Hoatzin (Opisthocomus) a heavily built bird occurring in
South America and belonging to the cuckoo family. Its young possess
The debate as to whether Archaeopteryx could or could not fly still continues. It
has been argued that Archaeopteryx was too heavy and that its muscles were to
light to power it, and that they used their feathers for gliding or cooling
themselves. Some researchers have argued that Archaeopteryx had the
muscles of a cold-blooded reptile. These are twice as powerful per unit weight as
those of warm-blooded animals, and may have allowed Archaeopteryx to fly short
distances which makes more ecological sense than a warm-blooded
Archaeopteryx
possessing wings and feathers but not the ability to fly.
Now with the discovery of a fossil bird in northeastern China which has provided
the first evidence that fairly modern tree-perching birds had evolved by 135
million years ago, only 15 million years after Archaeopteryx.
This sparrow-sized bird, which is as yet unnamed, has an opposable first digit
and slender claws on its legs. This would have allowed it to firmly grasp a tree
banch and to "perch" (the flat forward pointing claws of Archaeopteryx mark it as
a ground-dwelling animal). This small bird had a well developed keel on its
sternum which was the anchor site for strong flight muscles and also possessed
a pygostyle (fused cluster of tail vertebrae to which long tail feathers are
attached). This gave the bird a centre of gravity in the centre of the wings,
whereas the long-feathered tail of Archaeopteryx puts the centre gravity well back
of the wing and just above its feet which is a better position for an animal that
runs. This Chinese bird did, however, retain some primitive traits. These include
small remnants of claws and fingers, stomach ribs and the bird may have had
teeth. All of these are present in Archaeopteryx and carnivorous dinosaurs but
not in modern birds. This Chineese fossil does present problems such as how an
animal like Archaeopteryx could have evolved into this bird like animal within 10 15 million years?
It is now almost certain that Archaeopteryx was not a direct ancestor to the
modern birds, but would have been an offshoot. The fossil of a 4 metre long
coelurosaur called Deinonychus showed an anatomy almost identical to
Archaeopteryx except that it lacked wings and feathers and was around 50
million years older than Archaeopteryx. Other bird-like dinosaurs include
Avimimus, a 1,5 metre bipedal fossil found in Mongolia. This animal had a short
deep head, toothless beak, long neck and tail and possibly feathers, which would
make it the most ancient of feathered animals. Yet another fossil discovered in
North America called Protoavis, may have been a bird or a dinosaur, but certainly
pre-dates Archaeopteryx. However, the fossil that has attracted the most amount
of interest in relation to the links between birds and dinosaurs is Mononychus, a
turkey-sized predator equiped with sharp teeth and a long tail and looked very
similar to other theropods. Mononychus does share some anatomical features
with birds that are not found in any of the other bird-like dinosaur fossil including
Archaeopteryx. For example Archaeopteryx has a fibula (the thin bone in the
lower hind limb) that touches the ankle, in birds and Mononychus this does not
happen. All birds have a keeled sternum for attachment of wing muscles.
Mononychus also has a keeled sternum and some of its wristbones are fused
together which is also an adaptation for flight. This evidence suggest that
Mononychus evolved from a flying animal, just as ostriches are descended from
flying birds. If this is the case Mononychus probably had feathers and the real
ancestor of birds goes back still further in the fossil record.
Although we have a poor fossil record describing the evolution of birds there is
19
little doubt that they evolved directly from a small coelurosaurian dinosaur.
However, the conquest of the air by birds was not only achieved with the
adaptation of the feathers and powerful wing muscles, but also necessitated
considerable weight reductions. The bones of birds are extremely thin and
hollow inside, with structural strength being created by cross struts. The heavy
extension of the spine that supported Archaeopteryx's tail has been replaced with
stout quilled feathers. The heavy jaw with teeth has been replaced with a beak
composed of lightweight protein called keratin.
The basic bird plan of structure originating in the Jurassic has been modified to
produce over 8600 living species. The factors that promoted such species
radiation are unclear since there is a poverty of fossil records and it is not
possible to trace individual lines, which you can do for other vertebrate groups. It
is clear that the process of change has been radical and accomplished in an
extremely short evolutionary period.
In particular the bill structure appears to be easily and quickly moulded by
evolutionary processes. From an ancestral finch-like bird with a short straight
beak, the Hawaiian Honey-creepers (Family Drepanididae) have evolved bill
structures that are adapted to feeding on insects, nectar, fruit and seeds in a
period of a few thousand years. Darwin noted similar variation in the bills of the
finches of the Galapagos islands. Elsewhere in the bird world the evolution of bill
structure has occurred for a much longer time and we therefore see bills adapted
to seed-eating (sparrows; Ploceidae), fruit-eating (hornbills and toucans;
Bucerotidae and Ramphastidae respectively) insect-eating (nightjars;
Caprimulgidae), tearing (eagles and hawks; Accipitridae), probing (stilts;
Recurvirostridae), filtering (flamingoes; Phoenicopteridae) and capturing of fish
(cormorants; Phalacrocoracidae). The feet of birds also show adaptations to
scratching for food (pheasants; Phasianidae), wading (heron; Ardeidae), grasping
(eagles), perching (warblers; Muscicapidae) and swimming (ducks; Anatidae).
Feathers are also highly evolved in the differentiation of different feathers
(primary and secondary wing, tail, inner and outer contour feathers, down and
filoplume) as well as adaptations to meet different habitats due to the unequalled
insulation properties of feathers which permit the Emperor Penguin (Aptenodytes
forsteri) to be the only animal that can endure winter on the Antarctic ice cap.
Most birds have an oil gland near the base of the tail. The bird takes this oil with
its beak and coats individual feathers to waterproof them and maintain their
insulation. Other birds, including herons, parrots (Psittacidae) and toucans lack
this gland and condition feathers with a fine talc like dust, powder-down, that is
produced by the continuous fraying of the tips of special feathers. Cormorants
and darters, spend a great deal of their time diving in water, their feathers are not
waterproofed, permitting them to get completely wet. This is of advantage since
it reduces buoyancy and they can dive deeper and more easily in pursuit of their
fish prey. After foraging they stretch their wings to dry.
Feathers are unique to birds, but were derived from scales and arise to form
papillae. A papilla consists of a projection of vascularized dermal tissue that
grows out of an epidermal pit, called the feather follicle. A typical feather consists
of a stiff axial rod, or shaft. The proximal portion of the shaft, the quill is hollow
whereas the distal end is solid. The shaft bears two rows of branches, or barbs,
which in turn support two rows of smaller, numerous barbules. The feathery vane
is composed of a double series of barbs and barbules. The barbules on the side
of the barb towards the tip of the feather bear hooklets or barbicels, that form
bridges with ridges on the adjacent proximal barbules. The vane is thus
lightweight and pliable, but also extremely strong and resilient. At least once a
year each feather is shed and a new feather develops from the same papilla.
Birds usually shed, or moult, their old feathers during late summer. There may
be partial or complete moult in spring when the bird assumes a more colourful
breeding plumage. The acquisition of breeding plumage may also result from
wear or the breaking-off of feather tips, thus exposing different colours beneath.
Feather coloration is due to two basic pigments known as melanins which are
pigment granules of brown, black or yellow and the carotenoids which are either
red or yellow. Green, blue and iridescent markings on sunbirds and other
species are due to the peculiar surface and (Nectarinidae) internal structure of
their feathers. Absence of pigments result in partial or complete albinism.
The first juvenile plumage of birds is usually replaced before the first winter. This
winter plumage usually resembles that of an adult female irrespective of whether
the juvenile is male or female. Only in the second year does differentiation of
plumage between males and females occur. Mature male and female plumages
frequently differ in colour (sexual chromatic dimorphism), especially during the
breeding season, when the male may be particularly brightly coloured (eg Red
Bishop birds Euplectes orix). Such colour changes are used during courtship
with male birds advertising themselves. Breeding plumage may facilitate mate
recognization within a species, and is particularly important when many related
species coexist in the same area. In particular striking combinations of colour are
used in finches (Fringillidae) and parakeets/parrots. Worldwide ducks assemble
in multispecies flocks, but during breeding each drake (male) species will acquire
a unique colour and pattern combination particularly in the head regions which
will distinguish that species from other duck species in his quest to find a mate.
Colour may also be used to effectively camouflage birds. The most striking being
the ptarmigan (Lagopus mutus; Tetraonidae), this grouse is white during winter
when snow is on the ground, but mottled brown during the rest of the year.
Feathers have become enlarged and specialized and are used with or without
changes of plumage colour to attract mates. The Pennant-wing Nightjar
(Macrodipteryx vexillaria) acquires long pennants from the primary feathers. In
the Crested Grebe (Podiceps cristatus; Podicipedidae) both sexes develop
elongated chestnut-brown feathers on their cheeks, a deep brown ruff beneath
the beak and a pair of horn-like tufts of glossy black feathers on the head. Sexual
difference has been taken to the most extreme for any animal with the male
pheasants, peacocks, grouse, manakins, and birds of paradise all of which grow
feathers to a great size. The Great Argus pheasant (Argusianus argus) has wing
feathers that are over a metre long and are lined with huge eye spots. The
Peacock (Pavo cristatus), which is basically a pheasant, has tail feathers up to
1.8 m long, with a conspicuous pattern that resembles large eyespots.
The most spectacular bird plumages occur in the Birds of Paradise
(Paradisaeidae) from the island of New Guinea. The King of Saxony
(Pteridophora alberti)has two long quills from his forehead each bearing a line of
enamelled blue pennants; the Superb Bird (Lophorina superba) has an immense
emerald shield which it can expand until it is as broad as the bird is tall; the
Twelve-wired Bird of Paradise (Seleucidis melanoleuca) has a shimmering green
bib and a huge inflatable yellow waistcoat with bare quills, the wires of its name,
curling down behind it. The most celebrated birds of paradise are those
possessing plumes arising from beneath their wing coverts. There are several
species, each with a plume of a different colour (yellow, red or white). These
birds display communally, with dance displays being held in a prominent position
on a branch which has had twigs and leaves stripped off it. In this way a dull
coloured female is attract and she flits across to the branch where one of the
male birds jumps aggressively onto her back. Copulation is quick, and the
female returns to the nest that she has already prepared for her now fertilized
eggs. The male birds which had been burdened with the plumes for several
months now losses them.
20
Although bright colours are important for courtship in some birds. other birds
have used behavioural patterns to attract their mates. The Satin Bower Bird
(Ptilonorhynchus violaceus; Ptilonorhynchidae) bird Australia constructs an
avenue of twigs on which he attaches a variety of objects which are either yellowgreen, or preferably a shade of blue that closely matches his plumage colour.
The nature of the objects collected is unimportant and may include berries,
feather from other birds and even pieces of plastic. These birds are even known
to steal desirable objects from a neighbouring nest and certainly mash blue
berries with his beak and uses the blue-purple pulp to paint the walls of his
bower. With this bower he tries to lure the female bower bird for courtship and
copulation.
Copulation in birds is generally clumsy, since the male birds with few exceptions
have no penis. The mating birds cling and may twist about until the two vents are
brought together and sperm is transferred to the females. Unlike other tetrapods
birds only lay eggs, a characteristic inherited from the archosaurian ancestors. It
is possible that vivipary would have been too great a load for a female to carry in
flight throughout the weeks necessary for their development and therefore the
eggs within the females are laid soon after fertilization.
Birds now have to pay the penalty for being endothermic, for reptiles can bury
their eggs and abandon them. Bird's eggs like the adults themselves, need to be
kept at a constant temperature which is usually several degrees above ambient
temperatures. Birds therefore incubate their eggs. Some birds just before egglaying moult a group of feathers on their undersides and expose a bare patch of
skin which becomes distended with minute blood vessels. The eggs are kept
against this patch and kept at the same temperature as the parent bird. But not
all birds produce this patch by moulting. Ducks and Geese mechanically pluck
out their own feathers. The blue-footed Booby (Sula nebouxii; Sulidae), not only
uses its feet for display but also uses them as insulators.
The other disadvantage of egg laying is the need to build a nest, or in some way
to safe guard the eggs. This puts both eggs and parents at risk. Vertical cliffs
being almost inaccessible represent one safe site, providing the eggs do not roll
off. This is minimized by producing eggs that are pointed at the one end which
permits them to roll in a circular direction. Other birds, particularly those belong
to the plover group (Order Charadriiformes) lay their eggs on open fields and
gravel plains, but are usually highly cryptic and not easily found. More commonly
birds construct nests to provide some form of protection. Woodpeckers (Picidae)
excavate or enlarge holes in trees, kingfishers (Alcedinidae) use holes in river
banks. The Tailor bird of India, (Orthotomus sutorius), a warbler, sews together
the growing leaves of a tree by piercing holes in their margin and tying them
together with strands of plant fibre. The weaver bird weaves plant material
together to form an almost basket-like structure which is attached to a thin twig
and hangs upside down. Other species of weaver birds collaborate and build
elaborate community nests. The oven bird of Argentina (Furnarius rufus;
Furnariidae) builds its nest out of mud and against fence posts and bare
branches. Hornbills, also nests in holes in trees and incaserates using mud the
incubating female and feeds both the female and the young hatchlings through a
small hole in the mud wall. Cave swiftlets (Collocalia inexpectata; Apodidae) in
southeast Asia construct artificial nests from glutinous spittle which is attached to
the walls of the cave.
Several bird species, including the famous cuckoo (e.g. Cuculus carnosus;
Cuculidae) have escaped the labour of incubation and chick rearing by depositing
their eggs in the nest of other birds and allowing foster parents to rear its young.
Adaptations for such parasitism include close mimicry of eggs between the
cuckoo and its host, and the more rapid development of the cuckoo chicks so that
they hatch first and can dispose the legitimate eggs of its foster parents.
However, all hatchling bird species do have a small egg-tooth at the tip of their
beak which they use to break the egg. The egg has provided a small air sac at
the end of the egg to provide the first air for the chick. Hatchlings can be divided
into two categories. Chicks that can run away almost immediately from the nest
and are fully covered with down feathers and can feed on their own but still have
parental supervision are said to be precocial. This type of hatchling is most
common to birds that do not build nests, but lay their eggs in the open such as
the plover group. Chicks that at birth are naked and helpless and need to be fed
by the parents are said to be altricial and restricted to bird species that construct
nests.
The anatomy of birds is intimately connected to their ability to fly and this is
apparent in the bird shape which offers minimum resistance to the air. Several
adaptations result in a low centre of gravity, which tends to prevent the body from
turning over during flight. The wings are attached high up on the trunk, as are the
light organs such as lungs, whereas heavy flight muscles and muscular digestive
organs are positioned ventrally. The pattern, speed and endurance of flight are
reflected in the shape of wings. Highly aerial birds; which includes swifts
(Apodidae), swallows (Hemiprocnidae), terns (Laridae) and albatrosses
(Diomedeidae); have long pointed wings which enable them to soar in the air for
long periods using the minimum amount of energy. Other bird species have short
rounded wings that enable them to take off quickly and fly rapidly for short
distances (eg sparrows). Vultures (Accipitridae) which fly in circles at low
speeds using thermal air currents have broad rectangular wings that permit slow
flight. Humming birds (Trochilidae) are even able to achieve hovering flight, by
tilting their bodies so that they are almost upright and they can beat their wings
as fast as 80 times per second.
Flight has, however, permitted birds to be both the fastest moving animals and
the animals that travel the most distance. The Carrier Pigeon (Columba livia;
Columbidae) attains a maximum racing speed of 96 km/h, ducks can reach 145
km/h and the swift (Apus apus) 170 km/h in level flight. The swift may travel up
to 900 km each day to collect aerial insects which is its only source of food, and
this species even copulates in flight. The Peregrine Falcon (Falco peregrinus)
during a dive can achieve speeds of 267-290 km/h, and has swept its wings back
to reduce drag even further.
No other creatures can fly as fast or as far as birds. Many species of bird make
long journeys. The White Stork (Ciconia ciconia; Ciconiidae) travels every
autumn down to Africa and returns to Europe in the spring navigating with such
accuracy that the same pair, year after year will occupy the same nest on the
same roof top. However, the Arctic Tern (Sterna paradisea), holds the record for
long-distance migration. The extremes of its Arctic nesting and Antarctic
wintering ranges are 16 700 km apart. Since the routes taken are circuitous,
these birds may fly 40 300 km each year. During the autumn, many birds gather
in flocks and fly southward, returning the following spring. A lesser and opposite
movement occurs in the Southern Hemisphere, where the seasons are reversed.
Some other birds perform altitudinal migrations into mountainous regions for the
summer and return to the lowlands to winter. In Africa young Starred Robins
(Pogonocichla stellata; Turdidae) moves from the high interior forests to the
warmer river valleys with the onset of autumn and winter.
