Dog penis bones via Wikimedia Commons

Although we call erections “boners,” anyone who has basic familiarity with human anatomy knows our slang deceives us—for unlike most mammals, men have no penis bones. For decades scientists have wondered, “Why? No? Penis Bone?” A new study published in Proceedings of the Royal Society might finally answer the mystery of what killed the penis bone: monogamy.

To explain how, I gotta lay out some key baculum facts—that’s the scientific name for the penis bone—for all the PB n00bs out there. Penis bones are diverse in shape and size across many different mammals. The baculum of a walrus is two-feet long. In the bonobo, it is eight millimeters. In this new study, scientists set out to trace the evolutionary history of the baculum to account for the vast differences in penis bone length amongst animals.

The study ultimately discovered “a clear [relationship] between the bone’s length and a species’ promiscuity: more promiscuous species had longer bacula,” writes The Economist.

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Kit Opie, a postdoctoral research fellow at University College London who led the study, tells The Guardian that penis bone length was longer in males that engaged in “prolonged intromission”—meaning when species take more than three minutes to mate. Males use “prolonged intromission” to keep the female away from his competition while he impregnates her. Matilda Brindle, the study’s co-author, explains it like this on The Conversation:

Far from simply being a nice way to spend an afternoon, prolonging intromission like this is a way for a male to prevent a female from sneaking off and mating with anyone else before his sperm have had a chance to work their magic.

What scientists now theorize is that human lost their penis bones when monogamy became prevalent, about 1.9 million years ago. They no longer had the pressing need to keep away the competition by fucking for a long time. After all, “the average duration from penetration to ejaculation for human males is less than two minutes,” Brindle reminds us. (Dudes... get it together...) Nevertheless, despite the grumblings of lonely hearts, human males simply don’t have intense sexual competition because human females tend to mate with one male at a time.

And that’s why you don’t have a dick bone. Good night, boys.

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[The Guardian, The Economist, The Conversation]