Download Exercise #1 - UBC Zoology

Lesson 33
Lesson Outline:
Exercise #1 - Basic Functions
Exercise #2 - Phylogenetic Trends
Exercise #3 - Case Studies to Compare
• Reproductive Strategies- Energy Partitioning
• External versus Internal Fertilization
• Sexual Dimorphism
o Functional Characteristics
o Aids to Identification
o Copulatory Organs
• Timing - Copulation, Ovulation, Fertilization, Development
Objectives:
Throughout the course what you need to master is an understanding of:
1) the form and function of structures,
2) the phylogenetic and ontogenetic origins of structures, and
3) the extend to which various structures are homologous, analogous and/or
homoplastic.
At the end of this lesson you should be able to:
Describe the advantages and disadvantages of internal and external fertilization
Describe sexual dimorphism and the selection pressures that lead to it
Describe the trends seen in the design of copulatory organs
Describe the various forms of reproductive strategy for delaying development of
the fertilized egg and the selective advantage of them
References:
Chapter 15:
351-386
Reading for Next Lesson:
Chapter 16: 387- 428
Exercise #1
List the basic functions of the urogenital system:
The urinary system excretes the waste products of cellular digestion, ions, amino acids,
salts, etc. It also plays a key role in water balance along with numerous other structures in
different species living in different environments (i.e. gills, skin, salt glands).
The primary function of the system is to give rise to offspring, - to reproduce.
Exercise #2 Describe the evolutionary trends that we see in the urogenital systems
of the different vertebrate groups:
The phylogenetic trends that we see throughout the chordates were covered in detail in
lectures (lecture 31 and 32) and are summarized schematically in the next figures:
Exercise #3 – Comparisons –
Case 1
Reproductive Strategies - Energy Partitioning
Some would argue that the primary reason that organisms exist is to reproduce and make
more organisms. We are the sperm and eggs way of making more sperm and eggs.
As a consequence, all energy that does not go into growth and survival goes into
reproduction in most animals. Given the limited amount of energy available for this
purpose, every species is confronted with the question of how best to partition that
energy.
For example, the two extremes in energy partitioning are seen by comparing mating smelt
and humans. In the case of the fish, all energy goes into making gametes. The males and
females get together once a year on a spawning beach and when conditions are right, they
all release their eggs and sperm into the water synchronously. No energy goes into
mating, courting, sexual dimorphism, childcare, etc. - just producing gametes.
In mammals such as humans, on the other hand, large amounts of energy are spent on
sexual dimorphism, mating, courtship, pregnancy and child rearing.
One strategy is based on quantity. If you release enough gametes, some will meet, some
eggs will get fertilized and some of the fertilized eggs will survive. The other strategy is
based on quality. Put less energy into making gametes and more energy into making sure
that the sperm and egg met, and that the embryo will develop and reach sexual maturity.
There are numerous strategies in between. The diversity of reproductive structures,
physiology and behaviour seems to exceed that of any other system. This is in part due to
the freedom of reproductive structures and processes from the demands of other organs
and systems. There is a definite autonomy of the reproductive system; it exists for the
future of the species and not for the individual that houses it.
Case 2
External versus Internal Fertilization
In most water dwelling vertebrates, fertilization is external. Eggs and sperm are shed
simultaneously from the body into the water where fertilization occurs.
If the female uterus houses the embryo, or if a shell seals an egg, then sperm must
fertilize the egg before it descends the oviduct. In these instances, fertilization must be
internal.
The disadvantage of external fertilization in an aquatic environment is the possibility of
dispersion of the gametes before fertilization can occur combined with the release of the
gametes into a harsh environment (especially for freshwater fish).
Many mechanisms exist to minimize these effects:
- juxtaposition of genital openings during release of gametes.
- mass spawning - broadcast fertilization (the ultimate in promiscuity) common in
fish that release a large number of small eggs.
- nest building, with courtship and fertilization.
The disadvantage of external fertilization in a terrestrial environment is the problem of
desiccation of the gametes.
Internal fertilization usually takes place in the genital tract. It appears to have evolved
independently in every group.
An example where it does not occur in the genital tract is in Haplochromis where it takes
place in the mouth chamber. The female spawns and takes her eggs up into her mouth.
The male has egg shaped spots on his tail. The female tries to swallow these which
induces sperm release and the female takes up the sperm "accidentally".
Internal fertilization provides a better, controlled environment for fertilization, but to
ensure fertilization, ova and sperm must still meet at the right time. Mechanisms to
ensure this include:
- ova and sperm may be viable for prolonged periods - copulation can occur
anytime (but the female must nourish the sperm).
sperm in
fish
- 4-10 months
turtles - up to 4 years
snakes - overwinter
lizards - up to 6 years
birds - up to 45 days
rodents - up to 156 days
- copulation and ovulation may be timed together.
