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. 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.