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.

Male And Female

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.

Hermaphrodites

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."

Differences In Size

Another sexual difference among mammals that doesn't hold with other animals is size.

In most mammals the male is larger than the female, but in most other animals where there are two distinct sexes, the female is the larger one.

Producing Eggs Or Producing Sperm

Today scientists define female as the ability to produce eggs and male as the ability to produce sperm. But this is a relatively new distinction since until the 19th century no one knew whether mammals even produced eggs.

And it wasn't until the 20th century that we were able to see the similarities between mammalian eggs and other eggs.

So even though we talk about male and female in most species of animals, the two terms can have extremely different meanings.