Most species used established routes for migration and travel more or less on
schedule, arriving and leaving regularly. Migration, breeding, and moult are
phases in the annual cycle of birds that are regulated by the endocrine system.
Migration is a semiannual event, dependent especially on the pituitary gland in
21
the brain. Usually prior to migration fat reserves, not present at other times, are
accumulated rapidly for extra fuel during the long flights. Also, many strictly
diurnal birds become nocturnal during migration. Seasonal differences
(photoperiodism) influence migratory behaviour of some northern species.
Generally birds migrate close to the earth's surface, although some bird species
may migrate at more than 1 km altitude. Most birds migrate at between 50 and
80 km/h and stop and feed as they proceed along the migration front. Although
some birds use obvious landmarks such as coasts, rivers and mountain ranges
other birds will migrate without the aid of directional features. Evidence suggest
that migration in daytime is guided by the position of the sun and at night by the
patterns of stars. This would necessitate that migrations need to be done on
clear nights. On cloudy nights birds tend to get lost and if they are released in a
planetarium where the constellations have been rotated so that they no longer
match the position of the stars in the heavens, the birds will orientate with the
visible, artificial constellations. Still other bird species appear to be able to use
the earth's magnetic field as a guide.
Despite the large amount of adaptation required for flight, there are nevertheless
a large number of birds that have abandoned flight. The older bird fossils dating
some thirty million years after Archaeopteryx including gull-like forms
(Ichthyornis) which were skilled flyers with a keeled chest bone and no bony tail.
In essence they were modern birds. At the same time, however, lived huge
swimming birds Hesperornis, which were nearly as big as a man and had already
ceased to be able to fly. Fossils of those other non-flying birds, the penguins,
also appeared around this time. Fossils of another large flightless bird Diatryma
stalked the plains of Wyoming, while a similar bird Phororhacos. This bird was
about 2m tall, carnivorous, and equipped with a huge bill. It is possible that this
group was successful in the absence of other large carnivores representing either
reptile or mammal classes. Large carnivores in the former class were already
extinct in the former class and were yet to evolve in the latter class. Diatryma
may have been the early ancestor to Gruiformes group of birds (Rails and
Cranes) which even today have representatives (eg flightless rails of Gough
Island) that showed a marked tenancy to lose flight when they colonize islands
that have few or predators. The cormorants of the Galapagos Islands have such
small wings that they cannot fly any longer. On the Madagscarene Islands, the
dodo (Raphus cucullatus), was a very large pigeon that adopted a terrestrial habit
and was exterminated by the human introduction of dogs to the island in the
seventeenth century. The Elephant bird Aepyornis was about 3m tall and
possessed the largest known eggs for any bird species (148 times the size of a
hens egg by volume). Moas (Diornis) were another giant flightless bird over 3m
tall and occurred on New Zealand.
Currently four orders of birds species fall into the general category of wingless
and flightless terrestrial birds. These include ostriches (Struthioniformes), rheas
(Reiformes), cassowaries and emus (Casuariiformes) and kiwis
(Dinornithiformes)
ASSIGNMENTS
IN YOUR OWN WORDS WRITE A ONE TO TWO PAGE ESSAY ON THE
FOLLOWING TOPICS
Discuss the general adaptations birds have evolved for flight. Your answer
should include sections on anatomical modification, physilogical
adaptations, feathers and wings.
Describe the modifications that have occured in the beak and feet of birds.
Discuss how such a diversity of forms evolved, relating these forms to
environmental factors and or food items that they forage.
Birds have been described as the living relatives of dinosaurs, briefly
discuss the validity of such a statement.
EGGS, POUCHES AND PLACENTAS
The duck-billed platypus Ornithorhynchus anatinus (bird-billed) from Australia is
animal belonging to the most primitive order of mammals (Monotremata). This
animal is the size of a rabbit, possesses thick fur, webbed and clawed feet, a
cloaca combining both excretory and reproductive functions and a large pliable
flat beak like a duck's. It lives in the rivers of eastern Australia, swimming using
its webbed flat-feet and steering with its hind-limbs. When it dives, it closes its
ears and tiny eyes with little muscular flaps of skin and hunts for aquatic
invertebrates using its bill, which is rich in nerve endings and very sensitive. It is
also a powerful burrower, excavating tunnels up to 18 metres in length through
the river banks. These animals roll back the webbing of their fore-feet into their
palms and this frees the claws for burrowing. Within these tunnels the female
constructs an underground nest of grass and reeds and lays two eggs that are
nearly spherical, the size of marbles, and soft shelled and therefore similar to a
reptile's egg. Since platypuses have fur, they are warm blooded and possess
rudimentary mammary glands; they definitely belong to the class Mammalia and
is one of only two primitive living mammal families which lays eggs. The female
platypus develops on her belly special glands, that are similar in structure to
sweat glands but are enlarged and produce a thick rich milk which oozes into the
fur. The young platypus suck the fur. This is the beginning of the true mammary
gland found in all higher mammals. The other important mammalian feature of
endothermy is also incompletely developed and the platypus allows its body
temperatures to fluctuate more greatly than other mammals (viz can drop to
300C).
The only other animal that can parallel this mixture of primitive features is the
spiny ant-eater called Echidna which taxonomists renamed Tachyglossus (swifttongued). These animals are spiny with a long tube-like snout that has no teeth
but does possess a long tongue which flicks out to catch insects. Its front legs are
equipped with long digging claws. At the beginning of each mating season the
female develops a small pouch into which she later transfers her single egg. The
mammary glands discharge directly into the pouch and the milk is sucked of the
hairs.
The Echidna and Platypus are of great antiquity, but we have no hard evidence to
indicate which fossil reptiles were their ancestors. Our knowledge of many of the
candidates is based to a considerable degree on its teeth, one of the most
durable parts of any animal's anatomy. Fossilized teeth provide information
about an animal's diet and habits. They are also highly characteristic of a
species and similarities between teeth are strong evidence of genealogical
relationships. Both Platypus and Echidna became highly specialised for
underwater foraging and ant-eating respectively and consequently lost their teeth
(although young Platypuses still produce three tiny ones soon after birth which
are lost in a very short time). We therefore have virtually nothing to help us link
these creatures to any group of fossil reptiles. This is further complicated since
the features that characterize mammals are hair, warm bloodedness and
milk-producing glands which cannot be easily deduced from fossils.
22
like a platypus or gave birth to live young and suckled them by means of a breast.
We know that dinosaurs, such as Stegosaurus undoubtedly developed very
effective methods of absorbing heat quickly from the sun and thereby maintained
higher than ambient body temperatures. Mammals, however, evolved from an
earlier group of reptiles (the Synapsids, often referred to as mammal-like
reptiles). One of the earliest group of synapsids, the pelycosaurs, also had
similar adaptations to the Stegosaurus dinosaurs. Dimetrodon, grew long spines
from its backbone which supported a sail of skin which must have served as a
solar panel in a similar way to the Stegosaur's plates. Although the pelycosaurs
persisted for a considerable time their sail-like crests disappeared in later forms.
In seems extremely unlikely that even if there was a warming of the climate, the
forces of evolution would allow an animal to lose such a valuable method of heat
control unless it was able to replace it with an adaptation that is more efficient. It
has been hypothesized that the pelycosaurs and their successors, the therapsids,
were to some degree endothermic.
One of the therapsid lines were the theriodonts which were small carnivorous
animals (< 1m) and were almost certainly the evolutionary line that lead to the
mammals. There is some doubt as to whether theriodonts should be classified
as reptiles or as very primitive mammals. An example of which is Cynognathus,
an animal approximately a metre long, possessing a large dog-like skull with
highly specialized and differentiated teeth (remember reptiles are generally
characterized by simple, undifferentiated peg-like teeth). These teeth suggested
that Cynognathus teeth were for chewing and cutting food rather than swallowing
it whole. There is also a well developed secondary palate, which separates the
nasal passage from the mouth, which permits continued eating while the mouth is
filled with food. All these features suggest that the animal was very active and
probably requiring an endothermic metabolism. To maintain such a metabolism
would require some form of body insulation, possibly even fur.
These fossils indicate that some theriodonts were far advanced towards the
mammals in certain characters, but still remained comparatively primitive in other
respects. The mixture of conservative and advanced features makes it difficult to
identify the final line that evolved towards the mammals.
The environmental conditions that stimulated such changes may have produced
similar adaptations in more than one group of animals. It is likely that
mammalian traits were acquired by several separate reptilian groups. It was
originally hypothesized that the line of reptiles from which the platypus and
echidna stemmed was not necessarily the same as that which was to give rise to
other mammals. In other words mammals had a polyphyletic origin (derived from
more than one ancestor) rather than a monophyletic origin (derived from a single
ancestor). Recent evidence based on the skull morphology of Probainognathus
is argued for monophyletic origin for the mammals. Much of this debate depends
on whether the advanced theriodonts were reptiles or represented the first
mammals. What is certain that monotremes diverged from the main mammalian
line during the Triassic, whereas the other major division in the mammals,
namely differential of placental and marsupial forms only occurred during the late
Cretaceous period.
Whatever the exact shape of the genealogical tree, at least one group of the
reptiles completed the transition to a mammalian status some 200 million years
ago. A fossil from the upper Triassic of a small animal (Megazostrodon)
discovered in 1966 in southern Africa, is possibly the earliest true mammal. This
creature was only about 100 mm long and resembled in body form a modern day
shrew. Details of its jaw and skull link it firmly with true mammals and its teeth
were specialised for eating insects. There is little doubt that it must have been
both warm-blooded and fury. What we cannot determine is whether it laid eggs
Even with the advantage of warm bloodedness the first small mammals were
quite over-shadowed in both numbers and size by the dinosaurs until 65 million
years ago. Equipped with warm bloodedness, mammal were able to be active at
night when the great reptiles became torpid and therefore survived in the shadow
of the dinosaurs.
The earliest mammals were probably like the opossums that today live in the
Americas particularly those belonging to the genus Didelphis. The Virginia
opossum Didelphis marsupialis of North America is a large rat-shaped creature,
with small eyes and a long naked tail which it can wrap round a branch with
sufficient strength to support its own weight. It has a large mouth that opens wide
and is equipped with a great number of small sharp teeth. It is a tough adaptable
creature that has spread through the Americas, from Argentina in the south to
Canada in the north. One of the most extraordinary aspects of this animal is its
manner of reproduction. The female has a capacious pouch on her underside in
which she rears her young. The young are extremely small and without fur and
have attach themselves to the mother's teats. The method by which they get
there is one of the most fascinating. The opossums copulate and fertilization of
the female's eggs occurs internally. The young embryos, however, have only
enough yolk to maintain themselves for the first few days of their life. At twelve
days and eighteen hours the animals are expelled into the outside world. This
represents the shortest gestation period known for any mammal species. These
young are born so premature that they are no larger than bees, and so unformed
that they are not called infants but are rather referred to as neonates. As the
neonates emerge from their mothers cloaca, they haul themselves through the fur
of her belly to the opening the pouch, a distance of some 80 mm. Only about half
of the neonates reach the pouch and each animals attaches itself to one of
thirteen nipples and starts to take milk. If more than thirteen complete the
journey, only those that attach themselves to a teat will survive. Nine or ten
weeks later, the young clamber out of the pouch. They are now fully formed, the
size of mice, and cling to their mother's fur. In about three months they leave their
mother for an independent life of their own. Mammals that bread in this way (by
means of a pouch) are all placed in the order Marsupialia.
There are seventy-six species of opossum (Family Didelphidae) in America, with
the smallest (Marmosa murina) being mouse-sized and not possessing a pouch
(the young simply cling to the teats between their mother's hind legs. The largest
is the water opossum or Yapok (Chironectes minimus) and is almost the size of a
small otter, and possesses webbed feet for swimming. Its young are saved from
drowning in the pouch when their mother goes, a sphincter (ring-shaped muscle)
which closes the entrance the entrance of the pouch. The young inside are able
to endure several minutes of submergence and breathe air within the pouch that
has a higher concentration of carbon dioxide than most mammals could survive.
The earliest mammalian fossils that have been positively identified as being
marsupial were found in the Americas and this may be where the group
originated, however, the greatest assemblage of marsupials occuring today is in
Australia. The earliest marsupials (Alphodon and Eodelphis from Cretaceous
North America) closely resemble the living Didelphis opossums that occur in the
Americas.
From didelphid ancestors certain South American marsupials specialized into
aggressive carnivores during Tertiary times. These were the borhyaenids, of
which the Miocene genus Borhyaena was typical and resembled a large wolf.
The skull was very dog-like, with the canines enlarged as piercing and stabbing
teeth, and some of the molars modified into shearing blades. The body was long,
23
limbs exceptionally strong and the feet were equipped with exceptionally sharp
claws. Others such as Thylacosmilus was as large as a tiger; possessed a short
skull and tremendously elongated bladelike upper canine tooth, whereas in the
lower jaw there was a deep flange of bone to protect this tooth when the mouth
was closed. These carnivorous marsupials became extinct with the influx of
placental carnivores from North America.
In order to explain how the marsupials got from South America where they
originally radiated to Australia we have to return to the period when the dinosaurs
were still at the height of their dominance. At that time, the continents of the
world were grouped together in a single large land mass. Consequently fossils of
closely related dinosaurs have been found in all of today's continents. The early
mammal-like reptiles would have similarly widespread distributions. About 135
million years ago the large single land mass (Pangaea) split into two - a northern
supercontinent called Laurasia comprising today's Europe, Asia and North
America; and in the south, another super-continent called Gondwana made up of
South America, Africa, Antarctica and Australia.
The primary evidence for this grouping and the subsequent splitting and drifting is
geological. It comes from studies of the way in which today's continents fit
together, the continuities of the rocks between their opposite edges, the
orientation of magnetic crystals in rocks which shows the position that they held
when they were first formed, the dating of the mid-ocean ridges and their islands
and drillings taken from the ocean floors.
The distribution of many animals and plants adds corroborative evidence. Giant
flightless birds provide a particularly clear case since they appeared very early in
the history of the birds. One group which included the ferocious Diatryma,
evolved in the northern super-continent are all extinct. The other group called
ratites evolved in the southern supercontinent, and are represented by the Rhea
(Rhea americana) in South America, the Ostrich (Struthio camelus) in Africa, the
Emu (Dromaius novaehollandiae) and Cassowary (Casuarius spp.) in Australia
and the Kiwi (Apteryx) in New Zealand. These birds are so similar that it seems
very probable that they are descended from a single flightless ancestor which
had distribution right across the Gondwana supercontinent. When the land
masses separated the different groups of flightless birds continued to evolve
independently of each other into their present-day forms.
Other evidence for the splitting up of the super-continents comes from fleas,
which are parasitic and travel with the animals they live on but readily develop
into new species and move on to new hosts. Some families of highly
characteristic fleas are found only in Australia and South America, with the most
probable explanation being that they originated on group of animals that had a
wide distribution across Gondwana. Botanical evidence is found with the
southern beech, a forest-forming tree that flourishes only in the temperate lands
of the southern hemisphere. This distribution can also be explained by the breakup of Gondwana. During this break-up Africa separated and drifted northwards
and Australia and Antarctica remained joined to one another and were linked
either by way of a land bridge or a chain of islands, to the southern tip of South
America. At this point, it seems, the pouched animals (marsupials) were
developing from the early an mammal stock. If these developments took place in
South America, as some evidence suggests, then the early marsupials could
have spread across into the Australian-Antarctic block by way of these landbridges or by island hopping. Fossil evidence supporting this theory comes from
two very closely related marsupial animals; Polydolops and Antarctodolops being
found in South America and Antarctica respectively.
Meanwhile, primitive mammals were also evolving in the northern supercontinent. They were to develop a different way of nourishing their young.
Instead of transferring them at a very early stage into an external pouch, they
retained them within the body of the female and supported them by means of a
device called the placenta. We will examine this technique of reproduction later.
The South American marsupials flourished greatly while they had the continent to
themselves since the southern supercontinent was fragmenting and drifting apart
and South America was moving slowly northwards. In due course, it connected
with North America by way of a land bridge in the neighbourhood of Panama.
Down this corridor came the placental mammals to dispute the possession of
South America with the marsupial residents. In the course of this rivalry, most
species of marsupials disappeared, leaving only the tough, opportunistic
opossums. One of these has even invaded North America, the land from where
the placental invaders had come from. That marsupial invader is the Virginia
opossum.
