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Most other mammals have penis bones, so why don't we?

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If you only clicked this video because you saw the words “penis bones”: welcome.

I hope you learn something. Penis bones — a literal bone inside a penis, aka os penis, or a baculum — are a totally real thing, and they’re a legit scientific topic.

You may not be familiar with them, though, because no matter what anyone might claim, humans don’t have them. Very rarely, someone will form a bone in their penis because of a buildup of calcium, but that’s not a true baculum. Plenty of other mammals, though, do have a baculum: mice, cats, your dog...

In fact, it’s probably easier to list out the mammals that don’t have them than those that do. Even tiny pikas have real bone down there. So do our closest relatives, chimpanzees.

And some female mammals even sport a baculum equivalent called a baubellum, which is a clitoral bone. Just a handful of primates, including spider and woolly monkeys, are like us. So, what happened to our penis bones?

The question has puzzled scientists for years, and may say more about our sexual practices than you’d think. The first thing you might notice about bacula, if you took a bunch from lots of different animals, is how diverse they are. Some are small, like the length of a fingernail.

Others are massive, like one extinct walrus’s 1.5 meter penis bone. That record-holder sold for $8,000 at an auction, an amount that was actually considered disappointingly small. Bacula also differ wildly in shape.

Some are relatively straight, while others look like hooks or even tridents. Biologists think the baculum is the most diverse bone in the entire body, and they seem pretty important for reproduction. But exactly how they’re important isn’t so obvious.

One idea is that penis bones help stimulate ovulation, which is when females release oocytes, or egg cells, and are capable of becoming pregnant. While humans ovulate spontaneously, many mammals, including ferrets, cats, and mice, are induced ovulators, meaning they ovulate only once they’ve had sex. The only problem is, having a penis bone doesn’t correlate so well with being an induced ovulator.

Cats, for instance, which are notorious induced ovulators, have much smaller bacula relative to their size than dogs and wolves, which are spontaneous ovulators. Another possibility is that the penis bone makes it easier to get inside the female’s vaginal opening. But that’s also problematic, because penis bones aren’t more common in species with bigger size differences between males and females.

So, in a study published in 2016, two anthropologists decided to get to the bottom of the bacula mystery by looking at the evolutionary biology behind the penis bone. They thought a third possibility, known as prolonged intromission, was more likely. It had been tested before, although the earlier results had been mixed.

Really, “prolonged intromission” is just a technical term for very long sex. A longer penetration time prevents the female from going off and having sex with other males, so the male’s sperm is more likely to fertilize an egg. As you might expect, a bone is useful for this purpose because it can basically act as a supportive rod.

It keeps everything safely erect, including the urethra, which is important because that’s how the sperm get where they need to go. The researchers found that for primates, longer sex times correlated with having a baculum, and with how long the penis bones were. Generally, once animals had evolved long intromission times, or anything more than three minutes, they kept their penis bones.

It’s when those times were short that species lost them. Including us. Because yes, scientists have studied this, and the typical penetration time is only about two minutes.

All in all, the team concluded that bacula evolved some 95 to 145 million years ago — after the evolution of the first placental mammals, but before primates and carnivores split. So bacula were the default for our ancestors. They also noticed that primates that were seasonal breeders or polygamous had longer bacula.

This goes along with idea that bacula are most helpful in species facing high postcopulatory sexual selection, where there’s competition from other males’ sperm. Experiments in polygamous house mice, where researchers varied the level of sexual pressure, found that they could actually change the mice’s baculum length over 27 generations. When the mice were under more pressure, their penis bones got wider.

Humans, though, have much less of this sexual pressure, in part because we’re mostly monogamous. Monogamy evolved in early humans millions of years ago, and might have contributed to the loss of our penis bones. We just didn’t really need them.

But don’t worry. We appear to have compensated in other ways for our missing bones, at least in terms of making sure sperm can get through. The human penis has a piece of connective tissue called the distal ligament, which helps keep it stiff enough for sex — no bone necessary.

At least, not a literal one. Thanks for watching this episode of SciShow, and for more on how sex contributed to our evolution as a species, you can check out our video about the sex lives of early humans. Featuring 2012 Hank with a goatee!