Penis bones are a curious thing. Known as a baculum, a large number of male mammals, from bears and tigers to ferrets and elephant seals, have developed a rigid, mineralized, stick-like bone through the middle of their penis. It also happens to be one of the most morphologically diverse bones in the world. However, many species of mammals do not feature any such penis bone – most notably us, humans – leaving some researchers to wonder what role it serves. 

According to a new study, the penis bone in some species might have evolved for an act of “post-copulatory sexual competition” that involves scooping rivals’ sperm out of the female. Whether or not a species indulges in this dirty trick can be seen in the shape and size of the bone, and also appears to reflect the animals’ social structure.

As reported in the Proceedings of the Royal Society B, scientists from the Manchester Metropolitan University in the UK set out to understand the different shapes and sizes of penis bones found among carnivorous mammals. Using images obtained through 3D scanning and computer simulation techniques from their previous work, they studied the different baculums of 82 species of carnivores, including the red panda, snow leopard, southern elephant seals, brown bear, honey badger, European otter, and tigers.

Their analysis revealed some interesting trends. Generally speaking, they found that animals with penis bones that were simple, smooth, and straight were more likely to be multiple-mate species or “group living” species, such as elephant seals, grey seals, and walruses. In this scenario, just a small number of successful males are able to breed with a harem of females, meaning sperm competition is often pretty low. 

On the other hand, animals with a gnarly and complex-shaped baculum with odd tips were more likely to be socially monogamous. While you might assume a strong and stable relationship means sperm competition is low, it can also mean that males are more eager to ensure their paternity and sperm competition can remain high. Equally, socially monogamy doesn't necessarily mean genetic monogamy; extra-pair copulation, a promiscuous mating behavior, is found in many monogamous species. As such, the researchers think the complexly shaped bones of some species could be a tool for “postcopulatory sexual selection” involving the removal of others’ sperm from the female. 

"They’re very surprising and counterintuitive results, we initially expected the opposite," Dr Charlotte Brassey, lead study author and lecturer in Zoology at Manchester Metropolitan University, told IFLScience.

"When we find that dogs in ’socially monogamous’ pairs actually have really complex bacula, I think that’s because their bacula has anatomical adaptations that are actually useful when the males are trying to guard against another sneaky male trying the mate with the female. We know this can be quite common in canids," explains Dr Brassey.

"And likewise, when we find that ‘group living’ species like the land-breeding pinnipeds have simple bacula, I think that’s because their ‘harem-like’ mating system actually means the dominant males do a really good job of preventing any ’sneaky’ matings, so they need to worry less about another male’s sperm," they added.

There might be more to the story, however. The researchers note that many penises also feature a chunky piece of cartilage that surrounds the bone, altering the overall shape of the penis. Unfortunately, this is missing from most museum specimens, which only consist of an ossified bone. Very little is also known about the other half of the story: the three-dimensional shape of female genitalia seen in mammals. This means some of the assumptions about penis shape acting as poking rods or scoops may not be strictly correct.

Nevertheless, the long-standing mystery of the penis bone is starting to be revealed.