What purpose does the penis serve? Why is it that members of one species have no penis at all, while those of another have ...

"In species with internal fertilization -- like mammals, birds and insects -- sperm fertilize eggs inside the female's body, while in species with external fertilization -- like sea urchins and many ...

Sperm size varies dramatically among different animal species. But why is sperm size so variable when they share the same job - to fertilize eggs? In a new article published in Nature Ecology and ...