What purpose does the penis serve? Why is it that members of one species have no penis at all, while those of another have two? Why do they vary so much in shape and size? And where do vaginas, or the ...

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 ...

June 21 (UPI) --For every animal, sperm perform the same function -- egg fertilization. Yet sperm size varies dramatically from species to species. Most studies on the topic have focused on how sperm ...

"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 ...