1.8 Reproductive Adaptations in Animals

Evolution of Australian Biota
Topic 8: Reproductive Adaptations in Animals
Part of the Evolution of Australian Biota Module
Biology in Focus, Preliminary Course
Glenda Childrawi and Stephanie Hollis
DOT Points
 Describe some mechanisms found in Australian fauna to ensure
 Fertilisation
 Survival of the embryo and of the young after birth
Organisms in both aquatic and terrestrial environments are
successful in their varied reproduction methods. In this topic
we are going to investigate some of the mechanisms found in
Australian fauna.
Marine Animals
Many marine animals, such as the staghorn coral, achieve
fertilisation by simply shedding millions of gametes into the
sea. Environmental cues, such as water temperature, tide and
day length, help to synchronise the reproductive cycle.
Marine Animals
When one coral starts to spawn, phermones released along
with gametes will stimulate nearby individuals to spawn,
resulting in co-ordinated spawning over an area. During the
mass spawning of coral on Australia’s Great Barrier Reef, the
number of gametes shed is so great, that for a time, the sea
turns milky.
Marine Animals
Within one day fertilised eggs develop, forming into
swimming larvae. After a few days at the surface, the larvae
descend to find a suitable site to form a new colony. Millions
of staghorn coral larvae are produced and almost all of these
are eaten by predators. Of the few remaining, only a tiny
fraction develop into adults.
Southern gastric brooding frog eggs are fertilised externally in
a watery environment. The female releases eggs and, after
they are fertilised by sperm, the female swallows them.
Rather than leaving the eggs to develop alone unprotected, the
young develop internally in the females stomach.
In the stomach, digestive secretions cease and the eggs settle
into the stomach wall, where they are protected and absorb
nutrients from the mother. This gastric brooding appears to
last about 6-7 weeks, during which the female does not eat.
When the young frogs are ready, they
are regurgitated through the mouth.
Therefore these animals have external
fertilisation but internal development.
This is an extreme example of parental
care which was discovered in forests
north of Brisbane in 1974.
How would this effect the success of the
offspring surviving?
In birds, fertilisation is
internal but the fertilised egg
undergoes the majority of its
development externally.
Parental care is often needed
continuously. Most species
brood their eggs, with
parents taking turns so that
each can go and feed.
The megapodid birds, such as
the brush turkey, build a
mound from twigs, soil and
leaf litter, into which they
place their eggs. Heat from
decomposition of the leaf litter
keeps the eggs warm. The
male parent frequently tends
the nest adding or removing
material to control the
incubation temperature.
In crocodiles, fertilisation occurs internally , then the female
crocodiles lay clutches of eggs in sandbanks beside the sea or
river. When an egg hatches, the offspring resemble a miniature
adult, able to crawl up from the buried nest to the surface to
find its way to water and feed independently.
Duck-billed platypus and
echidna’s are oviparous, meaning
their eggs, after being fertilised
internally, are deposited outside
the mother’s body to complete
their development. Duck-billed
platypus incubate their eggs in a
nest or specialised pouch, and
the young hatchlings obtain milk
from their mothers mammary
glands by licking the skin
The echidna lays a single soft-shelled, leathery egg 22 days
after mating, and deposits it directly into her pouch. Hatching
takes place after ten days; the young echidna, called a puggle,
then sucks milk from the pores of the two milk patches
Marsupials from the kangaroo and wallaby family have an
extraordinary ability to control embryonic development.
Mothers are pregnant for a short period of time and can
become pregnant again just after giving birth.
Since milk production lasts much longer than the pregnancy, it
is necessary to delay development of the new embryo until the
pouch becomes vacant. This delay in development is
controlled by the suckling of the young in the pouch.
The red kangaroo carries out
both internal fertilisation and
internal development of the
young after birth. In good
conditions, it can have three
offspring at different stages of
development. The female may
have an older young out of the
pouch but still being suckled, a
newborn young in the pouch and
an embryo in dispause in the
At this stage, the mother will be producing two different types
of milk simultaneously. The newborn young will get low
fat/high carbohydrate milk from the nipples of the mammary
glands, while the young outside the pouch will feed from a
different nipple to get large volumes of high fat/low
carbohydrate milk.
In times of drought the mother may be unable to produce
sufficient milk to sustain a growing young in the pouch. If the
young dies, a new young will enter the pouch a month later.
Since the new young is so small it only needs a small amount
of milk. This ensures there is always a young ready when the
drought ends.
The ‘production line’ approach
allows very rapid population
growth when conditions are good.
However, under prolonged drought
conditions, breeding stop and only
begins again when rain triggers a
hormonal response in the female.
This very effective mechanism
controls the rate of reproduction
depending on the favourability of
the environmental conditions at the
-Students to complete DOT Point 3.6 (pg97 Prelim DOT Point
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