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Mammals
Characteristics of Mammals
Mammals are well adapted for terrestrial living
and are able to retain water more efficiently than
reptiles.
The mammalian kidney has an exceptional ability to
concentrate waste products in a small volume of urine.
Characteristics of Mammals
Hair
Of all animal species, only mammals have hair.
A hair is a filament composed mainly of dead cells filled
with the protein keratin.
The primary function of hair is insulation.
The dense coat of hair that covers most mammals
holds heat in.
The coloration and pattern of a mammal’s coat often enables
the animal to blend in with its surrounds.
Some animals show a seasonal change in the color of their coat.
The color of a mammal’s coat may also be a signal, such as the
black and white fur of a skunk to warn predators to stay away.
Characteristics of Mammals
In some animals, specialized hairs serve
a sensory function.
The whiskers of cats and dogs are stiff hairs
that are very sensitive to touch.
Mammals that are active at night or that live
underground often rely on their whiskers or
information about the environment.
Other specialized hairs can be used as a
defensive weapon such as the porcupines sharp,
barbed quills.
Mammalian Teeth
Mammals usually have only two sets of teeth.
The first set, commonly called baby teeth or
milk teeth, is replaced by permanent teeth,
which are not replaced if lost or damaged.
Animals use their teeth in a variety of ways:
To secure and chew food,
For protection,
As a threat signal,
To perform tasks (such as cutting down trees).
Mammalian Teeth
Most mammals have four types of teeth:
Incisors,
Canines,
Premolars,
Molars.
Each type of tooth performs a different function
in eating.
Mammalian Teeth
Incisors, the front teeth,
are for biting and cutting.
Canines are used for
stabbing and holding.
Lining the jaw are the
premolars and molars.
As the mammal chews its upper and lower molars fit
together, crushing and grinding the food.
A mammal’s teeth are specialized for the food it
eats, and it is usually possible to determine
a mammal’s diet by examining its teeth.
Body Temperature
Mammals are endotherms, generating heat internally
through the rapid metabolism of food.
Because a mammal’s body temperature remains
relatively constant regardless of the temperature
of its surroundings, mammals can be active at any time of
the day or night.
They also can live in very cold climates.
Endothermic metabolism permits mammals to sustain
activities that require high levels of energy, such as
running or flying long distances.
To maintain a high metabolic rate, mammals must eat
about 10 times as much food as an ectotherm
(of similar size) and have efficient respiratory and
circulatory systems for distributing oxygen.
Respiratory System
Mammalian lungs have a large internal surface
area that aids the exchange of oxygen
and carbon dioxide.
Respiration in mammals is aided by the diaphragm, a
sheet of muscle that separates
the chest cavity from
the abdominal cavity.
When the diaphragm contracts,
the chest cavity enlarges,
drawing air into the lungs.
Respiratory System
The lungs of mammals contain small,
grape-shaped chambers called alveoli.
Alveoli provide a very large respiratory surface area.
In more active mammals, the alveoli
are smaller and more numerous,
further increasing the surface
area for diffusion.
Heart and Circulatory System
Like crocodiles and birds, mammals have a fourchambered heart with a septum that completely divides
the ventricle.
The division of the ventricle creates two pumping
chambers, one for each loop of the mammal’s doubleloop circulatory system.
Heart and Circulatory System
One chamber pumps oxygen-rich blood
to the body, while the other pumps oxygen-poor blood
to the lungs.
Because the two do not mix,
only oxygen-rich blood is delivered
to the tissues.
Parental Care
Mammals are unique among vertebrates in the way that they
nourish their young after birth.
Mammary glands located on the female’s chest or abdomen
produce a nutrient-rich energy source called milk and give this
class its name.
Milk is rich in protein, carbohydrates (sugar lactose), and fat. It also
contains water, which prevents dehydration, and minerals, such as
calcium, that are critical to early growth.
Young mammals are nourished on milk from birth until
weaning, the time when their mother stops nursing them.
Unlike other vertebrates, young
mammals are dependent on their
mother for a relatively long period,
receiving milk and other food,
protection, and shelter from her.
Today’s Mammals
Diversity
Mammals range in size from tiny
shrews that weigh about 1.5 g
(less than .1 oz) to gigantic blue
whales that can weigh up to 136,000
kg (150 tons).
Reproduction
All mammals reproduce by
internal fertilization.
