Why do some animals change sex during their lives?

Some animals change sex because, in their species, being male or female works better at different sizes or social positions. A clownfish can become female when the breeding female is gone; some reef fish become male when they grow large enough to guard space and attract mates.

The change is run by the animal's own body. Signals from the group shift, hormones change, and organs develop in a new direction. In a reef family, one missing fish can alter the future of the next one in line.