The penis bone, or baculum, has long been a knobby issue for scientists. The bone, which dangles at the tip of a male’s reproductive organ detached from the skeleton, pops up in many placental mammals. Mice, bats, cats, dogs, and hedgehogs all sport structural reinforcement in their nether regions. Walruses possess startling two-foot-long models that resemble baseball bats. Most primates, including our closest relatives, also have members in the baculum club—but humans, oddly, do not.

Over the years, researchers have come up with a number of hypotheses for why man’s manhood is boneless, but a new evolutionary study offers some evidence. Analyzing anatomy and mating practices of thousands of mammals, anthropologists Matilda Brindle and Christopher Opie of University College London suggest that humans lost their baculum due to quick sex and relatively little competition.

The findings, published Wednesday in the Proceedings of the Royal Society B, may finally start to nail down our understanding of the puzzling purposes of penis bones.

For the study, Brindle and Opie started by constructing a large evolutionary tree of mammals, trying to get to the roots of the baculum. They found that it appeared in placental mammals after they split from non-placental mammals, about 145 million years ago. However, the bones were hanging around before carnivores and primates evolved, around 95 million years ago. This suggests that more recently evolved species in these groups that lack a baculum, such as humans, lost the bony package.

Next, the pair evaluated the mating practices of species with and without a baculum, including testes sizes, polygamy or monogamy, seasonal breeding, and the duration of vaginal penetration, called intromission.

Over the course of primate evolution, the presence of penis bones tightly hooked up with longer intromissions—those more than three minutes long, to be exact. And the longer the sex, the longer the bone, the researchers found. Moreover, long bacula were also linked with more competition for females.

Brindle, Opie, and other researchers speculate that the penis bone allows for extended lovemaking sessions—or at least ones that last for longer than three minutes. In this scenario, the bone may act like a supportive rod, strengthening the penis and protecting the urethra while keeping it open.

But this longer sex isn’t a romantic gesture; it’s likely a strategy to not just get busy, but keep busy when there’s a lot of other male competition around. In the words of Brindle, “prolonging intromission like this is a way for a male to prevent a female from sneaking off and mating with anyone else before his sperm have had a chance to work their magic.”

The theory isn’t perfect: Bonobos have maintained a baculum—a very tiny one—yet only have sex for about 15 seconds in one go. Brindle and Opie speculate that stiff competition between males keeps the bone around.

For men, the data points to a softer crotch. The average amount of time between penetration and ejaculation for men is less than two minutes. And with monogamy becoming popular among humans after our split from chimpanzees and bonobos, the evolutionary chances of keeping penis bones went limp.

Proceedings of the Royal Society B, 2016. DOI: 10.1098/rspb.2016.1736 (About DOIs).