The Intelligent Designer is a frickin' evil genius.
A mother gray nurse shark carries 40 or so embryos in her two wombs. But once an embryo develops jaws, it starts eating its siblings. Results: 1) Only one embryo survives in each womb. 2) The species is endangered.Update: Volokh reader ajf points us to more fetal shark-vs-fetal shark facts. The word is oophagous.
Update update: Some varieties of tadpoles eat their brethren to gain essential fatty acids. Yum.