- induced ovulation.
- copulation on or during estrus
- frequent copulation throughout the reproductive season.
Case 3
Sexual Dimorphism
These are external indications of gender. They fall into two categories - functional
structures essential for uniting gametes or nurturing the young and - characteristics to aid
individuals in identifying the sex or sexual condition of other individuals of the same
species.
Functional Characteristics
In animals with internal fertilization these include:
- various types of intromittent or copulatory organs in males.
- ovipositors in female fish, brood patches in female birds, mammary
glands in female mammals.
Aids to Identification
These include sight, sound and smell - which one usually being a function of
environment.
Visual:
- colour.
- shape.
- movement patterns (appropriate movement responses start the
behaviour chain leading to copulation in practically all vertebrates.
Sound:
- little used in aquatic species.
- well developed in frogs and birds.
Odor:
- particularly used in mobile, terrestrial vertebrates.
- allows them to identify gender in the absence of the individual
and helps bring the sexes together.
Allow sexual and species recognition at a distance. Allows animals to be more active and
mobile.
Copulatory Organs
If fertilization is internal, there must be a way to transfer sperm from the male to the
female cloaca or vagina.
In many vertebrates copulation simply involves
momentary apposition of the male and female
cloacae to transfer sperm. Often, however, the
male possesses an intromittent organ for
inserting the sperm into the female reproductive
tract.
In sharks and many fish, this may occur in the
form of specialized pectoral or anal fins. You
have seen the claspers on the male shark in lab.
In some fish the anal fin is designed in a similar
fashion and is referred to as a gonopodium.
During copulation, one clasper or gonopodium is inserted into
the female and the terminal cartilage is spread by muscle to
hold it firmly in place. Sperm are flushed along a groove in the
clasper or gonopodium into the female by water from a siphon
sac in the male.
Fertilization in most frogs is external. Usually it occurs in
water. Most salamanders, on the other hand, reproduce on land
and the males produce a spermatophore that consists of a
gelatinous capsule containing sperm. Following a courtship
display, the male deposits the spermatophore in front
of the female and the female gathers the
spermatophore up with the lips of her cloaca.
Turtles, crocodiles, birds and mammals all possess a
single penis while lizards and snakes possess
hemipenes. In all cases these structures are erectile.
Thus they are flaccid and often retracted into the
cloaca when not in use but become engorged with
blood making them erect and stiff when needed.
In many reptiles and amphibians, sperm are stored in a
spermatheca or pocket in the cloaca. The female
secretes nutrients to nourish the sperm and some species can store sperm for over 10
years. This decouples copulation and sperm transfer from fertilization allowing courtship
and mating to occur opportunistically and fertilization and egg deposition to occur when
conditions are right.
In birds, the intromittent organ consists of little
more than swellings of the edges of the male
cloaca while in others there is a true penis that is
quite elaborate.
All mammals have a single penis although in
male marsupials the tip is forked - to fit into the
two lateral vaginas of the females.
In many mammals the penis is strengthened by
bone - the baculum or os penis.
In all cases, these structures serve to enhance
sperm transfer and ensure that the sperm reaches
the oviduct of the female.
Case 4
Timing - Copulation, Ovulation, Fertilization, Development
Many strategies exist to match the timing of copulation to the timing of ovulation by the
female. These range from situations in which the female changes in sexual characteristics
around the time of ovulation to stimulate males to mate - to the situation in which males
always want to mate but females are only receptive when they are ovulating.
A more precise strategy is one in which the
act of copulation induces the female to
ovulate. This occurs in rabbits for instance.
If copulation does not occur at the time of
ovulation, then sperm must be stored until
ovulation occurs. We've discussed this
already in amphibians and reptiles. In bats,
fertilization may also be delayed (as in
amphibians and reptiles). Copulation occurs
in the fall and the sperm are stored
throughout hibernation. Ovulation takes
place in the spring when the eggs are
fertilized and the young are born when food
is most abundant.
In most species, there is an optimum time to give birth. This is usually at the start of
summer or the season when food is most abundant for the developing young. The timing
of mating and fertilization, however, are not always appropriate to ensure that the
offspring are born at the right time. Various strategies have developed to minimize this
problem.
In many other mammals, fertilization takes place but the developing embryo does not
implant in the wall of the uterus immediately but remains in a state of suspended
development. This occurs in some hibernating mammals as well as in marsupials. In the
case of the marsupials the female is adapted to maximize production of young. She often
has two developing fetuses or Joeys in the pouch and another fertilized egg ready to
implant in the uterus. Implantation does not occur until the oldest Joey stops suckling.
The act of suckling inhibits implantation and once this inhibition is removed,
implantation occurs. This minimizes the time between successives births.