None of the marsupials that lived in the central part of the southern supercontinent which became Antarctica survived. By that time Antarctica had drifted
over the South Pole where it was so cold that it developed an immense ice-cap
and life on the land became insupportable. The eastern section of the supercontinent, which became Australia had drifted in a north- east direction into the
emptiness of the Pacific basin and has since remained totally separate from any
other continent. The marsupials that occurred on this section of the supercontinent have continued to evolve without any invasion from placental animals
until man introduced them. During this time, the marsupials radiated into a great
number of different forms in order to take advantage of the wide range of
environments and opportunities available to them. Fossil remains of some
spectacular species that once existed have been discovered in the limestone
caves of Naracoorte, 250 kms south of Adelaide. Among them are the remains
of a huge marsupial the size of a cow, with a head like a small giraffe that
browsed on the branches of trees. One specimen Thylacoleo was originally
thought to be a carnivore due to the back teeth that were elongated into
formidable shearing blades, and called a marsupial lion. More recently the front
legs have shown that this animal was well suited for a tree climbing existence
and used its elongated back teeth to cut down hard fruits.
Australian marsupials still survive within a dozen main families and are
represented by nearly two hundred species. Many of these creatures parallel the
placental forms that evolved in the northern hemisphere. For example there are
carnivorous marsupials that will tackle reptiles and nestling birds and are called
marsupial cats (Dasyurus) and until very recently there was also a marsupial wolf
called a Thylacine. Since this animal took to preying on newly introduced sheep
it was hunted and eventually exterminated by local farmers.
Sometimes the resemblance between placental and marsupial forms is so close
that you need to examine the animals closely in order to distinguish them. The
sugar gliders Petaurus spp. are small leaf and blossom-eating marsupials that
live in eucalyptus trees. They have a parachute of skin connecting its fore and
hind legs which enable them to glide from branch to branch and resemble almost
exactly the North American flying squirrel (Petaurista alborufus). The similarity is
based on similar lifestyles requiring similar forms. For example in order to have
lifestyle that relies on gliding you will need to have structures that function as
parachutes. A burrowing lifestyle also demands particular structures that are
similar for marsupial and placental animals alike. Placental moles (e.g. Cape
Golden Mole Chrysochloris asiatica) and marsupial moles (Notoryctes) both have
short silky fur, reduced eyes, powerful digging forelegs and a stumpy tail. The
distinguishing feature is that the female marsupial mole possesses a pouch,
which unlike other marsupials opens from the rear and therefore does not fill with
earth when she burrows.
24
Not all marsupials have such close placental equivalents. The koala
(Phascolarctos cinereus) is a medium-sized tree-living creature that feeds on
leaves and is comparatively slow moving. Its ecological equivalent are monkeys
which are far more athletic, active and intelligent. The numbat (Myrmecobius
fasciatus) is an ant-eating marsupial possessing a long sticky tongue used to
collect its food items; a feature common to all ant-eaters. Further adaptations for
ant-eating are not nearly so extreme for the numbat as those of other ant-eaters,
e.g. the giant ant-eater (Myrmecophaga tridactyla) of South America which has
evolved a long curving tube-like snout and lost all its teeth. The numbat jaw is
are not nearly so elongated and it still possesses all its teeth.
Other marsupial forms are more unique in their adaptations for example the
boodie (Bettongia lesueur) a shy, strictly nocturnal rat kangaroo, possessing
small pointed canine teeth to help fed on other small animals. It makes its nest in
a burrow, industriously collecting material for it in a most ingenious way. It picks
up a few straws in its mouth, stacks them in a bundle on the ground and then
pushes them back over its long tail with its hind legs. The tail then curls up
tightly so that the straw is effectively baled and the boodie move away by
hopping. Boodies locomote using only their back legs which have very long feet.
An animal like the boodie may have been the ancestor to the spectacular
radiation of bipedalism that resulted in the kangaroos and wallabies
The development of the kangaroos may be related to Australia's continuing drift
northwards and the consequent drying and warming of its climate. This would
have caused a reduction in forest cover and replacement by grasslands. Living
in an open grassland would require that the herbivores feeding on the grass an
ability to escape predators. In kangaroos the hind legs have become enormously
powerful and the long muscular tail is held out stiffly behind to acts as a
counterbalance which gives the animals a potential to reach speeds of 60 kph
and to clear fences nearly 3 metres high.
The second difficulty that grass-eaters must overcome is the wear and tear on
their teeth. Grass is tough, due to the silicates that occur in them, and breaking it
down into a pulp in the mouth is very abrasive on the teeth. Grazers elsewhere
have molars with open roots so that wear can be compensated by continuous
growth throughout the animal's life. In kangaroos the roots of the teeth are
closed, and they have evolved a different system of tooth replacement. There
are four pairs of cheek teeth on either side of the jaws. Only the front ones
engage. As they are worn down to the roots, they fall out and those from the rear
migrate forward to take their place. By the time the animal is fifteen or twenty
years old, its last molars are in use.
There are some forty different species in the kangaroo family. The smaller ones
are usually called wallabies. The largest is the red kangaroo Macropus rufus
which is as tall as a man and the largest living marsupial. Kangaroos reproduce
in much the same way as the opossums. The egg which is still enclosed in the
vestiges of a shell a few microns thick and has only a small quantity of yolk within
it, and descends from the ovary into the uterus. There, lying free, it is fertilised
and begins its development. If this is the first time that the female has mated, the
fertilized egg does not stay there long. In the case of the red kangaroo it is only
thirty-three days before the neonate emerges. Usually only one is born at a time.
It is a blind, hairless an only a few centimetres long; its hind legs are mere buds,
but its forelegs are better developed and with these it hauls its way through the
thick fur on its mother's abdomen. The neonate's journey to the pouch takes
about three minutes. Once there, it fastens on to one of four teats and starts to
feed. Almost immediately, the mother's sexual cycle starts again. Another egg
descends into the uterus and she becomes sexually receptive and she mates and
the egg is fertilised. But then an extraordinary thing happens, the egg's
development is temporarily halted. Meanwhile, the neonate in the pouch is
growing prodigiously. After 190 days, the baby is sufficiently large and
independent to make its first foray out of the pouch. From then on it spends
increasing time in the outside world and eventually, after 235 days, it leaves the
pouch for the last time.
If there is a drought at this time, as happens often in central Australia the
fertilised egg in the uterus still remains dormant. But if there has been rain and
there is good pasture, then the egg resumes its development. Thirty-three days
later, another bean-sized neonate will emerge from the mother's cloaca. The
female will then immediately mate again. But the first-born does not give up its
milk supply so easily. It returns regularly to feed from its own teat. The female
kangaroo in effect has three young dependents on her, each at a different stage
of development. One active young-at-foot which grazes but comes back to
suckle, a second, the tiny neonate, sucking at her teat in the pouch; and a third
the fertilised egg waiting further development.
It is a commonly held notion that the marsupials are backward creatures, scarcely
much of an improvement on those primitive egg-layers, the platypus and echidna.
That is a long way from the truth. The marsupial method of reproduction must
certainly have appeared very early in mammal history, but the kangaroos have
refined it marvellously. No other creature anywhere can compare with the female
kangaroo who, for much of her adult life, supports a family of three in varying
stages of development.
The mammalian body is a very complicated machine that takes a long time to
develop. Even as an embryo it is warm-blooded and burns up fuel very quickly.
Both these characters demand that the developing young should be supplied with
considerable quantities of food. All mammals have found methods of providing
far more than could ever be packed within the confines of a shelled egg. We do
not know whether the early mammals in the northern supercontinent ever passed
through a marsupial stage before developing the placenta. It could be that they
sprang from a branch of the mammal-like reptiles that never acquired pouches.
The placental and marsupial forms probably arose independantly from a common
ancestor, and they evolved side by side. Certainly the fossil record of the
placentals is as ancient as that of the marsupials, and they arose sometime
during the Cretaceous period. During the early stages of their evolutionary
histories they were probably well matched, so that marsupial adaptations were
about as efficient in evolutionary terms as placental adaptations. However,
during the Cenozoic, the placental animals came to dominate in all areas of the
world except the large island of Australia, which until the advent of many had
never witnessed placental mammals. In Australia the marsupial animals
achieved the sophisticated levels of efficiency occurring in the Red Kangaroo.
In the northern continents the placental method of mammalian reproduction
evolved with many ensuing benefits. The placenta allows the young to remain
within the uterus for a very long time. It is a flat disc that becomes attached to
the wall of the uterus and is connected by the umbilical cord to the foetus. The
junction with the uterine wall is highly convoluted so that the surface area
between the placenta and the maternal tissues is very great. It is here than that
the interchange between the mother and foetus takes place. Blood itself does
not pass from mother to young, but oxygen from her lungs and nutrients derived
from her food both dissolved in her blood, diffuse across the junction and so
enters the blood of the foetus. There is also traffic in the other direction. The
waste products produced by the foetus are absorbed by the mother's blood and
then excreted through her kidneys.
25
All of this makes for great biochemical complications. But there are further ones.
The mammalian sexual cycle involves the regular production of a new egg. This
causes no problem to the marsupial, for in every species, the neonate emerges
before the next egg is due to be produced. In the placental animal the foetus,
however, stays in the uterus for a very much longer period. So the placental
foetus secretes a hormone which suspends the mother's sexual cycle for as long
as the placenta is in place so that no more eggs are produced to compete with
the foetus in the uterus.
There is also another problem. The foetus' tissues are not the same genetically,
as the mother's. They contain genetic material from the father. So when it
becomes connected to the mother's body, it risks immunological rejection in the
same way as a transplant does. Just how the placenta prevents rejection is not
completely understood.
So by these means, the babies of placental mammals can remain in the uterus
until, if necessary, they are so well developed that they can be fully mobile as
soon as they are born. The placental breeding technique spares the young the
hazardous journey outside their mother's body at a very early stage that a
marsupial neonate has to undertake, and allows their mother to supply their every
want during the long period they remain within her. So whales and seals can
carry their unborn young even as they swim for months through freezing seas.
No marsupial with air-breathing neonates in a pouch could ever succeed in doing
such a thing. It is possible that the placental technique of reproduction was to
prove one of the crucial factors in the mammals' ultimate success in colonising
the whole of the earth.
ASSIGNMENTS
IN YOUR OWN WORDS WRITE A ONE TO TWO PAGE ESSAY ON THE
FOLLOWING TOPICS
Compare the placental and marsupial modes of reproduction.
Briefly describe how mammals evolved from the synapsid reptiles.
Describe the process of continental drift and how it has influenced the
global distributions of marsupial and placental mammals.
Briefly discuss the adaptive radiation that has occured in the Australian
marsupial mammals and compare such adaptative radiation with that
found in placental animals.
THEME AND VARIATION
In the forests of Borneo lives a small, furry, long-tailed creature resembling a
squirrel but called a Tree Shrew, (Tupaia glis). Unlike a squirrel this creature
does not eat vegetable matter, but hunts small invertebrates. When first
discovered its phylogenetic relationships with other animals was much debated
and it was called a tree shrew based on its dental similarity (small, pointed
unspecialized teeth) to insectivores. Some scientists suggested that the structure
of its genitals indicated a relationship with marsupials, whereas others analysing
the structure of its skull noted an exceptionally large brain, and proposed that it
was a very distant ancestor of monkeys and apes. The debate is not over yet.
Currently the balance of opinion has swung away from viewing the Tree Shrew
as an ancestral monkey and favours classifying it within its own mammalian order
(Scandentia) but recognizing that its closely allied to other primitive mammals
such as shrews which are classifed within the order Insectivora.
The fact that characteristics of so many different kinds of mammals can be seen
in the Tree Shrew, suggests than it might well resemble the ancient creature from
which all placental mammals are descended. Certainly, judging from fossil
skeletons such as Megazostrodon, the first mammals to exist in the dinosaur
dominated forests must have looked very like it:- small, long-tailed and
pointed-nosed, and, by inference, furry, warm-blooded, active and insect-eating.
The reign of the reptiles had been a long one. They had come to dominate about
250 million years ago. They had browsed the forests, munched the lush
vegetation of the swamps, and carnivorous forms had evolved which preyed on
the plant-eaters. Still other species lived by scavenging carrion. The plesiosaurs
and ichthyosaurs were forms that returned to the seas and preyed upon fish;
while pterosaurs took to the skies. Then, 65 million years ago, they all
disappeared. In the void created by the demise of the dinosaurs a radiation of
the placental mammals began.
Tree shrews and other primitive insect-eating mammals (with representatives in
mammalian orders Insectivora and Macroscelida) have survived and are
scattered worldwide. In Malaysia, alongside the Tree Shrew, lives an unkempt
irritable creature with a long nose bristling with whiskers and smelling of rotten
garlic and is known as a moon rat (Echinosorex gymnurus). In Africa there is the
otter shrew (Potamogale velox), the biggest of all and a powerful swimmer; and a
whole group the size of rats which hop, have slender elegant legs and mobile thin
trunks and are called elephant shrews (Order Macroscelida). In the Caribbean
there is another insectivore called the Solenodon (Solendon paradoxus).
However, the most spectacular radiation of insectivores has occurred in
Madagascar and are called tenrecs. Some of these animal are striped and hairy
with stiffened quills (Hemicentetes semispinosus), whereas others have all their
hairs stiffened into spines on their backs (Echinops telfairi) and resemble
European hedgehogs (Erinaceus europaeus), and yet others have become large
and lost their tails (Tenrec ecaudatus).
Europe also has a number of insectivores including hedgehogs (Erinaceus
europaeus), shrews (e.g. Sorex araneus) and moles (e.g. Chrysochloris asiatica).
The spines of a hedgehog are no more than modified hairs. In many parts of the
world shrews are abundant animals and although small, are very ferocious,
attacking any small creature they encounter including one another. To sustain
themselves, they have to eat great quantities of earthworms and insects every
day. Among the shrew is one of the smallest mammals, the pygmy shrew
(Suncus etruscus) which weighs only 1.5 to 2.5 g). Shrews communicate with
one another by shrill high-pitched squeaks. They also produce noises of a
frequency that is far above the range of our ears, their eyesight is very poor and
there is some indication that they use these ultra-sounds as a simple form of
echo-location.
Several species of shrew have taken to water in their search for prey items. In
Europe, there are two near-relatives called the desmans - one lives in Russia
(Desmana moschata) and the other in the Pyrenees (Galemys pyrenaicus)which use long mobile noses as snorkels, turning them up so that they project
above the water as their owners swim about busily searching for food. One
insectivore group searched for its prey entirely underground, the mole. Judging
from the structure of its paddle-shaped forelegs and powerful shoulders, it is
possible that the mole's ancestors were once water-living shrews and the mole
26
has simply adapted the same sort of actions for moving along its tunnels. Fur,
underground, might be a mechanical handicap, but many moles live in temperate
areas and they need fur for insulation. So it has become very short and without
any particular grain so that it points in all directions and the animal can move
forwards or backwards along its tight tunnels with ease. Eyes are of little use
underground, would easily clog with mud, so they are much reduced in size.
Moles locate their prey using their nose which is an organ of both smell and
touch, since it is covered with many sensory bristles. At the rear, in has a short
stumpy tail also covered with bristles which make it aware of what is happening
behind it. The star-nosed mole of America (Condylura cristata) has an additional
device, an elegant rosette of fleshy feelers around its nose which in can expand
or retract. It may be simply a tactile organ or it may be a means of detecting
changes in the chemical content of the air.
Mole tunnels are not simply passageways but traps. Earthworms, beetles,
insect larvae, in the soil may suddenly fall into a mole's tunnel where the mole
harvests the food item. Incessantly active, it patrols its extensive network at least
once every three or four hours and consumes vast numbers of invertebrates each
day. On the rare occasions when so many worms collect in the tunnels that even
a mole's appetite is sated, it gathers up the surplus, gives each of them a quick
bite to immobilise them, and then stores them away in an underground larder.
Some of these stores have been found with thousands of paralysed invertebrates
in them.
A few insectivores specialised in eating one particular kind of invertebrate, ants
and termites. In order to do this a long, sticky tongue is required. Many
unrelated creatures, specializing on this diet, have independently evolved such
an organ. The numbat, the marsupial ant-eater from Australia, the monotreme
echidna and even ant-eating birds, woodpeckers and wrynecks, have developed
one that fits inside a special compartment of the skull and in some extends round
the eye-sockets.
