This is Why Male Humans Have No Penis Bones

Unlike other species close to our lineage, humans have no penis bones, and this could be because of the difference between our mating practices.

A study published recently at the Proceedings of the Royal Society B explains how humans lost their "bacula," the penis bones that are found in chimpanzees and most of our ancestor of all primates and carnivores.

According to the study, penis bones are most tightly correlated to increased "intromission duration," which is how long the penis is inside the vagina during sexual intercourse. The longer the intromission duration, the longer the bacula. However, longer intromission times often occur in species with polygamous mating practices. In present human's case, polygamy is not as extensive as what is seen in chimpanzees, which could be why bacula was lost as early humans evolved.

The Conversation reported that the walrus baculum is around a sixth of its body length, whereas ring-tailed lemur's is only around a 40th of its body length.

Wire reported that bacula's purpose is to act as a "supportive structure" during the prolonged penetration - protecting the urethra and preventing it from being constricted.

Another discovery as mentioned by the study is that males of species facing high levels of sexual competition for females have longer bacula than those facing lower levels of sexual competition.

"We think that is when the human baculum would have disappeared because the mating system changed at that point," Kit Opie who ran the study with Matilda Brindle at University College London told The Guardian. "This may have been the final nail in the coffin for the already diminished baculum, which was then lost in ancestral humans."

"With the reduced competition for mates, you are less likely to need a baculum," he added. "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."