When you go to the pet store to pick out a new puppy, one of the first questions you may be asked is whether you want a male or a female. And when your friends have a new baby the first thing you find out is whether it’s a boy or a girl. We may be familiar with the physical differences between male and female in humans and other mammals, but outside of the mammals, the terms “male” and “female” take on some very different meanings. Aristotle defined females as those who could produce life inside themselves and males as those who could produce life in others. But the eggs of some animals, such as fish, are fertilized after they are laid so that the embryos develop outside of both parents. In other animals, such as earthworms, each individual has both male and female reproductive organs. Animals with both male and female organs are called “hermaphrodites.”