The male releases sperm into the
female’s reproductive tract where one
or more eggs are fertilized.
Present-day mammals are divided into
three groups based on their pattern of
development.
Monotremes
Marsupials
Placental Mammals
Monotremes
The most primitive of the mammals are represented by the
order Monotremata.
All modern monotremes live in Australia,
New Guinea, and South America.
This distribution may be the result of the
movement of these landmasses by continental drift.
The three living monotreme species include the duckbill
platypus and two species of echidnas (spiny anteaters).
Monotreme Characteristics
Monotremes seem to resemble early mammals.
The monotreme species have shoulders and forelimbs that are
reptilian in appearance.
Monotremes reproduce by laying leathery, shelled eggs.
The female monotreme incubates her eggs with her body
heat, and at hatching, the newborns are only partially
developed.
Like birds and reptiles, monotremes have a cloaca, a common
passageway for the digestive, reproductive, and urinary
systems.
No other mammals have a cloaca.
Adult monotremes do not have true teeth.
Monotreme Characteristics
Two features monotremes share with
other mammals are hair and milk production.
Unlike other mammals, monotremes
do not have nipples and young
monotremes do not nurse.
The young lap up milk that oozes from
glands located on their mother’s belly.
The platypus inhabits lakes and streams.
Its broad, flat tail and webbed front feet
make it an excellent swimmer.
The platypus uses its flat bill to probe
for crustaceans, worms and snails.
Echidnas are terrestrial and have very strong, sharp claws and an
elongated, beaklike snout which they use for burrowing and
digging out insects and other invertebrates.
Marsupial Characteristics
Order Marsupialia includes kangaroos, wombats,
wallaroos, koalas, and the opossums.
In marsupial mammals, the young are born only days
or weeks after fertilization (tiny and incompletely
developed, except for their front limbs).
Without any parental help, the newborns crawl to
their mother’s nipples (usually located in a pouch, the
marsupium, on her abdomen).
Each newborn attaches
itself to a nipple and
continues to grow and
develop for several months.
When the young marsupials
Placental Mammal Characteristics
The young of placental mammals develop within the female’s
uterus, where they are nourished by nutrients from her
blood.
An organ called the placenta allows the diffusion of nutrients
and oxygen from the mother’s blood, across placental
membranes, and into the blood of the developing fetus.
Waste materials from
the fetus diffuse in
the opposite direction
and are eliminated by
the mother’s excretory
system.
Placental Mammal Characteristics
The period of time between fertilization and birth is called
the gestation period.
Most placental mammals have a longer gestation period
than marsupial mammals and their young are more
completely developed at birth.
Some placental animals like a foal
can stand and walk
within a few hours
of birth. Others like
a rabbit are born blind,
deaf, and helpless.
Placental Mammal Characteristics
Placental mammals vary greatly in size, shape, diet, and
habits.
They live in a variety of habitats, from hot, moist rainforests
to the frigid tundra.
Although mammals share many similarities, species have
different characteristics.
Some placental mammals are adapted for running, leaping,
swimming, or flying.
Placental Mammal Characteristics
Some placental mammals have hooves, horns, or antlers.
Hooves are specialized pads that
cover the toes of many running
mammals.
Hooves are made of keratin,
a protein that is a component
of many mammalian structures.
The horns of sheep, cattle, and
antelope are composed of a core
of bone surrounded by a sheath
of keratin.
This bony core is firmly attached to
the skull, and the horn is never shed.
The horn of rhinoceros is composed
not of bone but of hair-like fibers of
keratin that form a hard structure.
Placental Mammal Characteristics
Other placental mammals, such as deer and elk,
grow and shed a set of antlers each year.
Antlers grown only by the male, are composed of bone but are not
covered by a keratin sheath.
While they are growing, the antlers are covered
by a thin layer of soft skin called velvet.
When the antlers are fully grown,
the velvet dries up and comes off.
The male uses his antlers during breeding season
to attract females and to combat other males.
After the breeding season is over, the antlers are
shed, and a new pair grows the next year.
Because the male grows a larger pair of antlers
each year, antler size gives an indication
of a male deer’s age.
Domestic Animals
Domestic animals are animals that have been
kept and bred by people for special purposes.
Most domestic animals are placental mammals
whose association with humans dates back at
least 2,000 years.