But the most extreme version of such a tongue is that evolved by the placental
mammals including the pangolins (Order Pholidota) the aardvarks (Order
Tubulidentata) and South American ant-eaters (Edentata). In Africa and Asia,
there are seven different species of pangolin including the local species called
the Cape Pangolin Manis temminckii a medium-sized creatures 850 mm long
with short legs and long stout prehensile tails. The Giant Pangolin (Manis
gigantea) is 1,5 metre in length and has a tongue that can extend 400 mm
beyond its mouth. The sheath that houses it extends right down the front of the
animal's chest and is actually connected with its pelvis. The pangolin has lost all
of its teeth and its lower jaw is reduced to two slivers of bone. The ants and
termites collected by the mucus on the tongue are swallowed and then mashed
by the muscular movements of the stomach which is horny and sometimes
contains pebbles to assist in the grinding process.
Without teeth and without any turn of speed, the pangolin has to be well
protected. It has an armour of horny scales that overlap like shingles on a roof.
At the slightest danger the animal tucks its head into its stomach and wraps itself
into a ball with its muscular tail clasped tightly around it.
South America has evolved its own particular group of insect-eaters (Order
Edentata) and their ancestors were among those placental mammals that,
migrated down from north America through Panama and mingled with the
marsupials. However the land bridge did not, in this first instance, last long. After
a few million years, it became submerged beneath the sea and once more the
continent was cut off and its animals evolved in isolation. Eventually, contact
was re-established and there was a second invasion from the north as a
consequence of which many of the recently evolved South American placentals
disappeared, although not all. One of the less specialised of the survivors are the
armadillos. Like the pangolins, they are protected by armour which consists of a
broad shield over the shoulder and another over the pelvis, with a varying number
of half-rings over the middle of the back to give a little flexibility. Armadillos eat
insects, other invertebrates, carrion, and any small creatures, like lizards, that
they manage to catch. Their standard method of seeking food is to dig. They all
have an excellent sense of smell and when they detect something edible in the
ground, they start excavating with manic speed. When you watch them digging,
it seems impossibly that they are able to breathe while excavating, and in fact
they are able to hold their breath for up to six minutes while digging.
There are twenty living species of armadillo, a mere fraction of what formerly
existed. An extinct gigantic armadillo called the glyptodont (Glyptodon) that had
a single-piece domed shell as big as a small car. One such shell has been found
and it appears to have been used by early man as a tent. In the glyptodonts, not
only was the body heavily armoured, but the top of the head was covered with a
thick shield of bony armour, as was the tail. The ends of the tail were often
provided with an enlarged, spiked knob of bone, which was probably used for
defence. The biggest surviving species is the Giant Armadillo, (Priodontes
giganteus) the size of a pig, which lives in the forests of Brazil. Like all the group,
it is very largely insectivorous and consumes great quantities of ants. In
Paraguay the little three-banded armadillo (Tolypeutes matacus) trots about on
the tips of its claws and can roll into a neatly fitting impregnable ball. Down in the
pampas of Argentina there are small Hairy Armadillos (Chaetophractus villosus)
that are mole-like and seldom come to the surface except at night. All armadillos
have teeth. The Giant Armadillo has about a hundred, which is almost a
mammalian record, but they are small, simple and peg-like.
The specialist ant-eaters of South America, however, like the pangolin of Africa,
have lost their teeth entirely. There are three species of them, the smallest
being the Dwarf Ant-eater (Cyclopes didactylus) which lives entirely in trees and
exclusively on termites. A bigger version, the Tamandua (Tamandua
tetradactyla) is cat-sized has a prehensile tail and short coarse fur. It too is a
tree-dweller but it often comes down to the ground. On the open plains, where
termite hills stand as thick as tombstones in a graveyard, lives the Giant
Ant-eater (Mymecophaga tridactyla) which is about 2 metres long. Its forelegs
are bowed, and its claws are so long that it has to tuck them inward and walk on
the sides of its feet. With these claws it easily tear open termite hills. Its
toothless jaws form a tube even longer than its forelegs. When it feeds, its huge
thong of a tongue flicks in and out of its tiny mouth with great rapidity and probes
deep into the termite hill.
All ant-eaters are slow movers and are without teeth and armour to defend and
protect themselves. The Dwarf Ant-eater and Tamandua favour tree-living ants
and termites and spend most of their time up in the branches out of the way of
most predators. The Giant Ant-eater is less defenceless than might at first
appear. Its huge front claws can do severe damage even to a large predator
such as the jaguar.
The mammals that we have studied so far, almost all feed on invertebrates,
particularly insects, however a large number of insects fly and therefore are able
to escape such predators. Insects first took to the air some 300 million years ago
and had the skies to themselves until the arrival of the flying reptiles like the
pterosaurs, some hundred million years later. Whether the reptiles flew at night
is not known although unlikely bearing in mind the reptilian problem of
maintaining body temperature. Birds eventually succeeded them, but there is no
reason to suppose than there were any more night-flying birds in the past than
27
there are today - which is very few. Consequently the night skies offer the best
refuge from predation until another variation on the insectivore theme evolved:
the bats.
There were probably many mammalian attempts at flying before the success of
the highly specialized bats. In Malaysia and the Philippines there lives an odd
animal called the colugo or flying lemur (Cynocephalus volans) and has been
classified in its own order Dermoptera. It is about the size of a large rabbit but its
entire body, from its neck to the end of its tail, is covered by a softly furred cloak
of skin. When the animal hangs beneath a branch or presses itself against a tree
trunk, its camouflaged patterning on its fur makes it almost invisible, but when it
extends its legs, the cloak becomes a gliding membrane. The colugo's gliding
technique has several parallels. The marsupial sugar glider planes through the
air in just the same way. Two groups of squirrels have also independently
acquired the talent. But the colugo has the biggest and most completely
enveloping membrane and took to the habit early in mammalian history, for it is
certainly a very primitive member of the group and seems to be a direct
descendant of an insectivore ancestor. A few Palaeocene and Eocene fossils
from North America are very similar to the living Colugo and therefore it is
considered to be a fairly primitive animal. Having perfected a gliding life style, the
Colugo has remained unchallenged - and unchanged. Colugos cannot be
regarded as a link with the bats, for its anatomy is entirely different in many
fundamental aspects, but it is an indication of a stage that some early
insectivores may have passed through on their way to achieving flapping flight
that occurs in bats which are classified in the order Chiroptera.
The first fossil evidence of fully developed bats were dated at fifty million years
ago (Icaronycteris), so the evolution of flight started early on in the radiation of the
placental mammals. Bats are the only mammals that have mastered true,
flapping flight. The bat's flying membrane stretches not just from the wrist, like
the colugo, but along the extended second finger. The other two fingers form
struts extending back to the trailing edge. Only the thumb remains free and
small. This retains its nail and the bat uses it in its toilet and to help it clamber
about its roost. A keel has developed on its chest bone which serves as an
attachment for the muscles which flap the wings.
The bats have many of the modifications developed by birds in order to save
body weight. The bones in the tail are thinned to mere straws to support the
flying membrane or have been lost altogether. Though they have not lost their
teeth, their heads are short and often snub-nosed and so avoid being nose-heavy
in the air. They had one problem that birds did not face. Their mammalian
ancestors had perfected the technique of nourishing their young internally by
means of a placenta. Evolutionary developments can seldom be reversed so
bats have not been able to revert to egg-laying with the associated benefit of
weight saving that occurs in birds. The female bat must therefore fly with the
heavy load of her developing foetus within her. In consequence, bats usually
have one young born per breeding season. This, in turn, means that if the
population is to be maintained, the females must compensate by having long
reproductive lives, and bats are for their size, surprisingly long-lived creatures,
with a life-expectancy of up to twenty years.
Today, most bats fly at night and it is likely that this was always the case since
the birds had already laid claim to the day. To do so, however, the bat had to
develop an efficient navigational system. It is based on ultra-sound like those
made by the shrews and other primitive insectivores. The bats use them for
sonar, an extremely sophisticated method of echo-location. This is similar in
principle to radar, but radar employs radio waves whereas sonar uses sound
waves. These are frequencies that lie a long way above the range of the human
ear. Most of the sounds we hear have frequencies of around several hundred
vibrations a second. Some of us, particularly when we are young, can with
difficulty distinguish sounds with a frequency of 20 000 vibrations a second. A
bat flying by sonar, uses sounds of between 50 000 and 200 000 vibrations a
second. It sends out these sounds in short bursts, like clicks, twenty or thirty
times every second and its hearing is so acute that from the echo each signal
makes, the bat is able to judge the position not only of objects around it but of its
prey which is also likely to be flying quite fast. Most bats wait to receive the echo
of one signal before emitting the next. The closer the bat is to an object, the
shorter the time taken for the echo to come back, so the bat can increase the
number of signals it sends the closer it gets to its prey and thus track it with
increasing accuracy as it closes in for the kill.
Hunting success, however can mean momentary loss of it senses for if its mouth
is filled by an insect, a bat cannot squeak in the normal way. Some species
avoid this difficulty by squeaking through their noses and developed a variety of
grotesque nasal outgrowths which serve to concentrate the beam of the squeak
and act like miniature megaphones. The echoes are picked up by the ears and
these too are elaborate, extremely sensitive and capable, in many cases, of
being twisted to detect a signal. So the face of many bats is dominated by sonar
equipment - elaborate translucent ears, ribbed with cartilage and laced with an
intricate pattern of scarlet blood vessels; and on the nose, large protrusions to
detect sounds. The combination and patterns of protrusions on the nose and ear
structure is species specific so that each can produce a unique call. Receptors
synchronized to particular sounds filter out signals from other bat species. The
system, described in such terms, sounds simple but when you encounter several
million bats flying simultaneously in pitch darkness represented by eight species
as occurs in the Gomanton Caves in Borneo you realize that echolocation has
become a highly sophisticated sensory apparatus.
A few insects have developed systems to protect themselves from predation from
bats. In America, there are moths that have the ability to tune in to the frequency
of the bat's sonar. As soon as they hear a bat approaching, they drop to the
ground. Other species go into a spiralling dive which the bats find hard to follow.
Yet others manage to jam the signal or send back high-frequency sounds that
convince the bat that they are inedible or are objects to be avoided.
Not all bats feed on insects. Some such as the Pallas' long-tongued bat
(Glossophaga soricina) have discovered that nectar is very nutritious, and have
refined their flying skills so that they can hover in front of flowers, just like
humming birds, and gather nectar by probing deep into the blossoms with long
thin tongues. Just as a great number of plants have evolved to exploit the
services of insects as pollinators, so too some rely on bats. Some cacti, for
example, only open their blossoms at night. These are large, robust and lightcoloured, for in the darkness colour is valueless. Their scent, however, is heavy
and strong and the petals project well above the armoury of spines on the stems
so that the bats are able to visit without damaging their wing membranes.
The biggest of all bats live only on fruit. They are called flying foxes (e.g.
Pteropus giganteus), not only because of their size - and some of them have a
wing span of one and a half metres - but because their coats are reddish brown
and their faces are fox-like. They have large eyes but only small ears and lack
any kind of nose-protrusions and they are not equipped with any form of
echolocation apparatus. Whether this major difference between them and other
insectivorous bats indicates that the two groups derive from separate branches of
primitive insectivores is not yet agreed. Unlike insectivorous bats, fruit bats do
not live in caves but in the tops of trees in large communal roosts. In the
evening, they set of in parties to feed. Their silhouette is quite unlike that of
birds, for they lack a projecting tail and their flight is very different from the
28
fluttering of insect- hunting bats. Their huge wings beat steadily as long skeins of
them keep a level purposeful course across the evening sky. They may travel as
far as 70 kilometres in their search for fruit.
Other bats have taken to feeding on meat. Some prey on roosting birds, some
take frogs and small lizards. The Yellow-eared bat (Phyllostomus hastatus) even
feeds on other bats. An American species even manages to fish (Noctilo
leporinus). At dusk, it beats up and down over ponds, lakes, or even the sea.
The tail membrane of most bats extends to the ankles. In the fishing bat, it is
attached much higher up at the knee, so that the legs are quite free. The bat can
therefore trail its feet in the water, keeping the membrane out of the way by
folding up its tail. Its toes are large and armed with hook-shaped claws. When
they strike a fish, the bat scoops it up into its mouth and kills it with a powerful
crunch of its teeth.
The vampire bat (Desmodus rotundus) has become very specialised indeed. Its
front teeth are modified into two triangular razors. It settles gently on a sleeping
mammal, a cow or even a human being. Its saliva contains an anti-coagulant, so
that the blood, when it appears, will continue to ooze for some time before a clot
forms. The vampire then squats beside the wound lapping the blood. They fly
by sonar and it is said that the reason that dogs, whose hearing is also tuned to
very high frequencies, are so seldom attacked by them is that they can hear the
vampire bats coming.
The diveristy of bats is amazing with some 950 species. Possible the most
unique adaptation that has occurred is the Yellow-eared Bat (Uroderma
bilobatum). Unlike most bats, which make no nest or shelter of any sort, this bat
cuts a row of holes in a bannana leaf so that the edges drop and forms a tent
under which it hangs during the day.
Not only have mammals taken to the air, but they have also returned to an
aquatic environment. The mammals that are the most fish-like are Whales and
Dolphins and are classified in the order Cetacea. Despite their appearance they
are warm-blooded and milk-producing animals that have a long ancestry, with
fossils dating back to the beginning of the great radiation of the mammals fifty
million years ago. The earliest known cetacean is Pakicetus, the fossils of which
are found in river sediments, indicating that these primitive cetaceans had not
ventured into marine environments. The earliest fossil that resembled a marine
whale is Basilosaurus, which occured about 42 million years ago and had already
reached a length of 20 metres, possessed a very long tail and its forelimbs were
modified into paddles. The hind limbs were small, but still included a foot
possessing three toes.
The problems associated with a return to an aquatic existance include
locomotion, respiration and reproduction. Yet such adaptations were undertaken
in an extremely short period, although it is difficult to comprehend how such an
immense animals as the 130 ton blue whale (Balaenoptera musculus) really
descended from a tiny creature like the tree shrew. Their ancestors must have
entered the sea at a time when the only mammals in existence were the little
insectivores. But their anatomy is now so extreme in their adaption to swimming
that it gives no clue as to how the transition back to the seas was made. It may
be that the two main groups of whales; the carnivorous forms possessing teeth
(suborder Odontoceti) and the filter feeding forms using a baleen (suborder
Mysticeti) have different ancestries, those with teeth having come from
insectivores by way of primitive carnivores and the rest, the baleen whales, being
descended more directly.
The major differences between the whales and the early mammals are all
attributable to adaptations for a swimming life. The forelimbs have become
paddles. The rear limbs have been lost altogether, though there are a few small
bones buried deep in the whale's body to prove that the whale ancestors really
did, at one time, have back legs. Fur, that hallmark of mammals, functions as an
insulator due to air being trapped between hairs and is therefore of little use to a
creature that never comes onto dry land. Consequently whales have lost that
too, though there are a few bristles on the snout to demonstrate that they once
had a coat. Insulation, however, is still needed and whales have developed
blubber, a thick layer of fat beneath the skin that prevents their body heat from
escaping even in the coldest sea. The mammals' dependency on air for
breathing must be a considered a real handicap in water, but the whale has
minimized that problem by breathing more efficiently than most land-livers. Man
only clears about 15% of the air in his lungs with a normal breath. The whale, in
one of its roaring, spouting exhalations, gets rid of about 90% of its spent air. As
a result it only has to take air in at extended intervals. It also has in its muscles a
particularly high concentration of a substance called myoglobin that enables it to
store oxygen. This form of oxygen storage allows the fin-back whale, to reach
depths of 500 metres and swim for forty minutes without surfacing for air.
One group of whales has specialised in feeding on tiny shrimp-like crustaceans,
krill, which swim in vast quantities in the sea. Just as teeth are of no value to
mammals feeding on ants, so they are of no use to those animals eating krill.
These whales have lost their teeth and instead have baleen, sheets of horn,
feathered at the edges, that hang down like stiff parallel curtains from the roof of
the mouth. The whale takes a large mouthful of water in the middle of the shoal
of krill, half shuts its jaws and then expels the water by pressing its tongue
forward so that the krill remains and can be swallowed. Sometimes it gathers the
krill by slowly cruising where it is thickest. It also can concentrate a dispersed
shoal by diving beneath it and then spiralling up, expelling bubbles as it goes, so
that the krill is driven towards the centre of the spiral. Then the whale with its
jaws pointing upwards, rises vertically in the centre of the spiral it has created
and gathers them in one gulp. On such a diet, the baleen whales have grown to
an immense size. The blue whale (Balaenoptera musculus) the biggest of any
animal to inhabit our planet, grows to over 30 metres long and weighs up to 130
tonnes. There is a positive advantage to a whale being so large. Maintaining
body temperature is easier the bigger you are and the lower the ratio between
your volume and surface area. This phenomenon had affected the dinosaurs but
their dimensions were limited by the mechanical strength of bone. Above a
certain weight, limbs would simply break. The whales are less hampered. The
function of their bones is largely to give rigidity. Support for their bodies comes
from the water. Nor does a life spent gently cruising after krill demand great
agility.
