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Humans no longer have a penis bone because we don’t need to have as much sex

Use it or lose it

Jaume F. Lalanza via Flickr

Many of our primate relatives have penis bones. Humans don’t — and the reason is probably because we don’t need to have as much sex, researchers say.

In a study published this week in the journal Proceedings of Royal Society B, researchers traced the evolutionary history of the penis bone, or bacula. It’s been around for a long time (no pun intended), first evolving between 145 and 95 million years ago. That means the most recent ancestor that all primates and carnivores share had a bacula. And of course, chimps and bonobos still have theirs. But there are some key differences in sexual behavior between chimps and bonobos and us. They’re polygamous while humans usually aren’t, and being polygamous might make it more important to have lots of sex.

Scientists aren’t completely sure what the penis bone does. One theory is that it lets males have sex for longer by supporting the penis during sex. After tracing the evolution of the penis bone through 27 generations, the researchers found that polygamous species have much thicker penis bones than monogamous species, which supports the idea that the bacula is important for those who need to have more sex.

If you’re monogamous, you can usually trust that your partner isn’t having sex with lots of other people; being polygamous comes with no such promise. There’s more competition, and so it’s more important for the males to spend a lot of time having sex with their female partners so their DNA is the one that wins.

Basically, use it or lose it.


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