Broods and care.

Step 1 of 6

Inside or out

Almost all animals reproduce sexually: a male’s sperm joins a female’s egg, and the egg is fertilised. The Scene holds four mothers, each in a display case of her own: a cod, a frog, a blackbird and a red deer hind. Time is at Laid or born: a million cod eggs drift in the water, two thousand frog eggs sit in a clump of jelly, four blackbird eggs lie in a nest, and one calf lies in the grass.

In external fertilisation the female releases eggs into the water and the male releases sperm beside them. In internal fertilisation the male passes sperm into the female’s body, and the eggs are fertilised there.

Step 2 of 6

Almost all die

Larvae and tadpoles are eaten by fish, jellyfish and dragonfly larvae, starve, or drift somewhere too cold. Nests are robbed, and calves are taken. The chart beside the Scene shows each brood’s survivors on a logarithmic scale, where each line is ten times the one below, so a million and one fit in the same column.

Step 3 of 6

The first weeks

Losses are sharpest at the start, when the young are smallest, and for the cod and the frog nobody guards them. Time is locked at about $45$⁠ days after laying. Above each case is how many young are alive.

Step 4 of 6

One budget

A mother can put only so much food into breeding each year. She can split it into many tiny portions or a few large ones. Brood size scales every brood against what the animal has in nature, but the mother’s budget stays the same, so each young gets less.

Step 5 of 6

Care makes few possible

A blackbird that laid four eggs and flew off would raise no family: the eggs would chill, and the chicks could not feed themselves. A few young succeed only when the parents put the effort saved into them: warming, guarding, feeding and nursing.

Step 6 of 6

Two strategies

r-strategists make huge numbers of small young and leave them to chance: cod, frogs, most insects. K-strategists make a few large young and look after them: elephants, whales, deer. Most animals sit somewhere between. In any population that lasts, each pair of parents leaves on average about two young that grow up and breed, to replace themselves.