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SciShow recently began producing its fifth year of content. Something that we are extremely proud of. And we're also proud of the community that's grown around these videos. We've covered a lot of topics, both trivial and profound, and we worked very hard to capture both our fascination and excitement as well as our deep desire to always get things right. 

And over the last year, you may have noticed a comment on, we think, every single SciShow video, asking us one question. We have ignored this question long enough, it is time we took it on. litojonny wants to know, why does he have hair around his anus?

Well, jonny the reason we haven't answered you is because, you know, like despite the fact that everyone gets their own personal pocket sized supercomputer, and that we can send robots to Mars, and convert the entire face of a planet to human use, we still do not really know why humans have butt hair. And it may not surprise you to learn, that not a whole lot of research has been done on the "why" part of this question about butt hair.

But a fair amount of study has gone into the medical problems that butt hair can cause. For example, pilonidal disease is a chronic skin infection caused by hairs that get embedded near the top of the butt crack, which, if you want to impress your doctor, you can describe by its technical name, the intergluteal cleft. So as the owner of a butt yourself, you probably know that butt hair seems to have more downsides than upsides. So given that, what, if any, purpose does it serve?

Well there are a few theories out there and maybe some enterprising scientist out there, watching right now, can do some research on them. But here's what has been proposed. Theory number 1: butt hair exists, because there's just no significant evolutionary pressure against butt hair. Sure, it's sometimes inconvenient, and, depending on the moment in cultural history, it might be considered unsightly,  but it appears that butt hair has never been a significant reason for one human not to make babies with another human. It's important to keep in mind that not every bit of our physiology needs an evolutionary purpose, so butt hair might just be another side effect of unintelligent design. 

Theory number 2: scent communication. Body odor definitely has a negative connotation in today's world, but there's little doubt that communication through scent has played an important role in the evolution of humans. After all, that's likely why we have body hair in the same areas where we produce body odors. The hair is there to hold onto sebaceous, or oily, secretions that have their own smell and are also consumed by bacteria that create even more smells. Since we all produce different smell compounds, and have our own microbiomes, each individual human actually smells different. And if our early human ancestors were anything like other animals, and they probably were, their personal smell probably helped with everything from broadcasting territorial rights to attracting mates. Butt hair then may be just another way our oldest human ancestors enhanced their smell profiles. 

Theory number 3: friction. In addition to giving off smells, humans have also always done a great deal of walking and running. And skin rubbing on skin (especially in areas where skin may be moist and dirty) can cause irritation, rashes, and even serious, debilitating infection. It's even possible that those sebaceous or waxy secretions that help produce body odor, are held in place by body hairs to provide an added benefit, acting like a natural anti-chafing cream. Now this theory, of the ones that we have talked about, is most appealing to me, personally, but it's very difficult to test, because shaving or otherwise removing butt hair and then having someone run 20 miles on a treadmill is not a good experimental design. Because there's no way to know whether any irritation is caused by the lack of hair or whatever technique was used to remove the hair. None of which sounds fun to me. 

But I have come up with an alternative design that I like quite a lot. Just interviewing a few hundred runners about how much they need to worry about butt chafing, and then measure the density of their anal pelage, to see if there's any correlation between whether they chafe and how hirsute their butts are. Which is not an experiment I want to do personally. But if there's an expert out there, in anatomy and physiology, who is up for tackling this prickly problem, please, take it on. And if you get any useful data, definitely let us, and litojonny, know how it went. 

Thank you for watching this episode of SciShow, especially to litojonny, for being so persistent. Ah... What are gonna do now? We've answered you're question, you're gonna come up with a new one? I challenge you to come up with a new one. Thank you for watching and if you want to go to YouTube.com/SciShow and subscribe, that would be great. 

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