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Have you ever wondered why humans lost their penis bones? Especially when most mammals have one. Chimpanzees have them. Raccoons too. So, why they have the baculum and we don’t? Scientists thinks they found the answer.

Baculum bone in jar
Baculum/Didier Descouens/Museum de Toulouse/Wikimedia Commons

The baculum is the bone located above the male urethra. Penis bone is not attached to the rest of the skeleton and comes in the different sizes and shapes. The baculum first evolved between 145 million and 95 million years ago which means it was present in the most recent common ancestors. The purpose of baculum is to maintain sufficient stiffness during sexual penetration. Most primate males have one.  The fact that human males don’t have it is a rather exception than a rule.

Researchers from University College London (UCL) studies the evolution of a baculum and reasons why humans lost it. They brought a few hypothesis why baculum exists at all. One of the researchers, Matilda Brindle explains that in certain species, such as cats, a female’s body doesn’t release its eggs until she mates. So baculum may help to stimulate females and trigger ovulation. Another reason might be that the baculum acts as a shoehorn, enabling a male to overcome any friction and squeeze himself into a female.

This diagram compares the size the shape of the penis bones of the following bear species: A) sun bear, B) Asian black bear, C) Andean bear, D) American black bear, E) sloth bear, F) brown bear, G) polar bear and H) the extinct Indarctos arctoi.

Differences In Mating Practices

As all studies showed that penis bone main purpose is to maintain penetration to last as long possible, a scientist came to the logical explanation why we lost it.  During the history, people developed monogamy. The average duration from penetration to ejaculation for human males is less than 2 minutes. Due to monogamy relationships, human males minimized the sexual competition. Our dominant reproductive strategy makes the main difference from other mammal mating practices and this causes the baculum disappeared. In theory, the human male is safe with his female partner, as she is not likely to be leaped upon by other amorous males.

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