These animals include dogs, cats, cattle, horses,
donkeys, mules, rabbits, sheep, goats, pigs, camels,
llamas, and alpacas.
Different breeds of domestic mammals have been
developed through selective breeding.
For example, some breeds of goats produce more milk
than others. The milk is used to produce dairy
Modern Placental Mammals
There are 19 orders of placental mammals, which
include more than 90% of all mammal species.
Terrestrial placental mammals inhabit all
continents except Antarctica, and aquatic
placental mammals inhabit all oceans.
Order Rodentia
Over 40% of all placental mammals are gnawing
mammals called rodents.
Rodents are distinguished from most other mammals
by their teeth, which are specialized for gnawing.
All rodents have two pairs of large, curling incisor
teeth that grow continuously.
As the rodent gnaws, the back side of the incisors
wears away faster than the front,
creating a sharp chisel
edge on the
teeth.
The success of this
group of herbivores
can be attributed to their
intelligence, small size, and
Order Chiroptera
This order is composed of bats, the only mammals capable
of true flight.
A bat’s front limbs are modified into wings.
The thumb, which is not attached to the wing, has a curved claw that can
be used for clinging or grasping.
Bats generally live in groups and are active only at night.
During the day, bats hang upside down in caves or some other protected
place.
Most bats are carnivorous,
using echolocation to find
insects, which they catch
while in flight.
Other bats eat fruit or nectar
from night-blooming flowers.
Order Insectivora
Insectivores are the mammals most similar to the
ancestors of the placental mammals.
As their name implies, these small mammals eat mainly
insects, but their diet may also include fruit, small birds,
and snakes, as well as other insectivores.
Insectivores have an
enormous appetite,
and some such as the
shrew must eat more
than two times their
body weight daily to
fuel their extremely
high metabolic rate.
Order Carnivora
Some of the best-known animals are the flesh-eating hunters
called carnivores.
Scientists generally divide this order into two subgroups, the
cat family and the dog family.
Carnivores are strong and extremely intelligent, and they have
keen senses of smell, vision, and hearing, which have enabled
them to become successful hunters.
Their long canine teeth are specialized for capturing prey and
tearing flesh.
Some members of this order are no longer carnivorous.
Raccoons and bears are omnivores.
Pandas are herbivores.
Order Carnivora
Order Pinnipedia
This order of marine carnivores includes seals and
sea lions that feed at sea but return to land to mate,
rear their young, and rest.
All four limbs are modified as
flippers for swimming, and their
bodies are streamlined for rapid
movement through the water.
An insulating layer of blubber
protects them from the cold
ocean waters.
Most species live in large
colonies called rookeries which
are headed by a large male.
Order Primates
Humans belong to the order Primates.
Other members of this order include the prosimians, which are
active at night, and monkeys and apes, which are active during the
day.
Most nonhuman primates, like the monkey, are tree dwellers, and
many of their characteristic features are adaptations for living in
trees.
Flexible, grasping hands and feet aid tree-dwelling primates in
climbing.
Monkeys and prosimians
have a tail that is used
for balance.
The excellent depth
perception of the
primates is critical for
those that live in the trees.
Primates are extremely
curious and their ability
to learn is exceptional.
Order Artiodactyla
Mammals belonging to this order and the
Perissodactyla Order are classified as ungulates,
mammals with hoofs.
Ungulates walk, not on their entire foot as most
vertebrates do, but with their weight supported
by their hoof-covered toes.
Most ungulates are herbivores
that live together in herds.
The young are well developed
at birth and can move along
with the herd within a day or two.
Order Artiodactyla
Artiodactyls have an even number of toes within
their hooves.
Many of these mammals have a stomach chamber
called a rumen. Microbes in the rumen break
down the cellulose in the plant material
artiodactyls eat.
Mammals with a rumen regurgitate partly
digested food, called cud, rechew it, and swallow
it again for further digestion.
This order includes pigs, hippopotamuses,
Order Perissodactyla
Ungulates with an odd number of toes within
their hooves are classified as perissodactyls.
This order includes horses, zebras,
tapirs, and rhinoceroses.
Perissodactyls do not chew their cud.
Instead of a rumen, they have a cecum, a pouch
branching from their large intestine.
The cecum contains microbes that digest the
cellulose in their diet.
