Chickens Kangaroos Flowering plants Tapeworms Shore crabs

What came first, the chicken or the egg? The life cycle goes on....

Some further examples of interesting different life cycles, often discussed in primary classrooms, are outlined below (see also Life cycles A: General stages - Butterflies and frogs):

Chickens
The adult male (rooster or cock) mates with the hen by climbing on her back and placing his opening (cloaca) against hers, and his sperm swims up the oviduct to fertilise the eggs. The "white" and soft shell are then deposited around the fertilised egg as it passes down the oviduct, and the shell then hardens before the egg is laid. The embryo develops in the egg using food from the yolk, and oxygen from the air sac.

The egg must be regularly turned and kept warm in an incubator or by a brooding hen on the nest. After 21 days, the chick pecks its way out. The yellow fluff (down) is later replaced by feathers and continued growth produces an adult bird.

Kangaroos
Adult male roos fight to determine "dominance", with the strongest males winning the right to mate. Mating occurs by the male inserting his penis in the female's vagina and depositing sperm which fertilise the egg cells. The embryo develops in the womb until it is about 3 cm long. Then it emerges through the birth canal and crawls across the fur into the pouch following a scent trail licked by the mother. Here it grows, attached to a teat, until about four months old when it starts to come out occasionally and eat grass, returning to the pouch for protection and further nourishment. After growing to maturity, adult kangaroos will reproduce only if environmental conditions are favourable.

Flowering plants
The life cycle of flowering plants involves seed germination, seedling growth, formation of flower, gamete production (pollen and ovules), pollination, fertilisation, development of the seeds inside a seed pod or fruit and dispersal of the seeds.
(For full description see Plant systems C: Reproduction and flowering plants)

Tapeworms
Tapeworms are an example of a hermaphroditic organism which must self-fertilise for the species to survive.

Tapeworms are parasites in the intestines of warm-blooded animals such as dogs and humans. They are usually hermaphroditic, producing both eggs and sperm. The self-fertilised eggs pass out with the faeces for dispersal.

Pet dogs can easily become infected. Children should wash their hands after playing with dogs as the tiny eggs are easily picked up and passed to the mouth when eating, starting the growth cycle again.

Shore crabs
This is an example of an organism which shows several larval forms in its life cycle.

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