The toothed whales fed on different prey. The largest of them, the squid-eating
sperm whale (Physeter macrocephalus), only attains half the size of the blue
whale. The smaller ones, dolphins, porpoises and killer whales, hunt both fish
and squid and have become extremely fast swimmers, some reputedly being
able to reach speeds of over 40 kph. Moving at such speeds, navigation
becomes critically important. Fish are helped by their lateral line system, but
mammals lost that far back in their ancestry and the toothed whales have instead
a system based on the sounds used by shrews and elaborated by bats, sonar.
Dolphins such as Bottle-nosed (Tursiops truncatus) produce the ultra-sound with
larynx and maybe an organ in the font of the head, the melon. The frequencies
they use are around 200 000 vibrations a second, which is comparable to those
used by bats. With this aid, they can not only sense obstacles in their path, but
identify from the quality of the echo, the nature of these objects ahead. This can
be demonstrated easily enough, for dolphins flourish in oceanaria and eagerly
cooperate in training. Blindfolded dolphins demonstrate that they can, without
difficulty, pick out particular shapes of floating rings and will swiftly swim through
29
the water, with blindfolds on their eyes.
Discuss adaptive radiation in the order Insectivora.
Dolphins produce a great variety of other noises quite apart from ultra-sounds
and there has been considerable speculation as to whether these sounds
constitute a language. So far, we have identified some twenty different sounds
that dolphins make. Some seem to serve to keep a school together when they
are travelling at speed, other appears to be warning cries. But no one yet has
demonstrated that dolphins ever put these sounds together to form the equivalent
of the two-word sentence that can justifiably be regarded as the beginning of true
language, a phenomenon already demonstrated for Chimpanzees (Pan
troglodytes).
Discuss adaptations to eating ants and termites in the mammalian orders
Edentata, Pholidota, Tubulidentata, Marsupialia and Monotremata.
The great whales also have voices. Humpbacks (Megaptera novaeanglia), one
of the baleen whales, congregate every spring in Hawaii to give birth to their
young and to mate. Some of them also sing. Their song consists of a sequence
of yelps, growls, high-pitched squeals and long-drawn-out rumbles. And the
whales declaim these songs hour after hour in extended stately recitals. They
contain unchanging sequences of tones that have been called themes. Each
theme may be repeated over and over again - the number of times varies but the
order of the themes in a song is always the same in any one season. Typically, a
complete song lasts for about ten minutes, but some have been recorded that
continue for half an hour and whales may sing, repeating their songs, virtually
continuously for over twenty-four hours. Each whale has its own characteristic
song but it composes it from themes which it shares with the rest of the whale
community in Hawaii. The whales stay in Hawaiian waters for several months,
calving, mating and singing. Then, within a few days, the deep blue bays and
straits off the Hawaiian islands are empty. The whales have gone. Humpbacks
appear a few weeks later off Alaska. It is very likely that these are the Hawaiian
animals but more studies will have to be made before we can be certain that they
are. Next spring, they reappear in Hawaii and once more begin to sing. But this
time they have new themes in their repertoire and have dropped many of the old
ones.
We still do not know why whales sing although each individual whale can be
identified by its song, which may mean that whales can do the same. Water
transmits sound better than air so it may well be that sections of these songs,
particularly those low vibrating notes, can be heard several kilometres away
informing them of the whereabouts and activities of the whole whale community.
Ant-eaters, bats, moles and whales are all early descendants of the first
mammals and have developed elaborate specializations to eat other small and
large animals, But there are other sources of nutriment to be trapped as well plants. This is the next step in the radiation of the placental animals, the first of
which Some creatures developed that ate grass and moved from the forest
onto the plains to graze. They were followed by the flesheaters and in the open,
the two inter-dependent communities evolved, side by side, each advance in
hunting efficiency producing responses in defence from the hunted. A second
group of creatures established their lives in the tree tops.
Discuss adaptive radiation in the orders Insectivora and Chiroptera.
THE HUNTERS AND THE HUNTED
Forests offer an ever-renewing, inexhaustible supply of food for evolving animals.
The first vertebrate herbivores probably evolve to utilize and digest such
vegetation. Herbivorous dinosaurs had fed on them, smashing saplings in the
forests of ash, elm and beech in North America, crashing through the palms and
lianas of the tropics. With the extinction of the dinosaurs, only invertebrates such
as insects would continue, unobtrusively, to claim their share, gnawing at the
wood, scissoring the leaves into fragments. A few lizard species would have
teared away at leaf fronds, and birds, would have been acquiring a taste for the
newly evolving fruit, and obliging the plants with distribution of their seeds. About
50 to 60 million years ago there appeared to be no large herbivores using these
plants. Eating plants is no easy business. It demands particular skills and
structures just like any other specialized diet. For one thing, vegetable matter is
not particularly nutritious and great quantities of material needs to be extracted to
obtain enough calories to sustain a large animal. Some dedicated vegetarians
have to spend three-quarters of their waking hours foraging. This in turn would
expose an animal to risk by a predator. One way for an animal to minimise such
a risk is to grab as much as possible, as quickly as possible, and to run of with it
to somewhere safe a strategy that the African Giant Rat (Cricetomys gambianus)
employs. This rodent emerges cautiously from its burrow at night and when it is
sure that there is no danger, frantically loads its cheek pouches with anything that
looks remotely edible. Seeds, nuts, fruits, roots, occasionally a snail or a beetle
all go in. The pouches are very large and when they are crammed full it scurries
back to its burrow.
Plant-eaters have to have particularly good teeth. Not only do they use them for
very long periods but the material they have to deal with is tough. Rats, like other
members of the order Rodentia - squirrels, mice, beavers, porcupines cope with
that problem by maintaining open roots to their front gnawing teeth, the incisors,
so that they continue to grow throughout the animal's life compensating for wear.
They are kept sharp by a simple but effective self-stropping process. The main
body of the rodent incisor is of dentine, but its front surface is covered by a thick
and often brightly coloured layer of enamel which is even harder. The cutting
edge of the tooth thus becomes shaped like a chisel. As the top incisors grind
over the lower ones the dentine is worn away more quickly and this exposes the
blades of enamel at the front keeping a sharp chisel edge.
ASSIGNMENTS
IN YOUR OWN WORDS WRITE A ONE TO TWO PAGE ESSAY ON THE
FOLLOWING TOPICS
Discuss echolocation in bats and whales.
Describe how placental mammals have colonized land, water and air.
Once gnawed, ground and pulped, the food has to be digested. This too
presents major problems. Cellulose, the material from which the cell walls of
plants are built, is one of the most stable of organic substances. Digestive
enzymes produced by mammals are unable to break cellulose down, and this
can be achieved by either mechanical means through extended chewing or by
bacteria which are able to dissolve the cellulose through fermentation. Herbivore
digestive systems maintain bacterial cultures to break down this cellulose. Even
with bacterial help, digestion of an entirely vegetarian meal can take a long time.
30
stimulus comes from the fat reserves themselves. When the animal has
accumulated sufficient fat biochemical processes initiate hibernation.
In rabbits (order Lagomorpha) and rodents additional digestion is provided by reeating soft faecal pellets (coprophagy), so that the material is twice processed
and the last vestiges of nourishment are extracted. Only after this second
processing are the faeces deposited outside the burrow as the familiar dry
pellets.
The two members of the order Proboscidea; the African and Indian Elephant
(Loxodonta africana and Elephas maximus) have particularly acute problems for
they eat, in addition leaves, a great deal of fibrous twigs and woody material.
Apart from their tusks their only teeth are molars at the back of the mouth, which
form massive grinders. As they wear down they are replaced every few years by
new ones erupting from behind and migrating forward along the jaw. The molars
pulp and crush with enormous power, but even so, the elephants food is so
woody it requires a very long period of digestion to extract anything of value from
it. The elephant's stomach, however, is big enough to provide it. A meal taken
by a human being normally passes through the body in about twenty-four hours.
An elephant's takes about two and a half days to make the same journey and for
most of that time it is kept stewing in the digestive juices and bacterial broth of
the stomach. Much earlier in history some dinosaurs, eating ferns and cycads,
had encountered the same problem and solved it the same way by becoming
giants.
Elephant dung, even after all this protracted treatment, still contains a great deal
of twigs, fibres and seeds that have remained virtually un-touched. Some plants
that have been stripped by elephants for millennia have reacted to the treatment
by coating their seeds with rinds thick enough to withstand a prolonged soaking
in the digestive juices. The paradoxical consequence has been that now, unless
the rind is weakened by passing through an elephant digestive system, the seeds
are unable to germinate.
The most elaborate apparatus for digesting cellulose is the familiar one used by
the ruminants such as antelope, deer, buffalo as well as domestic sheep and
cows (order Artiodactyla). They clip grass from their pasture with the lower
incisors, pressing it against the tongue or the gums of the upper jaw, which has
no teeth in the front. They then swallow it immediately and it goes down to the
rumen, a chamber of the stomach which contains a particularly rich brew of
bacteria. There it is churned back and forth for several hours, squeezed by a
muscular bag, while the bacteria attack the cellulose. Eventually, the mash is
brought up the throat, a mouthful at a time, to be chewed in a particularly
thorough way by the molars. Ruminants can move their jaws not only up and
down but backwards, forwards and sideways. This ruminating can be done,
however, at leisure and in safety, when the animal has left the exposed feeding
grounds and is relaxing in the shade during the heat of the day. Eventually the
mouthful is swallowed for the second time. It goes past the rumen and on to the
stomach proper which has absorptive.
Leaves have one further shortcoming as food. In temperate parts of the world (viz
deciduous forests), many disappear almost entirely for months at a time. The
creatures dependent upon them must, therefore, make special preparations as
winter approaches. Asiatic sheep (Ovis ammon) turn their food into fat and store
it as cushions around the base of their tails. Other species not only feed and
fatten themselves as much as they can, but reduce the demands of the next few
months to a minimum by hibernating. The triggers to initiate hibernation have not
all been precisely identified. It is certainly not simply a drop in the temperature
since animals kept in a constantly warm environment will still hibernate. In some
cases it appears to related to shortening of daylight hours. It may be that the
A hibernating dormouse (Glis glis) is spherical, with its head tucked into its
stomach, its soft furry tail wrapped around itself. In this posture the amount of
heat that seeps away from the body is reduced. Its heart beat slows considerably
and the breathing becomes so shallow and infrequent that it is difficult to detect.
The muscles stiffen and the whole body feels cold, since body temperature is
reduced to save energy. In this state of suspended animation, the body's food
demands are so low that the fat store can provide enough to keep essential
processes ticking over for months. Extreme cold, however, will waken the animal
to prevent it being frozen alive. When awakened the animal begins to shiver
violently, warming itself by burning fuel in its muscles. It may even, in an
emergency, squander some of its remaining reserves of fat by trotting about until
the worst of the cold is past and it can go back to sleep again. Normally it is only
the warmth of spring that brings the dormouse and other winter sleepers out of
their hibernation. Their appetites are now huge and urgent, for during the winter,
they may have lost as much as half of their body weight.
With such methods as these, a great variety of animals nourish themselves on
the vegetable foods provided by the forests of the world. Up in the topmost
branches, rodents such as the grey squirrel (Sciurus carolinensis) scamper along
the twigs, collecting bark and shoots, acorns and catkins. Some species have
even developed furry membranes between their hind and fore legs so that they
can glide between the branches and thereby improve their foraging efficiency.
These are called flying squirrels, and there are over forty species of them, and
they are concentrated almost in the Asiatic region (e.g. the Red and White Flying
Squirrel Petaurista alborufus) with seven species occurring in Africa (e.g. Pel's
Flying Squirrel Anomalurus peli) two species occurring in North America (e.g.
Southern Flying Squirrel Glaucomys volans).
In the upper branches live the monkeys (order Primates). Many species will take
a wide variety of food - insects, eggs, nestlings and fruit; but others will only take
the leaves of particular trees and have complicated stomachs to deal with them.
Life in the forest canopy has lead to a high degree of co-ordination, particularly
with respect to the grasping manipulative hands and a quick intelligences,
features that ultimately lead to the evolution of the human being.
However, one of the first creatures to make an existence high up in the tropical
forest canopy of South America was the sloth, a distant relative of the ant-eaters
and a member of the order Edentata, and it adopted a solution almost exactly
opposite to those of the monkeys. There are two main kinds of sloth, the twotoed (belonging to the genus Bradypus and the three-toed (genus Choloepus).
Of these, the three-toed sloths are considerably more slothful. It hangs upside
down from a branch suspended by hook-like claws at the ends of its long bony
arms. It feeds on only one kind of leaf, Cecropia, which happily for the sloth
grows in quantity and is easily found. No predators attack the sloth - few indeed
can even reach it - and nothing competes with it for Cecropia leaves. Without
fear of predation and plentiful food sources without competition from other
predators allows them to spend up to eighteen hours each day asleep. A green
algae grows on its coarse hair and communities of a parasitic moth live in the
depths of this coat producing caterpillars which graze on the alga-covered hair.
Its muscles are such that it is quite incapable of moving at any speed
whatsoever. It is virtually dumb and hearing poor. Even its sense of smell,
though better than ours, is less acute than that of most mammals.
These animals live a solitary life except when finding a mate to breed with? With
its poor senses, it is no easy matter to find one, however, since the sloth's
31
digestion also works as slowly as the rest of its bodily processes it defecates and
urinates once a week. To accomplish these processes it descends to the ground
and habitually uses the same spot. This is the one time in its life that it is
exposed to predators such as jaguars (Panthera onca), but also provides
opportunities to meet mates and to breed with them. Its dung and urine have
extremely pungent smells, and the sense of smell is the only one of the sloths
faculties that is not seriously blurred. So a sloth midden is the one place in the
forest that another sloth could easily find a mate.
The forest floor is not rich in vegetation. In some areas the shade is so dense
that there is nothing but a deep layer of decomposing leaves with the occasional
fungi. Where the canopy is thinner, there may be small bushes, a few herbs on
the ground and some spindly saplings. In Africa and Asia such plants provide
food for small antelope e.g. duiker (Cephalophus species). These animals are
extremely shy and difficult to observe as the forage for leaf material in the
dappled light. These animals are very similar to the primitive ruminants that were
among the first-leaf eating specialists that evolved some fifty million years ago.
In South American forests, the major herbivores are not hoofed animals but
rodents such as the paca (Cuniculus paca) and agouti (e.g. Dasyprocta leporina).
They have body forms, shy habits and a solitary life style. Browsing on the taller
shrubs and saplings requires greater stature and most tropical forests have some
form of large herbivore, which are secretative, generally uncommon and difficult
to observe. In Malaya and South America, there are nocturnal tapirs Tapirus
indicus and Tapirus terrestris), which belongs to the order Perissodactyla (oddtoed ungulates). In parts of Southeast Asia, another odd-toed ungulate occurs,
the Sumatran Rhinoceros (Didermocerus sumatrensis), with a slightly hairy hide.
In the Central African basin forests occurs the even-toed ungulate called the
Okapi (Okapia johnstoni; order Artiodactyla), and is a short-necked primitive
cousin of the Giraffe (Giraffa camelopardalis). It is an amazing fact that so large
and conspicuously marked a creature as the Okapi was unknown to science until
1901.
All these ground-living forest dwellers, large and small, are solitary since the
forest floor seldom produces sufficient leaves to sustain a large group in one area
for any length of time. Further if several animals are to maintain a relationship
they require some kind of communication. It is not possible to see far into the
forest and signalling by sound would attract the attention of potential predators.
These animals also maintain territories which they mark with dung or secretions
of a gland close to the eye and rely on concealment to protect themselves from
predation.
The hunters that seek them such prey are also solitary. Examples are the jaguar
preying on the tapir, and the leopard (Panthera pardus) preying on the duiker. A
wandering Brown Bear (Ursus arctos) will eat most things including a small
antelope. The smaller hunters such as genets (Genetta species), jungle cats
(e.g. Felis chaus), civets (e.g. Viverra species) and weasels (e.g. Mustela) prey
on small rodents as well as birds and reptiles.
Of all the carnivore hunters (order Carnivora), the cats (Family Felidae) are the
most specialized for meat-eating. Their claws are kept sharp by being retracted
into sheaths. When they attack, they hook their victim with them and then deliver
a piercing bite to the neck that severs the spinal cord. The long dagger-like tooth
on either side of the mouth, just behind the front teeth, typical of a meat-eater, is
used to slash open its prey. The jagged teeth further back in the jaw shear
bones. They are all the tools of butchery. None of the dogs or cats can really
chew. Most simply bolt their food down in chunks. Flesh is far easier to digest
than leaves and twigs and the hunters stomach is not so elaborate.