Perissodactyls are far less numerous than
Order Cetacea
Cetaceans are divided into two groups:
The predatory toothed whales, dolphins, and
porpoises
The filter-feeding baleen whales.
Whales, such as the orca, are
probably descendants of land
mammals that returned to the sea
about 50 million years ago and
have adapted to a fully aquatic life.
Their streamlined bodies have front limbs
modified into flippers, no hind limbs,
and a broad, flat tail for swimming.
A nostril called a blowhole is located on
Order Lagomorpha
This order is composed of rabbits and hares.
Like rodents, they have one pair of long,
continually growing incisors, but they also have
an additional pair of peg-like incisors that grow
just behind the front pair.
Rabbits and hares have long hind legs and are
specialized for hopping.
Rabbits build nests that the female lines with fur.
The young are born furless and their eyes are
closed.
Order Sirenia
These somewhat barrel-shaped marine animals
include
the dugongs and manatees.
Like whales, they have front limbs modified as
flippers
and no hind limbs.
A flattened tail is used for propulsion through the
tropical oceans, estuaries, and rivers where
sirenians live, grazing
on aquatic plants.
Despite their
Order Proboscidea
The two living species of this order are
the African elephant and the Asian elephant.
These species are the largest land animals alive
today.
Their long boneless trunk is really an elongated
nose and upper lip and is used for a variety of
tasks.
An elephant’s upper incisor teeth are modified
into
long ivory tusks.
Elephants live in
Order Edentata
Order Edentata includes sloths,
anteaters and armadillos.
These mammals are toothless
or with poorly developed teeth
that lack enamel, in the ant-eaters
they are absent entirely.
The feet have well-developed
claws, used for digging or hanging,
and some edentates walk on the
outsides of the feet.
They are found only in the
Western Hemisphere.
Order Macroscelidea
The order Macroscelidea includes the elephant shrews.
These are ground-dwelling insect eaters with long, flexible
snouts, large eyes, and a long tail.
They hop about somewhat like small kangaroos.
The elephant shrews are active mainly in daytime, but in hot
weather they may be nocturnal.
The smaller species of elephant shrews feed on ants,
termites, and slender shoots, roots, and berries, while some
larger species prefer beetles.
Elephant shrews live in thorn bush
country, grassy plains, thickets,
the under-growth of forests, and
on rocky outcrops on continental
Africa and on the island of Zanzibar.
Order Scandentia
The order Scandentia includes the tree shrews.
Scandentia are omnivorous, small, squirrel-like
mammals with long snouts, sharp teeth and a bushy
tail.
Tree shrews are territorial, omnivorous, and extremely
active; they dart about constantly in the trees
screaming and fighting with one another.
They are found in Asia and
live mainly on the ground,
despite their name.
Order Pholidota
The order Pholidota includes the pangolins also known as the
scaly anteaters.
Their body is covered with
overlapping, triangular scales.
Pangolins break open logs
with their large, powerful
claws and use their long, slender tongues to lap up insects.
When threatened, the animal rolls into a ball and erects the
scales, points upward, so that it resembles a large pinecone.
It also secretes a foul-smelling liquid.
Members of some species are tree dwellers and have
prehensile, or grasping, tails; others are terrestrial.
Order Hyracoidea
The order Hyracoidea includes the hyraxes of Africa and
SW Asia.
These mammals have a rabbit-like body, short ears, four
hoofed toes on their front feet and three hoofed toes on
their back feet with moist padded soles that cling to
steep surfaces by suction, making it an excellent climber.
They feed on seeds, fruit,
and leaves, and in large
numbers can be serious
agricultural pests.
Order Dermoptera
The order Dermoptera includes the flying lemurs native
to the tropical lowland forests of S Asia, Malaya, and the
Philippines.
These mammals are squirrel-like that glide
on a sheet of skin stretching between their
forelegs and hind legs.
Although its teeth resemble
those of carnivores, the flying
lemur's diet consists of fruit
and leaves.
It sleeps by day and forages
at dusk.
Order Tubulidentata
The order Tubulidentata includes the aardvarks found in
Africa and Europe.
These mammals are nocturnal, with a pig-like body, big
ears, and a long snout that is used to feed on ants and
termites.
The animal is able to bury its snout in earth and locate
and swallow prey while continuing to breathe.
The limbs are specialized
for digging, which is used
as a means of escape from
danger as well as to
obtain food.