In a group of clownfish, size determines rank, and rank determines sex. Young clownfish are sexually immature. As they mature, they develop first as males. The largest and most dominant fish in the group becomes female, while the second-largest becomes the breeding male. Smaller fish remain sexually immature until a higher position in the hierarchy opens up.

If the female dies, the hierarchy shifts. The breeding male, now the largest fish, begins changing sex. Hormonal changes alter his reproductive system, and over time he becomes a fully functioning female. The largest of the remaining immature fish then matures into the new breeding male.

Clownfish are known as protandrous sequential hermaphrodites, meaning they mature first as males and can later become females. The change only goes one way: once a clownfish becomes female, it cannot change back.

This unusual system is particularly useful for an animal that spends most of its life close to a particular sea anemone. Instead of leaving the protection of its host to search for a new mate, the social hierarchy allows a new breeding pair to form within the group.