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.