Why Don’t Humans Have a Penis Bone?

There are 206 bones in the human body, and it’s the same for men and women. Women don’t have fewer ribs than men, and men don’t have one extra bone. But several mammals, including our close relatives like chimpanzees, do have that extra bone. So, why do other animals have penis bones while humans don’t?

It wasn’t always this way. Researchers who traced its evolutionary timeline found the penis bone, also known by its scientific name, the baculum, first evolved around 145 million years ago after placental and non-placental mammals split. This means that way back, our ancestors had a baculum, and on the way to being human they lost it. The researchers put that about 1.9 million years ago.

The reason why we lost our baculum may be down to our mating strategy. Baculum length in primates is closely correlated with intromission time, which is how long the penis is inside the vagina. What constitutes a long intromission? According to the researchers, 3 minutes. Humans don’t quite make the cut.

Our average time from penetration to ejaculation is less than two minutes, so it looks like we just don’t need the extra support. There are primates like bonobos who copulate for 15 seconds at a time, and yet they still have a baculum. This might be because they’re polygamous primates, where many males mate with many females, meaning competition is stiff so they have to be, too.


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