Unlike humans, many of our male mammalian kinfolk have a baculum, a.k.a. the penis bone. Cats, skunks, dogs, rodents, bears, otters, primates, whales, walruses, raccoons; they've all got it. Elephants, dolphins, kangaroos, whales, horses, cows, pigs, kangaroos; they do not. So, why do some mammals have it when others don't, and what is its purpose? While researchers have struggled to answer these questions, University of Southern California biologist Matthew Dean and his colleagues decided to look at how many times the penis bone evolved.

After looking across 954 mammal species to check for the presence or absence of a baculum, Dean and his colleagues determined that the enigmatic bone independently evolved nine times and was subsequently lost in 10 different lineages. This means that the baculum is not an ancestral trait, but something that has popped up over and over again in mammalian history.

This only raises more questions. Why gaining, or losing, a penis bone is advantageous remains an open question. “There is nothing in common among species with a baculum versus species without,” Dean says. And solving the mystery “is not some weird niche of science.” The rapid and repeated evolution of bacula, Dean says, “is an absolutely fundamental pattern of evolution in almost all sexually reproducing organisms. This pattern demands our attention.” More than that, understanding the ways in which the baculum forms may have practical applications that extend beyond natural history. “The answers to this could actually help us understand, for example, how to bio-engineer bone in a petri dish," Dean says.

Head over to the Washington Post to read more about the baculum and its female equivalent, the baubellum.

Feature image via Museum of Toulouse