does evolution slack off in the bedroom?

A widely covered study alleges that humans lost the penile bone and a certain measure of endurance in mating. But that finding seems to be at odds with other studies.

tattooed pixie model

Many things in life are named somewhat deceptively in common slang, and one of them is the human penis, which evolved in some bizarre ways that saw it dramatically grow in size in relation to the rest of the body, and lose the bone it once had. Calling an erection a boner, in other words, is kind of like calling an NBA player short stuff. But while we believe that the change in its size over millions of years was simply female preference and a proxy for physical prowess in competition for mates, what happened to the bone itself? Why did it shrink until it finally disappeared for good? One paper is trying to find an answer to that and what it has so far is a tad bizarre, and somewhat unsatisfactory, to put it mildly. It posits that lack of competition rendered it irrelevant and our bodies found a different use for the requisite minerals. And as a consequence, it made sexual intercourse shorter. Sorry ladies. Nature can often behave like an evil genie like that, giving you what you wanted but with one hell of a catch.

According to the research, a penis bone is supposed to support an erection during intercourse, pushing the average time from start to ejaculation over three minutes. This gives a male a chance to make sure his companion isn’t going anywhere, giving him a better chance to impregnate her and continue the species, quite literally keeping her busy after she’s made her choice. But with humans, less competition meant there was little need for it so the bone was allowed to shrink and a new average of two minutes of intercourse was now enough to make sure the species went on. Nature, it seems, cares more about efficiency than satisfaction, and since serial monogamy and organized social groups made humans far more efficient at reproduction, that penile bone was allowed to vanish. Or at least, that’s the paper’s angle, which left some glaring questions unanswered. There are still apes with penile bones out there, and for most of them, intercourse lasts an average of 15 seconds to under a minute, and human males have a wide disparity in the duration of intercourse as any sexually active adult can attest.

While the average intercourse time for humans is frequently said to be two minutes, a definitive study on the subject found that to be 5.4 minutes, with data points as short as 33 seconds and as long as 44 minutes. Not exactly all that surprising when you consider a paper which shows that this time falls well within the bounds of what people seem to consider satisfactory: 3 to 13 minutes. Keep in mind that this study comes with a caveat, it used a rather small convenience sample of 50 psychology professionals, but it does sound like it’s not too far off most people’s anecdotal experience and is in line with the study on actual intercourse durations. In light of this evidence, the idea that shorter intercourse in humans rendered the penile bone irrelevant isn’t exactly iron-clad to put it mildly. Unlike you’ll see in pop sci news, the paper actually admits as much, saying that this hypothesis holds in certain models but falls apart miserably in others, leaving us with few conclusions.

To bottom line this, no, we do not know why humans lost the penile bone in the last million years or so and this paper sheds light on a couple of possible correlations between sexual behavior and bone presence and length, but it’s ultimately a hypothesis-fishing study that has nothing definitive to say. This doesn’t mean it’s a bad study, in fact I really appreciate the refusal to p-hack the results, and the admission that the predictive model they wanted didn’t live up to their expectations. But it does have that social media shareability as a study on sex, and joke fodder for a comedian or two if it gets covered in enough places, so that’s the scientists’ upside of studying the disappearance of the penile bone: fun and catchy PR for their research.

# sex // evolution / health / research

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