The relationships between predator and prey are very different on the open
grassy plains. Grass may look to be a simple almost primitive plant, little more
than leaves with roots. In fact, it is a highly advanced one, bearing tiny,
unobtrusive flowers which rely not on insects to distribute their pollen but on wind.
It produces horizontal stems running close to the surface or just below it. When
fire sweeps across the plains, consuming the old dry leaves, the stems and the
root stocks are unharmed and resprouts almost immediately. Grass leaves grow,
not from the tip as do those of bushes and trees, but from the base. This is of
benefit to the grazing animals for it means that even though the leaves have been
cropped, they will continue to grow and new leaves will become available to be
eaten.
The grass itself benefits from the presence of the grazing herds for they trample
and eat the seedlings of woody plants that might take root on the plain and
eventually displace the grasslands. It seems likely therefore that the spread of
the grassland and the evolution of grazing animals proceeded together, and that
the grassland maintains the herbivores and the herbivores maintain the grassland
by preventing woody species from colonizing it.
On an open plain such as an African grassland a single herbivore is an easy
target for a predator unless you are very large such as an Elephant (Loxodonta
africanus), Black and White Rhinoceroses (Diceros rhinoceros and Ceratotherium
simum) and Buffalo (Syncerus caffer). The dense vegetation of a forest makes it
easier for a herbivore to move around without being seen, and a smaller size
would tend to be favoured. On the plains a small size is not an advantage, in fact
a large size may reduce the risk of predation. Great bulk with a tough skin may
be deterrents to predation. However, for smaller animals, the dangers of
predation are high.
Some sought safety in burrows, and in grassland which are free of roots of large
trees, it is easy to construct extended tunnel systems without hinderance. One of
the most specialized of burrowers is the naked mole-rat (Heterocephalus glaber;
order Rodentia) of East Africa. It eats the roots of grasses together with bulbs and
tubers. Mole-rats live in families and excavate elaborate underground quarters
with special dormitories, nurseries, larders and lavatories. Life spent entirely
underground in the warm, dry earth of the African plains has changed them
dramatically. They have lost use of their eyes and are now hairless. These
naked sausage-shaped animals have huge incisor teeth that project clear of the
head in a bony semicircle in front of the face. They are used for both feeding and
as burrowing tools. Gnawing one's way through earth could clearly be a
distasteful business, but the mole-rat avoids mouthfuls of soil by pressing back its
lips behind the protruding teeth and the mouth is kept tightly shut while the teeth
excavate through the soil.
When they dig, they work in teams. The one at the front gnaws away dislodging
the soil behind it where the second member of the team hurls the soil back
between its legs onto the third member of the team. The soil is passed in this
way until the last member of the line receives it and throws it vigorously out of the
entrance of the tunnel. A patch of ground colonized by mole-rats is riddled with
small heaps of earth which demarcate the entrance to the burrows.
Few, if any, predators are able to make a meal of a mole-rat. It can dig faster
than any predator and it has no need to come to the surface. But those
burrowers than eat not grass blades must emerge from their holes and then
become targets for predation. The plains of North America are colonized by
rodents called prairie dogs or Marmots (e.g. Cynomys ludovicianus). They not
only graze above ground but do so during the day when coyotes, bobcats, ferrets
32
and hawks are about, all predators of the prairie dog. These animals have
developed defences which depend upon a highly organized social system. They
live in huge concentrations called towns which may contain up to a thousand
animals. Each town is divided up into a number of communities called coteries of
about thirty individuals, all of whom know one another well. Many have
interconnecting burrows. The coteries always have some members on sentry
duty, sitting upright on the mound of excavated earth beside the burrow entrance
where they can get the best view of what is going on. If a potential predator is
spotted the sentry lets out a series of whistling barks. Different kinds of predators
elicit different calls so that the other prairie dogs know where the danger comes
from. The call is repeated by others nearby and so spreads through the town,
putting every-one on guard. The inhabitants do not immediately take to flight but
take up strategic positions close to their holes. From there, standing on their hind
legs, they stare at the intruder, watching its every move. So as a coyote trots
through the town, the alarm spreads from coterie to coterie and the intruder is
met with fixed glares from the citizens who let it come tantalisingly close before
they duck into their burrows.
The social life of the prairie dog is not limited to defence. The adults, sitting
outside their burrows, proclaim their ownership by giving yet another kind of
whistle, accompanied by a small leap into the air. During the breeding season,
the coterie members keep very much to themselves and defend their boundaries
against any intruder. The prairie dogs tend the vegetation within the town with
great care. Their grazing is so intense that many of the plants they favour
become eaten out. The animals then move to a different part of their territory and
let the old pasture recover. They also cultivate selectively. Sage, although one
of the commoner plants is not a favoured food item. If a seedling of one takes
root or if there is one growing in a newly colonized patch of territory, they do not
simply ignore it but deliberately cut it down and so allow more room for the plants
they prefer.
On the pampas of Argentina, the role of the prairie dog is taken over by another
rodent, the viscacha (Viscacha maximus). It, too, lives in dense communities but
it grazes only at dusk and at dawn. Like many creatures that are active in the
twilight, they have prominent recognition marks, broad horizontal black and white
stripes across the face. They build cairns over their burrows. If they find any
sizeable stone during their excavations they drag it up to the surface and dump it
in the pile on the top.
The viscacha is another descendant of the first mass placental migration from
North America which invaded grasslands and forests of South America. This
invasion included some strange herbivores, most of which are now extinct.
Which the separation of South from North America some of herbivores evolved to
great sizes and included an animal that resembled a camel (Alticamelus) but
stood over 3 metres tall. Another called the Ground Sloth, Megatherium a
relation of the sloth, was 7 metres tall and lumbered across the ground, feeding
on bushes and trees.
When the Panama bridge was re-established for a second time, creatures from
the north again invaded South America many of these animals such as the giant
camel and the sloth died out. In Patagonia, at the southernmost tip of the
continent, the remains of a ground sloth were found. The cold temperatures had
virtually freeze-dried the large bones and shaggy coated hide of this animal.
Grass stems in the dung left by the animal appeared to have clean edges as if
they had been cut by artificial means. This evidence has given rise to the
hypothesis that the prehistoric Indians kept this animals in caves and feed them
bales of grass.
At the time that the sloths and other members of the Edentates (e.g. Glyptodon
were evolving in the south, on the other side of the Panama strait in North
America, another different group of grass-eaters were developing on the prairies.
Their ancestors were forest-living creatures, not unlike tapirs but far smaller.
Their molar teeth were rounded and suited to forest browsing. On the plains, in
order to escape their predators, they began to run faster. The earliest forms
(Hyracotherium) run on four toes on their frontlimbs and three toes on their
hindlimbs. The longer the limbs, the better they serve as levers and, properly
muscled, the faster they can propel their owners. As time passed these grazers
lengthened their legs by rising off the ground onto their toes. The side toes
started to dwindle and the animal, an early horse the size of a dog, was running
on a single elongated middle toe (Mesohippus). The reduction of the side toes
continued (Merychippus). The ankle bones thus became placed halfway up its
legs, the side toes were reduced to internal vestiges called the splint bones, and
the nail thickened to form the protective shock-absorbent hooves (Pliohippus).
These changes in the limbs were accompanied by others changes. The grasses
of the plains were becoming tougher to chew and contained within their leaves
tiny sharp crystals of silica which wore teeth badly. So the proto-horses changed
their rounded molars into bigger and bigger grinders with hard ridges of dentine in
them. One of the problems of the grazing life is that an animal, with its head on
the ground for such long periods, cannot keep a good lookout for predators. The
higher the eyes are placed on the head the better the visibility. This requirement,
together with the necessity to provide room for the enlarged molars, resulted in a
considerable elongation of the skull. So the early horses evolved into the forms
we know today (e.g. Equus). They spread across the plains of America and
eventually, at a time when the Bering Strait was dry and connected North
America with Asia, they reached Europe. From there they spread south and
colonised the plains of Africa. Later, they died out in North America and only
reappeared when they were introduced by European man. In Europe and Africa,
they flourished as horses (Equus), donkeys (Equus asinus) and zebras (Equus
burchelli).
The zebras share the African plains with other running grazers which, during the
same period, had been evolving along lines of their own. They were the
descendants of the forest dwelling antelopes, like the duikers of today. They had
already elongated their legs for running within the forest though in a slightly
different way from that of the horses, retaining not one toe on the ground but two.
Now, out on the plains, their legs grew even longer and they became the clovenhoofed grazers - antelope, gazelle and deer. Today they flourish in such
numbers that they constitute some of the most spectacular assemblages of
wildlife to be seen anywhere in the world.
On the edges of the plains in the open bush, where a small amount of vegetation
cover still occurs, antelope such as the dik-dik (Madoqua) live alone or in pairs
within territories that they mark and defend very like their forest-dwelling relations
do. Farther out in the open, where concealment is no longer possible, the
antelope seek safety in numbers, gathering together in large herds. They lift their
heads regularly from grazing to look around, and with so many sharp eyes and
sensitive nostrils on the alert, it is more difficult for a hunter to take the herd by
surprise. If an attack does eventually come, then the fleeing herd makes it
difficult for a predator to target onto an individual prey item.
Keeping together in such numbers makes great demands on the pasture and the
herds have migrate regularly over great areas. Wildebeest (Connochaetes
taurinus) seem able to detect a shower of rain falling as far away as 50
kilometres and will move off to find it and crop the newly sprouting grass. But this
33
nomadic existence complicates the social arrangements for breeding that in the
forest, based on a single pair, had been so simple. For some - the Impala
(Aepyceros melampus), Springbok (Antidorcas marsupialis) and Kudus
(Tragelaphus strepsiceros):- territory remains nonetheless the basis of their
arrangements. Males and females form separate herds. A few dominant bucks
leave the bachelor herd to establish individual territories for themselves. Each
marks the boundary of its land, defends it against other males and tries to attract
females into it and mate with them. This however is a demanding business and
most of the bucks who undertake it are exhausted and badly out of condition after
three months or so. Eventually, they are then forced to yield to stronger, more
rested rivals and they go back to join the bachelor herd.
The eland (Taurotragus oryx), the largest of the antelopes, and the plains zebra
are among the few that have finally broken the bond with territoriality altogether.
They form herds in which both sexes are always present and the males settle
their problems over females by battling between themselves wherever the herd
happens to be.
In order to catch these grazers, predators need to improve their own running
abilities. Instead of elongating limbs and running on their toes, they have
increased their strides by making their spines extremely flexible. At full stretch,
travelling at high speed, their hind and front legs overlap one another beneath the
body. The cheetah (Acinonyx jubatus) has a thin elongated body and is said to
be the fastest runnner on earth, capable of speeds, in excess of 110 kph. But
this method is very energy-consuming and great muscular strength is needed to
keep the spine springing back and forth and the cheetah cannot maintain such
speeds for more than a minute or so. Consequently this method of locomotion is
fine for an attacking animal but would not be suitable for a fleeing animal.
Lions (Panthera leo) are nowhere near as fast as the cheetah. Their top speed is
about 80 kph. A wildebeest can do about the same and keep it up for much
longer. So lions generally hunt as a team. They set off in line abreast creeping
close to the ground and as they approach a group of prey - the lions at the ends
of the line move a little quicker so that they encircle the herd. Finally, these break
cover, driving the prey towards the lions in the centre of the line. Such tactics
often result in several of the team making kills.
Hyenas (Crocuta crocuta) are even slower runners than lions and in
consequence their hunting methods have to be even more subtle and dependent
on teamwork. The females have separate dens where they rear their pups, but
the pack as a whole works together and holds and defends a territory. They have
a rich vocabulary of sound and gestures with which they communicate among
themselves. They growl and whoop, grunt, yelp and whine as a means of
communicating amongst themselves. They also use their tails as a means of
communication. Tails are normally carried pointing down. An erect tail indicates
aggression; pointed forward over the back, social excitement; held between the
legs tight under the belly, fear. By hunting in well-co-ordinated teams, they have
become so successful that in parts of the African plains, they make the majority
of kills and the lions merely use their bigger size to bully their way on to a
carcass.
Hyenas usually hunt at night. Sometimes they set off in small groups of two or
three and then a wildebeest is likely to be their intended prey. They test the
herds by charging them and then slowing down to watch the fleeing animals
closely, as if trying to detect any weakness among individuals. In the end, they
appear to select one animal and begin to chase it doggedly, cantering after it,
snapping at its heels until it is finally goaded into turning and facing its
persecutors. When it does that, it is doomed. While it faces one hyena, the
others lunge at its belly, sinking their teeth into the unfortunate animal. The
wildebeest is soon crippled, and disembowelled.
Zebra are a more difficult prey. To hunt them, the hyenas unite to form a large
team. Through behavioural gestures they reaffirm bonds between one another.
When they are in groups like this, they will trot straight past herds of wildebeest,
paying no attention to them. At last they sight a small group of zebra, led by a
dominant stallion. This usually raises the alarm with a braying danger call and
the herd gallops away the dominant stallion taking the rear, placing himself
between the pursuing hyenas and his mares and foals. The hyenas follow in a
crescent behind. The stallion will swerve and attack the pack with his powerful
kicks and bites and even chase the leading hyena, who may be forced to drop
back and allow others to make the running. But eventually one of the pack will
get past the stallion and begin to snap at a mare or a foal. As the chase
relentlessly continues, one gets a tooth-hold on a leg or the belly or the genitals
and the animal is dragged down. While the rest of the herd canters to safety, the
hyenas leap on the fallen zebra, ripping it to pieces.
ASSIGNMENTS
IN YOUR OWN WORDS WRITE A ONE TO TWO PAGE ESSAY ON THE
FOLLOWING TOPICS
Discuss why herbivore occurring in the forest environment live alone or in
pairs, whereas those herbivores occurring in an open grassland
environment live in gregarious groups. Give examples of herbivores living
in both environments.
Discuss why a herbivorous diet imposes certain problems for digestion
and how some animals have overcome such problems. Also explain why
some herbivores have become such large animals.
Discuss the identifying characteristics of each order of placental mammal.
Your answer should include a list of animals representing each order.
Ungulates have increased their running speed by increasing the length of
their limbs, whereas many carnivore hunters have increased their running
speed by increasing the flexibility of their backs. Discuss these adaptions,
giving examples of animals that have evolved them.
Discuss how underground social mammals organize their communities
and protect themselves from predation.
A LIFE IN THE TREES
In order to live in trees, two abilities are extremely useful, a talent for judging
distances, and a capacity for holding on to branches which requires a pair of
forward-facing eyes that can both focus on the same object and hands with
grasping fingers. Only members of the order primates (monkeys, apes and
humans) have these characteristics.
There is no doubt that the early insect-eating shrew-like mammals which were
34
the ancestors of such diverse creatures as bats, whales and ant-eaters, also
gave rise to the primates. Indeed, an animal like the Tree Shrew (Tupaia glis)
could have been an ancestor to the primates. The Tupaia has two characteristics
which it shares with the primates; its eye-sockets are completely encircled by
bone and its tongue is underlain by a cartilaginous sub-tongue; other insectivores
do not possess these characteristics. But the Tree Shrew does not have the
other primate hallmarks, namely hands with thumbs that are opposable to the
fingers which is required for a true grasping hand, and eyes that face forward with
overlapping fields of view so that distances can be judged.
Another group of animals with unmistakably monkey-like characteristics are
called the prosimians or 'pre-monkeys'. Typical of them is the Ring-tailed lemur
(Lemur catta) of Madagascar. These animals spend a lot of time on the ground
in troops. Scent plays a very important part in their lives. Their nose is nowhere
near as well developed as that of a Tree Shrew, but it is still very fox-like in
proportion and it too has a moist muzzle with bare skin around the nostrils.
These animals also possess three kinds of scent glands. One pair on the inside
of the wrist which opens through spurs; another high up on the chest, close to the
armpits and a third around the genitals. With these, the males and to a lesser
extent the females produce signals. Such signals are often left on particular
plants. Typically a lemur will come upon a sapling, smell it carefully, checking
whether it has been visited before, then put its hands on the ground, hoist its rear
as high as it can and rub its genitals several times on the bark. Often, within a
minute or so, another individual will come and repeat the performance. Males
also grasp a sapling with both hands swing their shoulders so that they twist from
side to side. Their wrist spurs rub against the bark, making deep scratches that
are impregnated with their musk.
branch and then reinforce the effect by dribbling urine over the bark, wriggling
their hips and slowly drawing themselves up the branch as they do so.
