penis bone in a vial
December 14, 2016

Why don’t humans have penis bones?

Many male mammals have a penis bone, or os penis, yet humans do not.

According to a new study in the Proceedings of the Royal Society B, the relatively short length of human copulation time and a comparative lack of mating competitions are the two main reasons why guys don’t have a bone down there.

Past research has shown a penis bone offer the advantage of faster mating as the male doesn’t need to wait for a penis to fill with blood to become erect. Some researchers have concluded the bone stimulates the female reproductive tract in a way that increases the chances of egg fertilization.

penis bones

Penis bones are common when creatures spend more time having sex (Credit: University College London)

Studying Intercourse for Science

In the new study, scientists looked at multiple sexual attributes of primates and mammalian carnivores, including polygamy, testes bulk, seasonal mating and average penetration time – scientifically referred to as "intromission" time. For primates, the most effective predictor for the presence of a penis bone was if intromission continued for three minutes or longer. There was also a connection between lengthy intromission and size of the bone for both primates and carnivores.

Study author Matilda Brindle recently blogged that “humans don’t quite make it into the ‘prolonged intromission’ category. The average duration from penetration to ejaculation for human males is less than two minutes.”

These long primate intromission times are so female does not mate “with anyone else before his sperm have had a chance to work their magic,” Brindle said.

However, the three-minute-plus intromission time does not explain the presence of a penis bone in great apes like chimpanzees, which do the deed in seconds. Rather, the driving factor appeared to be competition, the study researchers concluded.

Chimpanzees are polygamous and mating rites are intensely fought over, the researchers noted. Conversely, humans are monogamous animals that mate year-round, the researchers argued, and therefore don't need a bone to copulate quickly.

“After the human lineage split from chimpanzees and bonobos and our mating system shifted towards monogamy, probably after 2 (million years ago),” study author Kit Opie, a University College London anthropologist, said in a news release. “This may have been the final nail in the coffin for the already diminished (penis bone), which was then lost in ancestral humans.”

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Image credit: Didier Descouens/Muséum de Toulouse/Wikimedia Commons