Despite some incredibly misleading euphemisms, human penises are entirely boneless. But that’s not the case for many of our mammalian relatives. From cats and dogs to primates and rodents, the males of many mammal species have a special genital bone called the baculum. Researchers have long been unsure whether this mysterious structure evolved just once, early on in the history of beasts, or many times. After examining the membership of the mammal family tree, scientists now believe they have the answer.

Genital bones are common in both males and females of many mammalian species. Females often possess a bone analogous to the baculum called the baubellum or os clitoris. The purpose of these bones, however, remains unclear.

What good is a baculum (or, for that matter, a baubellum)? “That is the most important question,” says University of Southern California biologist Matthew Dean, “and one that keeps me up at night.” Mammalogists have proposed several functions — from keeping the penis stiff during copulation to signaling the fitness of a male — but, so far, the only idea with strong support comes from a study of mice in which the width of the baculum was tied to how many offspring the males sired.

Then there’s the question of how many times the baculum evolved. “If you asked a mammologist,” Dean says, “they would look up to the sky, think for a bit, then tell you that it must have evolved multiple times,” given how many different lineages of beasts have the bone. It's unlikely that such a variety of creatures would have sprung forth from a common, penis-boned ancestor. It would make more sense for the perplexing adaptation to have arisen in different lineages at different times, perhaps even for different reasons.

But there was no reference for this until Dean and his colleagues decided to write one. The results, presented as part of “The Morphological Diversity of Intromittent Organs” symposium that took place this year, are published in Integrative and Comparative Biology.

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.

This isn’t just about the males, either. Very little is known about the female equivalent of the baculum, the baubellum. Dean and his colleagues are looking at the history of this neglected bone as well, although it won't make the story any simpler. “If you thought the evolution of the baculum was difficult to understand,” Dean says, “the baubellum is just evil.”

Brian Switek is a freelance science writer and the author of "My Beloved Brontosaurus." For more, read his Scientific American blog and follow him on Twitter or Instagram

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