The Indris (Indri indri) is the most arboreal of all the lemurs and hardly ever
comes down to the ground. It is the biggest of all living lemurs with a head and
body nearly a metre long, and its legs are even longer in proportion than those of
a Sifaka, the big toes are widely separated from the rest and about twice the
length, so that each foot resembles a huge calliper with which the animal can
grasp thick trunks. Indris also use scent in marking the trees, though to a much
lesser extent than the lemurs. Instead territories are established using their
voices. Every morning and evening, a family fills its patch of forest with an
unearthly wailing chorus.
Although the Ring-tail, Sifaka, Indris and several other Madagascan lemurs are
active during the day, their eyes have a reflecting layer behind the retina which
increases the ability to see in very dim light. This is a characteristic of animals
that move at night and strong evidence that these lemurs were nocturnal until
quite recently. Many other lemurs and their relatives are, however, nocturnal.
The Grey Gentle lemur (Hapalemur griseus), which is about the size of a rabbit,
lives in holes in trees and only comes out at night. The smallest of the group is
the mouse-lemur, with a snub nose and large eyes. The Indris has a closely
related nocturnal equivalent the Wooly Indris (Avahi laniger). Oddest and most
specialised is the Aye-aye (Daubentonia madagascariensis), an animal the size
of an otter, with a black shaggy fur, a bushy tail and large membranous ears.
One finger on each hand is enormously elongated and seemingly withered, so
that it has become a bony articulated probe. With this the Aye-aye extracts
beetle larvae, its main food, from their holes in rotting wood.
The male Ring-tail uses scent not only as a signature but as a means of offence.
When he prepares for battle with a rival, he vigorously folds his arms several
times and rubs his wrists against his armpit glands. Then he brings his tail
forward between his hind legs and in front of his chest and draws in several times
between his wrist spurs so that it is loaded with scent. Thus armed, rivals face
each other on all fours, lift their haunches high and thrash their splendid tails over
their backs with the fur bristling, so that the smell is fanned forwards. Troops
meeting on the frontier between territories may do battle in this way for as long as
an hour, hopping and skipping, squealing and yawning, and excitedly marking
saplings with their wrist spurs.
Fifty million years ago, there were lemurs and other prosimians not only in
Madagascar, but in Europe and North America. Around thirty million years ago,
Madagascar became separated from the continent of Africa, where more
advanced primates evolved. These primates never reached Madagascar, and
lemurs survive today. Elsewhere, lemurs died out, being unable to compete with
the monkeys. Since monkeys with the single exception of the South American
Douroucouli (Aotus trivirgatus), are diurnal, other prosimians which are nocturnal
have been able to co-exist with the monkeys.
The Ring-tail also spends a lot of its time in trees. Here, its behaviour is more
monkey-like. The eyes on the front of its head give it a binocular view and their
hands with their mobile fingers and opposable thumbs grasp branches. The
fingers ending in short nails rather than claws are sufficiently dexterous to enable
the animal to pluck fruit and leaves from the tips of branches. Although this lemur
is quite big it can leap safely from tree to tree.
In Africa, the prosimian group is represented by the Bush Babies (Galago and
Euoticus species), the Potto (Perodicticus potto) and the Angwantibo (Arctocebus
calabarensis). In Asia, the prosimians are represented by the Loris (Loris and
Nycticebus species) and the tarsier (e.g. Tarsius syrichta). The Loris have large
eyes and sign post their trees with scent and use it for route-finding in the dark.
They use urine to signpost, but because they live in the tops of trees, they urinate
on their hands and feet, rub them together and then on to the topmost branches
in their territory.
The ability to grip is put to good use by infant lemurs. which cling to their
mother's fur and thereby travels with her wherever she goes and is provided with
parental protection at all times. As a consequence of this intensive parental
investment Ring-tails usually have only one baby at a time.
In Madagascar there are 21 species of lemur and its relatives, with most of them
spending much of time in the trees. The Sifaka (Propithecus verreauxi), a little
larger than the Ring-tail, has become a specialist jumper. Its legs are
considerably longer that its arms and enables it to leap four or five metres from
one tree to another. However, when these animals come to the ground they
cannot use all four feet but have to hop using two feet. Sifakas have scent
glands beneath their chins; they mark their territory by rubbing them on an upright
The Tarsier, is the size and shape of a tall Bush Baby. It has a long near-naked
tail tufted at the end, greatly elongated leaping legs and long fingered grasping
hands and gigantic glaring eyes (150 times bigger in proportion to the rest of its
body, than our own) which face directly forward. If this animal needs to see
something to one side, it has to turn its whole head. Together with these
spectacular eyes, the tarsier has paper-thin ears, like those of a bat, that can be
twisted and crinkled so as to focus on a particular sound. With these two highly
developed sensory organs it hunts at night for insects, small reptiles and even
fledgling birds. It also marks territories with urine although its sense of smell is
not likely to be good. A look at its nose not only confirms this but reveals that the
animal is quite distinct from all other prosimians. For one thing, the eyes are so
35
huge that there is little room in the font of the skull for the nose itself and the
internal nasal passages are very much reduced in caparison with, say, a Bush
Baby's. The nostrils are not comma-shaped nor are they surrounded by bare
moist skin, as are the noses of lemurs and other prosimians. In this it resembles
monkeys and apes and it is tempting therefore to see the tarsier as representing
an ancestral form from which all the higher primates are descended. Indeed, this
was once held to be the case. Today it is argued that this little creature is so
specialised a leaper and nocturnal hunter that it could hardly have given rise
directly to monkeys. Nonetheless, it is seen as a close relative of those early
primates which, fifty million years ago, spread widely through the world displacing
most of the prosimians and ultimately populating both the Old and New worlds
with monkeys.
Monkeys differ significantly from all the prosimians, except the tarsier in that their
world is dominated not by smell but by sight. Clearly it is important for creatures
of any size living in trees and, on occasion, jumping between them, to be able to
see where they are going. So daylight suits them better than darkness and all
monkeys, except for the South American Douroucouli, (Aotus trivirgatus) are
active at that time. Their eyesight is better than that of the prosimians. Not only
do they see in depth, they have greatly improved colour perception. With
accuracy of vision they can judge the ripeness of distant fruit and the freshness of
leaves. They can detect the presence in the trees of other creatures which, in a
monochrome world, might be invisible. And they can use colour in their
communications between one another; monkeys because their colour-vision is
so good, have themselves become the most highly coloured of all mammals.
In Africa there lives de Brazza's Guenon (Cercopithecus neglectus) which has a
white beard, blue spectacles, orange forehead and black cap, the Mandrill
(Mandrillus sphinx) with a scarlet and blue face, and the vervet monkey, the
males of which have startling blue genitals; in China, the Golden Snub-nosed
Monkey (Pygathrix roxellana) with a metallic golden coat and an aquamarine
face; in the Amazon forests, the Red Uakari with a scarlet naked face (Cacajao
calvus). With these colourful displays they advertise and threaten and proclaim
both their species identity their sex.
They also use sound in a similarly extravagant way, for up in the trees they are
beyond the reach of most predators. Howler monkeys (e.g. Alouatta seniculus) in
South America sit morning and evening, and sing in chorus. Their larynx is
extraordinarily large and their throats swell into resonating balloons. The
resulting chorus can be heard for several kilometres.
The monkeys that reached South America and became isolated there when the
isthmus of Panama sank beneath the sea, have developed very much along their
own lines. That they all are derived from one common stock is deduced from the
number of anatomical features they share e.g. all have fat noses with widely
spaced nostrils opening to the side whereas monkeys elsewhere have thin noses
with forward or downward pointing nostrils.
One South American group, the marmosets (e.g. Callithrix penicillata) and
tamarins (e.g. Leontocebus leucopus), still use scent a great deal in
communication even though they are active during the day. The males gnaw the
bark of a branch and then soak it with urine. But they also have extremely
elaborate adornments - moustaches, ear-tufts and wig-like crests - which they
display during their social encounters; and they threaten one another with highpitched twittering calls. Their manner of rearing their young is less specialized
than for the old world apes.
Marmosets are the smallest of all true monkeys and seem to have moved from
the basic monkey life style to that of a squirrel; eating nuts, catching insects and
licking sap from bark gnawed by their special forward-pointing incisors. The
pygmy marmoset (Cebuella pygmaea) has a body length of 100 mm and runs
along branches, keeping a foothold on the bark with claws. Use of claws is a
recent reversion, for the embryonic marmoset begins to develop monkey nails on
its fingers and only later do they develop into claws.
Generally the primates have tended to evolve larger sizes, with the marmosets
being an exception. Greater weight, however, places greater demands on the
grasping hands and the South American monkeys have developed a unique way
of supplementing them. Their tails have turned into a fifth grasping limb. It is
equipped with special muscles so that it can curl and twine, and at the end its
inner surface has lost its hair and developed a ridged skin like that on its fingers.
So powerful is it that a spider monkey (e.g. Saimiri sciureus) can hang by its tail
while foraging for fruit with both hands.
Old world monkeys have not developed the prehensile tails, however, they do
extend them horizontally when they run along branches, as a balancing aid. The
failure of the African monkeys to use a prehensile tail meant that if they did grow
larger, they would find an arboreal life increasingly awkward and consequently
spend more time on the ground. This is clearly evident by the lack of ground
living New World monkeys, whereas in the Old World there are many. The
primate's tail seems of less value for terrestrial life and there has been a
tendancy to reduce and even lose the tail. The mandrill (Mandrillus sphinx) and
drill (Mandrillus leucophaeus), have tails that are reduced to a tiny stump.
The Macaque monkey (Macaca) is one of the most adaptable of primates
capable of surviving in extreme conditions. There are about six different species
and subspecies distributed from the Atlantic Ocean to the Pacific. One group
(Macaca sylvana) lives on Gibraltar, the only non-human primate living naturally
in Europe.
The Rhesus Macaque (Macaca mulatta) is one of the commonest monkeys in
India, often living close to urban areas. In Indonesia the crab-eating Macaque
(Macaca fascicularis) has become a competent swimmer and dives in the
mangrove swamps for crabs and other crustaceans. In Malaysia, the pig-tailed
macaque (Macaca nemestrina) has been trained to harvest coconuts. The
Japanese Macaque (Macaca fuscata) is the most northerly living monkeys and
has a shaggy coat to protect it from the cold winters.
Macaques spend most of their time on the ground. Their hands and eyes,
inherited from an arboreal existence, together with adaptive learning abilities
have permitted a successful transition to a terrestrial existence.
The adaptability of the Japanese Macaques is illustrated by their use of hot
volcanic springs to provide relief from the cold winters, by washing dirt off food
items such as sweet potatoes and even separating rice grains from dirt by
throwing them into water and scooping off the floating grain. This ability to resolve
problems is usually mastered by one individual and the behaviour patterns
associated with these are spread to all members of the troop.
This ability and readiness to learn from your companions results in the
community having shared skills and knowledge, shared ways of doing things - in
short, a culture. The word is normally used only in the context of human
societies, but the beginnings of a culture can be seen in the way the Japaneese
Macaques communicate amongst themselves and organize their communities.
However, one of the most significant behavioural patterns that occurred in the
36
evolving primates was bi-pedalism. Moving on to two legs, would free the upper
limbs, and paticularly the hands to explore objects which eventually lead to the
use of tools by ape-men. To trace the origins of these animals, we have to go
back some thirty million years.
At that time, one group of lower primates were increasing in size. This brought a
change in the way they moved through the trees. Instead of balancing on the top
of a branch and running along it, they began to swing along beneath it. Swinging
successfully involves physical changes. Arms lengthened, a tail that was used
for balancing, disappeared; and the musculature and skeleton of the body
changed so that the backbone and abdomen was supported in vertical rather
than a horizontal planes. Those changes produced the members of the Family
Hominidae and include Gibbons (Hylobates); and the Great Apes which includes
the Orang-Utan (Pongo pygmaeus) from Asia, the Gorilla (Gorilla gorilla), the
Chimpanzee (Pan troglodytes), the Bonobo (Pan paniscus) from Africa and
Humans (Homo sapiens).
The great red-haired Orang Utan of Borneo and Sumatra is the heaviest treedweller in existence. A male may stand over one and a half metres tall, have
arms with a spread of two and a half metres and weigh a massive 200 kilograms.
The digits on all four limbs have powerful grips, so that the animal is best
described as being four-handed and the ligaments of the hip joints are so long
and loose that an orang utan, particularly when it is young, can stick its legs out
at astonishing angles. Plainly, they are excellently adapted for the arboreal life.
At the same time, their size does seem to be something of a handicap to them.
Branches break under their weight. Often they are unable to get fruit they relish
because it is hanging far out on a branch that would never support them. Moving
from tree to tree can also cause problems. There is little difficulty if substantial
branches from each tree overlap, but that is not invariably the case. The Orang
Utan deals with that problem either by reaching out until he can clasp a stout
branch, or by rocking the tree that he is in until it bends over far enough for him to
climb across.
Ingenious though these techniques may be they can hardly be reckoned easy or
swift. Indeed, sometimes an old male gets so large that he apparently finds the
whole process too exhausting and whenever he wants to travel any distance, he
comes down and lumbers across the forest floor. There is also evidence that the
arboreal way of life is fraught with danger for the Orang Utan. A study of adult
skeletons showed, rather pathetically, that 34 percent had, at one time or
another, broken their bones.
The males, as they grow old, develop immense pouches which hang down from
the throat like gigantic double chins - not simply fat, but true pouches that can be
inhaled with air. They extend far down the chest across into the armpits and right
over the back to the shoulder blades. Although they may have been used by
ancestral Orang Utans as resonators to amplify their voice like howler monkeys,
the modern Orang Utan does not sing. His most impressive sound is his `long
call', a lengthy sequence of sighs and groans which continues for two or three
minutes. To produce it, he partly inflates his throat pouch and the call ends with
a number of short bubbling sighs as the pouch deflates. But he makes this call
infrequently, and most of his vocalisations consist of grunts, squeaks, hoots,
heavy sighs and a sucking
noise made through pursed lips. It is a varied repertoire but a quiet one that can
only be heard fairly close by. The animal more often than not is alone and during
these monologues he gives the impression of a recluse, mumbling and grumbling
to himself in an absent-minded way. Males take up this solitary life as soon as
they leave their mothers, travelling and eating by themselves and only seeking
company when they briefly come together with a female to mate.
Female Orang Utans are about half the size of their mates but they too are
solitary animals and travel through the forest accompanied only by their young.
This preference for solitude may well be connected with their size. Orang Utans
are fruit-eaters, and being so big have to find considerable quantities of it every
day to sustain themselves. Fruiting trees, however, are uncommon and widely
scattered through the forest, at widely varying intervals. Some only bear fruit
once every twenty-five years. Others do so almost continuously for about a
century but only on one branch at a time. Yet others have no regular pattern and
are triggered irregularly by a particular change in the weather such as the sudden
drop in temperature that proceeds a heavy thunder storm. Even when they do
produce fruit, it may only be on the tree for a week or so before it becomes overripe, falls or is exploited. So the Orang Utans have to make long journeys,
continually searching, and may well find it more profitable to keep their
discoveries to themselves.
The gibbons, also fruit-eaters, have followed a very different line of development.
lncreasing size may have been the stimulus that made apes start to swing
beneath branches but the ancestral gibbons subsequently exploited the new style
of locomotion to the full by becoming smaller again. In the end they developed
into even more accomplished acrobats than any balancing, branch running
monkey. A gibbon in motion in the tree tops is one of the most glorious sights
the tropical forest has to offer. With a supple grace that is breath-taking, they hurl
themselves nine or ten metres across space, grabbing isolated branches and
swinging themselves off again in another dazzling swoop through the air. The
arms that enable them to be acrobats in the air are as long as their legs and torso
combined, and if they do come to the ground, they have to be held above its
head out of the way. Its versatile grasping primate hands have also become
specialised at the cost of some of their manipulative abilities. Swinging at gibbon
speed requires that the hands be used as hooks that can be latched swiftly on to
a branch and then detached almost instantaneously. Thumbs get in the way, so
they have moved down towards the wrist and become much reduced.
Because Gibbons are small, there is usually enough fruit on a tree to satiate
several of them, so it is practical for them to travel together and they live in tightly
knit families. A pair is accompanied by up to four of their offspring of varying
ages. Every morning, the family sings in chorus. The male starts with one or two
isolated and tentative hoots, others join in, the group launches into a ecstatic
song and finally the female takes over with a rising peal that gets faster and
faster and higher and higher until it becomes a trill of tonal purity that no human
soprano could ever challenge.
