You may not believe this, but the engorged member—the thing you might call a “boner”—lacks any sort of bones at all (at least in humans).

This is peculiar, considering most mammals (and nearly all primates) have a penis bone or baculum (as the scientific community calls it). We prefer to call it a penis bone. Say it out loud: penis bone.

Why humans (and whales and dolphins) lack a penis bone is a hard question to answer. To understand why this is, we have to take a penetrating look at animal evolution.

Why would one even need a penis bone you may wonder? Well, sex is violent. Or it can be. Rough sex can potentially hurt a fragile penis. This is especially true for walruses, an animal that is known for aggressive mating behaviors (those kinky bastards).

In fact, walruses possess the largest baculum out of all animals, measuring up to 60 cm long, writes biologist Carin Bondar in The Nature of Sex.



Walrus penis bone (Nick Nyugen/CCBY)

A penis bone can essentially protect that vulnerable member. Though you must remember—the baculum is still a bone, which means it can sometimes fracture (ouch).

Bondar suggests, however, that another function of the penis bone may be to “keep the ladies happy.” As you would expect, a baculum can help males maintain longer erections.

Some primates even possess “penile spines.” While this sounds, um, painful, scientists suggest that it may stimulate the female, says Bondar. But spines can also damage a female’s reproductive tract, which stops her from doing the nasty for a short time, ensuring the original suitor’s seeds are sowed.

Diversity of penis bones in mammals (William Brudon/Current Biology)

Other animals evolved their very own set of vaginal accessories to help them deal with the baculum. Ground squirrels have vaginal folds that interlock with protruding teeth on a baculum.

If the penis bone can benefit both sexes, why do most humans lack this curious quirk of nature? (Notice the word most? There are 40 known cases in the world of human penises hardening into bone).

Let’s take a look at primates, shall we? Primates have a small penis bone. Incredibly small. Tiny. In fact, the baculum of chimpanzees is the same size as a grain of salt. It’s not a stretch to say that human’s ancestors may have had a baculum that disappeared through years of evolution.

While scientists are not sure why this um, shrinkage, occurred, they think it may have had something to do with monogamy.

In a study in which scientists bred 27 generations of mice, males that were polyamorous evolved thicker penis bones than those that were monogamous.

There is another alternative (and hilarious) theory on why humans lost their boners. As one Reddit user explains in a post in the Today I Learned community:

Yep—God almighty wanted your penis bone. In a article that was surprisingly published in the American Journal of Medical Genetics, theologians wrote (please take this with a grain of salt):

“Ribs lack any intrinsic generative capacity. We think it is far more probable that it was Adam’s baculum that was removed in order to make Eve. That would explain why human males, of all the primates and most other mammals, did not have one.”

Redditors, of course, were amused by this revelation.