Reproduction (What you don't know)

Earthworms are mating. Unless disturbed, they will remain above the ground and joined like this for several hours. Each worm produces both sperm and eggs, each donates and receives sperm during mating, and each will produce fertilised eggs. In a few weeks, sexual reproduction will be completed when new worms hatch.

A male Caribbean bluehead wrasse and two smaller females feed on a sea urchin. All wrasses of this species are born female, but the oldest, largest fish complete their lives as males.

These fish live in harems consisting of a single male and several females. If the male dies or is removed in an experiment, the largest female in the harem becomes the new male. Within a week, the transformed individual is producing sperm instead of eggs. In this species, the male defends the harem against intruders, and thus larger size may give a greater reproductive advantage to males than it does to females. In contrast, there are male–first animals that change from male to female when size increases. In such cases, greater size may increase the reproductive success of females more than it does males.

External fertilisation. Many amphibians shed gametes into the environment, where fertilisation occurs. In most species, behavioural adaptations ensure that a male is present when the female releases eggs. Here, a female frog, clasped by a male (on top), has released a mass of eggs. The male released sperm (not visible) at the same time, and external fertilisation has already occurred in the water.