How animals reproduce

Animals fertilise eggs in two ways:

• External fertilisation (frogs, most fish): the female lays eggs in water and the male releases sperm over them. Many eggs are washed away or eaten.
• Internal fertilisation (reptiles, birds, mammals): it happens inside the female's body, where the embryo is better protected, so more young survive.

More eggs where fewer survive (Table 11.4):

"Why do butterflies start as caterpillars?" Fish, amphibians and insects lay huge numbers of eggs. A mother can't pack much yolk (stored food) into thousands of eggs, so each has just enough to make a larva: a young stage that feeds itself, often on waste matter. Later it changes completely into the adult. This change is metamorphosis:

• butterfly: egg → larva (caterpillar) → pupa → adult;
• frog: egg → tadpole → adult frog.

Reptiles and birds lay fewer eggs, each with enough yolk to feed the embryo until it hatches. In mammals, the baby grows inside the mother's body.

After birth, some young can find food straight away; others need long feeding and care. Mammals feed their young on breast milk.