The parallel with the indri of Madagascar is an obvious one. Because of different
ancestral histories, one creature uses its fore limbs as its major propellant, the
other its hind. Otherwise, the tropical rain forest in diffent parts of the world has
produced creatures that are remarkably similar- families of singing, vegetarian
gymnasts.
The African apes, in great contrast to their Asian relations, are much more
terrestrial in their habits. Gorillas live in central Africa, one form in the forests of
the Congo basin, another slightly larger one in the cool sodden moss-forests that
cover the flanks of volcanoes on the borders of Rwanda and Zaire. Young gorillas
often climb trees, but they do so rather carefully without the confidence of Orang
Utans. This is not surprising since the gorilla foot cannot grasp in the way that an
Orang Utan's can, so the arms have to provide the main means of hauling up the
body. When gorillas descend, they do so feet-first, lowering themselves with their
arms, sometimes sliding down, braking by pressing the soles of their feet flat on
37
the trunk and showering moss, creepers and bark all around them.
The big adult males are so huge, weighing up to 275 kilograms, that only the
stoutest trees can support them. They climb rarely and do not have much reason
to, for although the shape of their teeth and the nature of their digestive system
suggest that they were once primarily fruit-eaters, like the Orang Utan, they now
subsist very largely on vegetation that can be reached without climbing, such as
nettles, bedstraw creeper and giant celery. Usually, they also sleep on the
ground, making a bed among the flattened vegetation on which they have fed.
They live in family groups of a dozen or so, each being led by a silver-backed
patriarch, who has several adult females attached to him. They sit quietly
grazing, ripping huge handfuls of stems from the ground with slow, irresistible
sweeps of their immense hands, lolling among the dense nettles and celery,
sometimes grooming one another. For the most part they sit in silence.
Occasionally they exchange quiet grunts or gurgles and if an individual wanders
away from the main group it makes a belching sound every now and then so that
the rest know where it is. While the adults doze, the young play and wrestle and
occasionally rear up on their hind legs to beat a quick tattoo on their chests,
rehearsing the gesture the adults use in display. The silver-back leads and
protects his group. If he is frightened or angered by intruders he may roar
defiance and even charge. A blow of his fist can smash a man's bones.
Pestered by a younger rival, who may be trying to lure away one of the females
of his group, he will even fight although this is a rare event.
Several groups of Gorillas have been studied for many years and, through the
patience and understanding of the scientists, have come to accept other people,
provided they are properly introduced and behave in a proper fashion.
Encountering a gorilla family and being allowed to sit with them is a moving
experience. They are in many ways so like us. Their sight and sense of hearing
and smell are closely similar to our own, so that they perceive the world in very
much the same way as we do. Like us, they live in largely permanent family
groups. Their life expectancy is about the same as ours and they move from
childhood to maturity and from maturity to senility at very similar ages. We even
share the same kind of gestural language and one that you must observe when
you are with them. A stare is rude or, put in a less anthropocentric way,
threatening - a challenge that invites reprisal. Keeping the head low and the eyes
down is a way of expressing submission and friendliness.
The placid disposition of the gorilla is connected with its diet and what it has to do
to get it. It lives entirely on vegetation of which there is an infinite supply growing
immediately to hand. As it is so big and powerful it has no real enemies and
there is no need for it to be particularly nimble in either body or mind.
The other African ape, the Chimpanzee, has a very different diet - and
temperament. Whereas a Gorilla may eat two dozen kinds of leaves and fruit,
the Chimpanzee samples two hundred or so and in addition, termites, ants,
honey, birds' eggs, birds and even small mammals like monkeys. To do this, it
has to be both agile and inquisitive. Several groups of chimpanzees, living in the
forests on the eastern shores of Lake Tanganyika, are being studied by a
Japanese scientific team and are now so accustomed to the presence of human
beings that you can sit among them for hours at a time. The size of their groups
varies, but they are very much bigger than those of the Gorilla and may contain
as many as fifty animals. Chimpanzees are adept climbers, sleeping and feeding
in trees, but they habitually travel and rest on the ground, even in thick forest.
There they move on all fours, their hands knuckle-down and their long stiffly-held
arms keeping their shoulders high. Even when the group is settled and at ease
on the ground, there is constant activity.
The sexual bonds between individuals are variable. Some females and some
males are monogamous. Other males will mate with many females, and the
females themselves, when their hind-quarters inflame into pink fleshy cushions
and they become sexually receptive, often court and mate with numerous males.
The tie between the young and their mothers is very close. Immediately after
birth, the infant clings to its mother's hair with its tiny fists, though at first it is not
strong enough to stay there for long without maternal support. It will remain close
to its mother, riding on her back like a jockey when the group travels, until it is
about five years old. This close dependence, made possible by the baby's
grasping hands, has a profound effect on Chimpanzee society, for as a result the
young learn a great deal from their mother and she is able to keep a close eye on
them as they grow up, supervising what they do, pulling them back from danger,
showing them from her own example how to behave.
There is a constant interplay between adults in a resting group. New arrivals will
greet one another, by offering the back of their outstretched hand to be sniffed
and touched with the lips. Elderly males, grey and balding with bright eyes and
wrinkled faces, often sit away from the main activity. They may be as much as
forty years old and they often give expression of short-tempered irascibility. They
are treated with considerable respect, the females rushing up to them smacking
their lips and effusively hooting. All of the group, young and old, spend hours
grooming one another, carefully sorting through the coarse black hair, scratching
the skin with a fingernail to remove a parasite or a scale. So anxious are they to
perform this service to one another and so pleasurable do they find it that
sometimes a chain of five or six individuals may form, each absorbingly grooming
another. It has become a truly social activity and a gesture of friendship.
One way or another, the group investigates everything around it. A log smelling
odd is carefully sniffed and probed with a finger. A leaf may be plucked,
scrutinized with the greatest care, and explored with the lower lip and gravely
handed to others for a similar examination and then thrown away. The group
may visit a termite hill. On the way there, an animal will break off a twig, trim it to
a particular size and strip it of its leaves. On arrival at the termite hill it pokes the
twig into one of the holes. When it pulls it out again, it is covered with soldier
termites than have gripped it with their jaws in an attempt to defend the nest
against the intrusion. The Chimpanzee draws the stem through its lips, taking off
the insects and eating them with relish. Although other animals use tools,
Chimpanzees like humans make tools.
The move made so long ago by the early primates from a ground-based scentdominated often nocturnal existence, to a life in the trees, led to the development
of grasping hands, long arms, stereoscopic colour vision an increased brain size.
With the aid of these talents, the monkeys and ape have made a great success
of their arboreal life. But those of them that subsequently returned to the ground,
whether it was because of an increase in body size or some other reason, found
that these very talents could be deployed in their new situation in a manner that
opened up fresh possibilities and led to further changes. The enlarged brain led
to an increase in learning and the beginnings of a group culture; the manipulative
hand and the coordinated eyes made possible the use and manufacture of tools.
The primates that are practising these skills today, however, are in essence
repeating a process that another branch of their family started soon after the
ancestral apes first appeared in Africa. It was this branch that eventually stood
upright and developed their talents to such a degree that they came to dominate
and exploit the world in a way that no animal had ever done before.
ASSIGNMENTS
38
IN YOUR OWN WORDS WRITE A ONE TO TWO PAGE ESSAY ON THE
FOLLOWING TOPICS
Describe the differences between New World and Old World monkeys.
Describe the adaptive radiation of lemurs that has occured in Madagascar.
Describe why the primitive Primates (Prosimians) are generally nocturnal
except for lemur species occurring in Madagascar.
Briefly describe all the members of the Family Hominidae.
Discuss how similar humans are evolutionarily, biochemically and
behaviourally to other members of the Family Hominidae.
THE COMPULSIVE COMMUNICATORS
Homo sapiens has suddenly become the most numerous of all large animals.
Ten thousand years ago, there were about ten million individuals in the world.
They were ingenious, communicative and resourceful, but they seemed, as a
species, to be subject to the same laws and restrictions which govern the
numbers of other animals. Then, about eight thousand years ago, their number
began to increase rapidly. Two thousand years ago it had risen to three hundred
million; and a thousand years ago, the species began to overrun the earth.
Today, there are over four thousand million. By the turn of the century, on
present trends, there will be over six thousand million. These extraordinary
creatures have spread to all corners of the earth in an unprecedented way. They
live on the ice of the Poles and in the tropical jungles of the equator. They have
climbed the highest mountains where oxygen is cripplingly scarce and dived
down with special breathing devices to walk on the bed of the sea. Some have
even left the planet altogether and visited the moon.
Humans evolved from ape-like creatures about the size of Chimpanzees. They
were descendants of a forest-living ape that had been widespread through not
only Africa but Europe and Asia about ten million years ago. The first fossils of
the plains-living ape were discovered in southern Africa and in was accordingly
named Australopithecus, Southern Ape, but now several more kinds have been
discovered in other parts of Africa.
They were not abundant and their fossilised bones are rare, but enough have
been found to give a fairly clear idea of what they were like in life. Their hands
and feet resembled those of their tree-climbing ancestors and were very good at
grasping things with nails on the digits, not claws. The limbs were not particularly
well suited to running. Their skulls also show clear signs of their forest-dwelling
past. The eyes, as can be judged from the sockets, were well developed by
contrast their sense of smell would have been relatively poor since the nasal
clefts were short. The teeth are small and rounded and not well suited to grinding
grass or pulping fibrous twigs nor did they have shearing blades, like those of a
carnivore. It is probable that they excavated for roots and gathered berries, nuts
and fruit, and despite the inadequacies of their anatomy, they became hunters.
The structure of their hip bones shows that they were well onto to evolving
bipedalism and being able to survive on the African plains. Although these
ape-men were small defenceless and slow, compared with the predators of the
plains, they were able to compete with the other predators. The ape-men had
hands with a precise and powerful grip, developed by their ancestors in response
to the demands of a tree-climbing life. If they stood upright, these hands could be
ready at all times to compensate for the lack of teeth and claws. If the ape-men
were threatened by enemies they could defend themselves by hurling stones and
wielding sticks. Faced with a carcass, they might not have been able to open it
with their teeth as a lion could do, but they could cut it open using the sharp edge
of a stone, held in the hand. They could even take one stone, strike it against
another and so shape it. Stones deliberately struck in such a way have facets on
them that are quite different from those on stone that have been chipped by
rolling in streams or split by frost. They can thus be identified and many such
have been found associated with the skeletons of ape-men. The animals had
become tool-makers. So ape-men claimed a permanent place for themselves in
the community of animals on the plains.
This state of affairs lasted for a very long time, probably as much as three million
years. Slowly, generation after generation, the bodies of one line of ape-men
became better adapted to the plains-living life. The feet became more suited to
running, lost their ability to grasp and acquired a slight arch. The hips changed,
the joint moved towards the centre of the pelvis to balance the upright torso, and
the pelvis itself became more bowl-shaped and broader to provide a base for the
strong muscles running between the pelvis and spine that were needed to hold
the belly in its new upright position. The spine developed a slight curve so that
the weight of the upper part of the body was better centred. Most importantly, the
skull changed, the jaw became smaller and the forehead more domed. The brain
of the first ape-men was similar to that of a gorilla, around 500 cubic centimetres,
but by this time had doubled in size and these ape-men had grown to a height of
over a metre and a half and were called Homo erectus, Upright Man.
Homo erectus was a much more skilled tool-maker than previous ape-men.
Their stones were carefully shaped with a tapering point at one end and a sharp
edge on either side, and were of a size that fitted neatly into the hand. Evidence
of one of his successful hunts has been unearthed at Olorgesailie in southwest
Kenya. In one small area, lie the broken and dismembered skeletons of a giant
baboon species now extinct and with these bones are the remains are hundreds
of chipped stones and several thousand rough cobbles. All are of rock that does
not occur naturally within 30 kilometres of the site. The fact that the stones come
from a distant site suggests that the hunts were premeditated and that the
hunters had armed themselves long before they found their prey. Baboons, even
the smaller living species (Papio species), are very formidable creatures with
powerful fanged jaws. Few people today, without fire-arms, would be prepared to
tackle them. The numbers killed at Olorgesailie suggest that such hunts were
regular team operations demanding considerable skill. Homo erectus was
clearly, a very formidable hunter.
Although impossible to establish Homo erectus must have possessed a language
to discuss their plans and carry out such attacks? Attempts have been made to
deduce from their skulls and neckbones the structure of the soft parts of their
throats and the current view is that although they were probably capable of
making noises considerably more complex than the grunts and screams of
modern apes, their speech, would probably have been slow and clumsy.
However, Homo erectus had another medium of communication at their disposal
- gestures - and we can make some confident guesses as to what they were and
what they meant. Human beings have more separate muscles in their face than
any other animal. They make it possible to move the various elements - lips,
cheeks, forehead, eyebrows - in a great variety of ways that no other creature
can match. There is little doubt, therefore, that the face was the centre of Homo
erectus's gestural communication.
39
One of the most important pieces of information it transmits is identity. We take it
for granted that all our faces are very different from one another yet this is a very
unusual characteristic among animals. If individuals are to cooperate in an
organised team in which each person has their own responsibility then it is crucial
for those taking part to be able to distinguish one from another immediately.
Many social animals, such as hyenas and wolves, distinguish each other by
smell. Human's sense of smell, however, is much less well-developed than their
sight, so recognition should be based on the shape of the face.
Since the features of the face are extremely mobile, they can also convey a great
deal of information about changing moods and intentions. We still have little
difficulty in understanding expressions of enthusiasm and delight, disgust, anger
and amusement. But quite apart from such revelation of emotion, we also send
precise messages with our faces. Are the gestures we use today arbitrary ones
that we have learned from our parents and share with the rest of the community
simply because we have the same social background? Or are they deeply
embedded in us and are an inheritance from our prehistoric past. Some gestures
do vary between societies and are clearly learned yet others appear to be more
universal and deep-seated.
With this improved talent for communication and skill in making tools Homo
erectus became more successful. Their numbers increased and they spread
from southeastern Africa into the Nile valley and northwards to the eastern
shores of the Mediterranean. Their remains have been found further east in
Java, and in China. Whether they migrated into Asia from Africa or whether
these people were the descendants of an Asiatic ape-man is unknown. Some of
the African groups reached Europe. A few crossed over a land bridge that once
connected Tunisia, Sicily and Italy. Others travelled eastwards round the
Mediterranean and up north through the Balkans.
Homo erectus was in Europe in some numbers about a million years ago. But
about 600,000 years ago the climate changed. It started to get very cold. The
shift was gradual but the overall trend was of great cooling. With so much water
being locked up in the ice caps caused the sea-level to drop and land bridges
connected the various continents, so that eventually these people were able to
spread into the Americas across the Bering Straits and down the island chains of
Indonesia towards New Guinea and Australia.
In Europe, Homo erectus must have felt the increasing cold very keenly. They
had evolved in the warmth of the African plain and did not have the protection of
thick fur, like the mammals that had lived in these cold regions for a long period.
Doubtless, many creatures, in such circumstances, would have moved to warmer
parts or died out. These humans being dexterous and inventive hunted for furred
animals, stripped the skins from their dead bodies and used the skin for
themselves. They also found shelter in caves.
These human's living sites have been discovered in great numbers in southern
France and Spain. Along the great limestone valleys of central France such as
the Dordogne and in the foothills of the Pyrenees, the cliffs are riddled with
caves. From the archaeological evidence there appears to be no significant
difference between these people who lived in the caves of France 35 000 years
ago and ourselves. Anthropologists, accordingly, have given these people the
same name as they use, somewhat immodestly, for all modern humans - Homo
sapiens, Wise Man.
The difference between the life of a skin-clad hunter leaving a cave with a spear
over his shoulder to hunt mammoth, and a smartly dressed executive driving
along a motorway in New York, London or Tokyo, to consult their computer printout, is not due to any further physical development of the body or brain during the
long period that separates them, but to a completely new evolutionary factor;
culture.
People have credited themselves with several talents to distinguish themselves
from all other animals. Once we thought that we were the only creatures to make
and use tools. We now know that this is not so. Chimpanzees do so and so do
finches in the Galapagos that cut and trim long thorns to use as pins extracting
grubs from holes in wood. Even our complex spoken language seems less
special the more we learn about the communications used by chimpanzees and
dolphins. But we are the only creatures to have painted representational pictures
and it is this talent which led to developments which ultimately transformed the
life of mankind. That skill is the use a written information in order to communicate
between ourselves and to create our own cultural identities.
ASSIGNMENTS
IN YOUR OWN WORDS WRITE A FOUR PAGE ESSAY ON THE
FOLLOWING TOPIC
Discuss how communication, co-operation and tool-making contributed to
the evolution of the species Homo sapiens.