Monkeys have them. In walruses, they might be up to two feet long. Mice have teeny, tiny ones. So why don't human men have a penis bone? Scientists have a theory, and gentlemen, it might hurt your ego a bit. While researching the bone—known as the baculum—researchers at the University College London discovered it first appeared in mammals about 95 million years ago and in primates about 50 million years ago, reports the Guardian. That's when it began to change size, stretching in those who engaged in sex that lasted longer than three minutes. Humans (sorry, guys) couldn't count themselves among that group. After all, "the average duration from penetration to ejaculation for human males is less than two minutes," researcher Matilda Brindle writes at the Conversation.
Researchers suspect human sex was so speedy that the human baculum, which they have yet to prove existed, essentially disappeared. But there's more to the equation. Brindle explains the baculum—which "floats" at the tip of the penis rather than being anchored at the base—provides structural support during lengthy mating sessions, which serve to reduce the risk that another male might steal the female away to breed. For humans, that risk was diminished with the rise of "monogamous or polygynous (one male, multiple females) mating systems" around 1.9 million years ago, Brindle tells Seeker. "With the reduced competition for mates, you are less likely to need a baculum," adds study co-author Kit Opie, whose work is published in the journal Proceedings of the Royal Society. (In some cases, penis size matters.)