A new study suggests that monogamy and how long humans have sex may be the reason why men don’t have bones in their penises, while other male mammals like bears, chimps and gorillas do.

The study, conducted by a pair of researchers from the University College London, found that prolonged intromission — penetration lasting longer than 3 minutes — was correlated to the presence of a penis bone, or baculum, which vary dramatically in length, width and shape among male mammals that have them.

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The research, which discovered that the penis bone first evolved in mammals between 145 million and 95 million years ago, found “prolonged intromission” predicted a longer baculum in primates and carnivores. The findings were published Wednesday in the Proceedings of the Royal Society, which noted the baculum’s description as the “most diverse of all bones” due to extreme variances across mammals.

“Our findings suggest that the baculum plays an important role in supporting male reproductive strategies in species where males face high levels of post-copulatory sexual competition,” one of the study’s authors, Matilda Brindle, said in a news release. “Prolonging intromission helps a male to guard a female from mating with any competitors, increasing his chances of passing on his genetic material.”

Monogamy may have also played a role in why humans do not have a baculum, as there’s a lack of the sexual competition present in other species, where mating occurs between multiple males and females.

“Interestingly, humans have neither prolonged intromission durations, nor high levels of post-copulatory sexual competition,” Brindle said. “Given the results of our study, this may help to unravel the mystery of why the baculum was lost in the human lineage.”

The researchers suggest that polygamous mating systems — like those among chimpanzees and bonobos, the closest relatives to humans — may be the reason why those species have retained a baculum that’s roughly 6 to 8 millimeters (about ¼ inch) long. For comparison, a fossilized, 4.5-foot baculum of an extinct walrus species was sold for $8,000 at an auction in 2007, according to the Washington Post.

Brindle’s research partner, Kit Opie, told The Guardian that humans might have lost their penis bones about 1.9 million years ago, when monogamy emerged as the dominant reproductive strategy.

“We think that is when the human baculum would have disappeared because the mating system changed at that point,” Opie said. “This may have been the final nail in the coffin for the already diminished baculum, which was then lost in ancestral humans.”

Put simply, with reduced competition, there’s less of a need for a baculum, Opie said.

“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,” he said.