The penis. It comes in so many different shapes and sizes … and that’s just in humans. As you would imagine, different species have very different penises.

The males in most mammal species, including cats, dogs and rats, have a bone in their penis called a “baculum”, or “os penis”.

Of course human males don’t possess an os penis – in fact, humans are the only primate species besides the spider monkey to be lacking in this department.

But we aren’t the only mammal not to have one: whales, horses, rhinos, rabbits, elephants, marsupials and hyenas all go without.

So, why do some species have a penis bone?

Well, simply put, to help males maintain an erection long enough to penetrate a female’s reproductive tract and deliver sperm. The baculum is generally kept in the male’s abdomen until it is required, at which point abdominal muscles push it out into the penis, thus causing an erection.

The other function of a baculum is speed. Sliding an already-erect bone into the fleshy penis is much easier and more reliable than waiting for the penis to fill with enough blood to maintain an erection long enough to deposit sperm into a female (as is the case with us ever-romantic human beings).

This speed is of real importance in many species, as mating often has to be quick and opportunistic. It also allows for quantity-over-quality mating. A male lion’s baculum, for instance, allows him to engage in an impressive 250 copulations in four days.

Sure, each copulation only lasts a minute or so, but the male’s ever-ready baculum makes it easy to get geared up for the next willing lioness shortly after his previous ejaculation.

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So, inevitably, this brings us to the question of why humans are the only apes to lack a penis bone.

Well, the reason is not entirely clear, but it’s believed to be down to our mating systems and strategies. In the 30th anniversary edition of his book, The Selfish Gene, evolutionary biologist Richard Dawkins suggested the lack of a human baculum is the result of “sexual selection” by female humans looking for healthy males.

That is, having a penis that relies on “hydraulics” to become erect (rather than a bone) means there will be some males with poor erectile function. If Dawkins is right, an innate female desire to guage a male’s suitability as a mate was responsible for “selecting” a penis that shows such suitability (or lack thereof).

Interestingly, our closest living relatives, including the chimpanzee, possess penis bones, but those bones are very small. It is possible that our primate relatives may too eventually lose their bacula.

Indeed, perhaps it’s more a question of why the other great apes still have bacula, rather than why we humans lack them. Complete loss of a baculum in humans seems to just continue a trend towards baculum size reduction which is found among the great apes.

It is also thought the presence of a baculum is associated with longer mating or perhaps just much more mating (as in the case of the lion discussed above). Perhaps the mating systems of humans are such that they don’t require this additional help.

I say “perhaps” here because so little has been published about the baculum that we really can’t say for sure. It would certainly be interesting to learn more.

And while we’re on the subject of learning more, did you know it’s not just males that have genital bones? There is also a female version of the baculum in some species which has a rather lovely name – the “baubellum”, or “os clitoris”.

While very little research has been done in this area as well, it seems to be generally accepted that the baubellum (which means “little gem” in latin) is essentially an equivalent of male nipples – a non-functional, under-developed version of the functional male counterpart.

So, will such bones, penile or otherwise, stand up to sexual selection? It’s hard to know.

A version of this article first appeared on PygmyLoris.