Planet Earth and Its Environment A 5000-million year Journey

Part of the Evolution of Australian Biota ModuleBiology in Focus, Preliminary CourseGlenda Childrawi and Stephanie Hollis

Evolution of Australian BiotaTopic 8: Reproductive Adaptations in AnimalsDOT PointsDescribe some mechanisms found in Australian fauna to ensureFertilisationSurvival of the embryo and of the young after birth

IntroductionOrganisms 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.

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Marine AnimalsMany 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.

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Marine AnimalsWhen 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 Australias Great Barrier Reef, the number of gametes shed is so great, that for a time, the sea turns milky.

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Marine AnimalsWithin 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.

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Amphibians 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.

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Amphibians 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.

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Amphibians 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?

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BirdsIn 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.

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BirdsThe 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.

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ReptilesIn 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.

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MonotremesDuck-billed platypus and echidnas are oviparous, meaning their eggs, after being fertilised internally, are deposited outside the mothers 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

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MonotremesThe 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 thelife-animal.blogspot.com

MarsupialsMarsupials 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.museumvictoria.com.au

MarsupialsSince 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.en.wikipedia.org

MarsupialsThe 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 uterus.southernson.com

MarsupialsAt 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.kidsblogs.nationalgeographic.com

MarsupialsIn 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.arkive.org

MarsupialsThe 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 time.

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Activity-Students to complete DOT Point 3.6 (pg97 Prelim DOT Point Text)