Why Don't Humans Have a Penis Bone? Baculum Study Shows Where Ours Went

For better or for worse, we may no longer need it.

Compared to the rest of animals on Earth, human beings are weird when it comes to sex. Walruses, chimps, and many other mammals have a sexual tool that human men lack: a penis bone. Scientists know that men have boners but, for a long time, didn’t know why boners didn’t actually have bones. This year, researchers came one step closer to understanding why.

In a study published in the September issue of the Proceedings of the Royal Society B, a team of scientists began their study by investigating the sex lives of animals that have a penis bone, technically called a baculum. Their data support a theory called the “prolonged intromission” hypothesis, and yes, it means exactly what it sounds like.

This story is #2 on Inverse’s 25 Most Surprising Human Discoveries Made in 2018.

The baculum is a mineralized bone that varies in size across animals. In a walrus, it can be two feet long, and in a chimp, it’s about the size of a grain of rice. Previously, scientists explained its existence with the “vaginal friction hypothesis,” which posited that the bone stiffens the penis and helps it get into small female vaginal tracts. The new study, in contrast, proposed that the penis bone’s function is to increase the amount of time it can stay inside a vagina.

The stronger the baculum, the longer the intromission, and the better chance the male has of impregnating and continuing his line.

A walrus baculum.


Case in point is the coati, a member of the raccoon family that has a baculum and an intromission duration of one hour. Humans, who lack a penis bone, last an average of 5.4 minutes. In light of the example of the coati and similarly well-boned animals, the scientists argue that penis bones must increase the length of time a penis stays in a vagina, which in turn is good for an individual animal’s fitness.

The team came to this conclusion after creating 3D models of different baculums and virtually crash-testing them into virtual vaginas. They determined that “the size and shape of the carinovran baculum have evolved in response to selective pressures on the duration of copulation and the protection of the urethra.”

The team did not, however, venture to guess why human penis bones disappeared during our evolution. In 2017, however, another group of scientists proposed an idea that the new findings are consistent with: We don’t need to have super long sex because the cultural invention of monogamy means you get to have more than once chance at having sex. It’s an idea that’s sort of sweet, if a little stiff.

As 2018 winds down, Inverse is highlighting 25 surprising things we learned about humans this year. These stories told us weird stuff about our bodies and brains, uncovered insights into our social lives, and illuminated why we’re such complicated, wonderful, and weird animals. This story was #2. Read the original story here.

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