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The Curious History Of The Mammalian Penile Bone

A team of evolutionary biologists based out of the University of Southern California have recently published new research on the baculum, the mammalian penile bone. In a study of hundreds of different mammalian species, the scientists found that this curious bone has disappeared and reappeared throughout evolutionary history, sometimes at several points the same species’ genetic history.

Canus dirus, the dire wolf, proudly displaying its baculum

Canus dirus, the dire wolf, proudly displaying its baculum

The baculum has puzzled evolutionary scientists for years due to its seemingly vestigial purpose and the fact that it has appeared across a wide range of mammals at different points in their phylogenesis, or evolutionary history. Today, many mammals possess the bone, from bears to dogs and cats. Some primates even have penile or clitoral bones, while humans have lost the structures entirely.

The 22-inch walrus baculum in all its glory

The 22-inch walrus baculum in all its glory

The exact purpose of the bone is also still unknown, as mammals are just as capable of reproducing without the bone as they are with it, especially since some species have lost the bone over time but still manage to propagate their species. Some female mammals also possess an analogous bone called the baubellum, or os clitoris, which is just as shrouded in mystery.

With so much left unknown about the baculum, the team of scientists studied the phylogenetic histories of 954 different species, examining fossil records and genomic evidence. They discovered that the bone has appeared and disappeared around ten times in several species, implying that the penile bone does not in fact stem from a common bone-wielding ancestor, but instead develops independently among species for evolutionary reasons yet unknown.

A collection of mammalian penile bones of all shapes and sizes

A collection of mammalian penile bones of all shapes and sizes

According to the team’s published findings, the baculum does not fit with current theories of evolution:

The evolution of baculum size and shape does not consistently correlate with any aspects of mating system, hindering our understanding of the evolutionary processes affecting it. One potential explanation for the lack of consistent comparative results is that the baculum is not actually a homologous structure.

According to Matthew Dean, head of this study and principal researcher of USC’s Molecular and Computational Biology Dean Lab, understanding the curious evolutionary pattern of the baculum could have ramifications far beyond understanding mammalian reproduction. Because the baculum has disappeared and reappeared throughout history, it implies that some sort of evolutionary trigger or environmental stimulus has affected mammalian evolution. By studying the mechanism by which this has occurred, scientists might gain a better understanding of the forces driving evolutionary adaptations of all kinds.