Even though many mammals, including our closest primate relatives, exhibit the presence of penis bones, humans seem to be the exception. Recently, scientists have announced that they have discovered the cause of the phenomenon: monogamy.

All Sorts Of Shapes And Sizes

The penis bone, or the baculum, has been around for a very long time. It developed especially in primates and carnivores between 145 and 95 million years ago and became one of the most spectacular feats of evolution.

In fact, many scientists consider it to have reached the biggest variety among all the other bones found in mammals. Thus, some mammals, like humans, don't have it at all, while others have very small ones (a few millimeters or less than an inch long) and other species exhibit quite impressive bacula (2 feet long in walruses).

In light of these diverse shapes and sizes, scientists were curious to find out why humans don't have this adaptation.

It's All About Competition

Researchers at University College London conducted an extensive study regarding this subject and reached the conclusion that a species' reproductive strategy is very likely to influence the size or even the existence of the baculum. It all depends on postcopulatory competition.

In some species, numerous males mate with numerous females, sometimes within a very short period of time. Thus, even if penetration occurs, a male cannot be certain that the offspring will be his if another male immediately engages the female.

The best solution for this particular problem is to prolong the intercourse as much as possible. The threshold identified by researchers seems to be around three minutes.

In species that mate for more than three minutes, the baculum is more developed, providing support to the male during penetration and also increasing the chances that his sperm will reach the egg.

In the case of species where this competition is not as fierce, like bonobos, penetration lasts less than half a minute and as a result their bacula are just 6-8 millimeters (0.2-0.3 inches) long. In the case of humans, competition is even more reduced because of monogamy becoming the dominant reproductive strategy.

Thus, a female is much less likely to be impregnated by another male after having intercourse with her partner. As a result, the existence of the baculum is no longer necessary, which may disappoint some, as explained by Kit Opie, who coauthored the study with Matilda Brindle.

"With the reduced competition for mates, you are less likely to need a baculum. Despite what we might want to think, we are actually one of the species that comes in below the three-minute cut-off where these things come in handy," Opie said.

The findings were published in the Proceedings of the Royal Society B on Dec. 14.