No other bone, arguably, has been the subject of such debate as the human penis bone. Mainly, why other animals have them - in diverse sizes and varying lengths - and we don’t.
Monkeys’ penises are as short as a fingernail, while walruses’ can be two feet long. The absence of the bone in men has long perplexed scientists, but a new study appears to have shed light on the strange anomaly.
Scientists at UCL have reconstructed the evolutionary story of what is called the “baculum” by tracing the bone back through history.
It first evolved in mammals between 90 and 145 million years ago and from that point on either shrunk or grew, depending on the species.
The study found that animals engaged in sex for longer than three minutes, or “prolonged intromission”, tended to have longer penis bones.
If animals also faced high levels of sexual competition during the act, they also tended to be longer.
The researchers believe that humans mating system became more monogamous about 1.9 million years ago.
It was at this point that the study indicates humans may have lost the penis bone after the evolutionary pressures dried up.
UCL’s Dr Kit Opie, co-author of the study, said: “This may have been the final nail in the coffin for the already diminished baculum, which was then lost in ancestral humans.”
Chimpanzees and bonobos, humans’ closest relatives, have very small bacula, because they have such short intromission durations.
The former spend just seven seconds in intercourse, while the latter take just 15 seconds.
However, both face high levels of competitions from other males during intercourse, which researchers believe explains why they retained the bone.
UCL’s Matilda Brindle, first author, said: “Our findings suggest that the baculum plays an important role in supporting male reproductive strategies in species where males face high levels of postcopulatory sexual competition.
“Prolonging intromission helps a male to guard a female from mating with any competitors, increasing his chances of passing on his genetic material.”
The study was published in the Royal Society journal Proceedings B.