Human males lost penis bone because sex was too short, researchers say

Male walruses have penis bones up to 2ft long, while those of monkeys are the size of a finger. But the human male does not have one at all, and scientists say they may finally know why.

A University College London (UCL) study has examined how the penis bone, known as the baculum, has evolved, and why humans do not have one when other mammals do.

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The penis bone has previously stumped researchers. It sits at the top of the organ, not connected to any larger skeletal structure.

According to the study, the first bacula appeared in mammals between 145 and 95 million years ago. From then, the baculum became larger in some animals and smaller in others.

The stump-tailed macaque, an animal that weighs just 10kg, has an extremely long penis bone for its size, extending for 5cm. In chimps, the penis bone is no longer than a human fingernail.

Researchers found that the penis bone length was longer in males that engaged in what is called “prolonged intromission.” That means the act of penetration lasts for more than three minutes, a strategy that helps the male impregnate the female while keeping her away from competing males.

During the time of Homo erectus about 1.9 million years ago, when monogamy emerged as the dominant reproductive strategy, human sex was over so quickly that the baculum became redundant, researchers say.

In monogamous relationships, the male does not need to spend a long time penetrating the female, because she is not likely to be leapt upon by other amorous males, researchers claim.

“With the reduced competition for mates, you are less likely to need a baculum,” Kit Opie, who helped run the study, told the Guardian.

“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.”

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According to scientists, the average duration from penetration to ejaculation for human males is less than two minutes.

The researchers also found that high levels of sexual competition between males also predicted longer bacula. As humans are monogamous or polygynous, where one male mates with multiple females, sexual competition between males is absent or very low.

Chimpanzees and bonobos - humans’ closest relatives - have very small bacula (between 6-8mm) and short intromission durations (around seven seconds for chimpanzees and 15 seconds for bonobos).

They are, however, characterized by polygamous mating systems, so they experience high levels of competition between males. Researchers suggest that might be the reason why the species have retained a baculum, even if it is tiny.

There has been another theory as to why humans do not have a penis bone. One reading of the Bible’s Genesis offers an explanation for the disappearing bone by way of a creation myth.

In 2015, Biblical professor Ziony Zevit suggested God made Eve from Adam’s baculum, not his rib. He argues that the Hebrew word ‘tsela’ does not translate as ‘rib’ but instead “refers to limbs sticking out sideways from an upright human body.”