DK Science: Life Cycles
The life cycle of an animal consists of all the stages from the start of one generation to the beginning of the next. For many insects, it takes only a few weeks for the young to become adults and reproduce themselves, but for larger animals it can take years. Some animals reproduce once and die, but many reproduce repeatedly during their adult life. A number of animals undergo a transformation, known as METAMORPHOSIS, as the young animal changes, gradually or directly, into the adult form.
The schistosome fluke is a parasite – it lives in another animal, known as the host. This fluke uses suckers to anchor itself in human veins and feeds on blood cells. Flukes have a complicated life cycle, with a number of larval stages that live in different hosts. The larvae often live in molluscs, but the adult stage usually lives in a vertebrate animal, often causing serious diseases.
An African elephant’s gestation – the time it takes for the baby to grow in the womb before it is born – is 22 months, the longest of any mammal. When the baby elephant is born, all the herd take good care of it. Adult elephants have no natural enemies and can live to be 60 years old.
Metamorphosis involves a radical change from the young animal to its adult form. The young, known as larvae, live in a different way to the adults. Incomplete metamorphosis, seen in the transformation of a tadpole to a frog, involves a number of gradual changes. Complete metamorphosis, seen in the change from a caterpillar to a butterfly, takes place inside a pupa and totally rearranges the body parts.