One of the weirdest and most wonderful products of evolution is the penis bone or baculum. The baculum is an extra-skeletal bone, which means it is not attached to the rest of the skeleton but instead floats daintily at the end of the penis. Depending on the animal, bacula range in size from under a millimeter to nearly a meter long, and in shape, varying from needle-like spines to fork-like prongs.
The walrus baculum, which could easily be mistaken for a 2ft-long club, is around 1/6th of its body length, whereas the diminutive centimeter-long baculum of the ring-tailed lemur is only around 1/40th of its body length.
Bacula is found in certain species of mammal, but not all. Most primate males have a baculum, so humans are rather an oddity in that they don’t have one. In a handful of extraordinary circumstances human males have formed bones in the soft tissue at the end of their penises, but this is a rare abnormality, rather than a baculum.
In a study published in 2016 in Proceedings of the Royal Society B, my colleague Kit Opie and I examined how the baculum developed in mammals by studying how it is distributed across different species in light of their pattern of descent (known as phylogenetics).
We showed that the baculum first evolved after placental and non-placental mammals split, around 145 million years ago, but before the most recent common ancestor of primates and carnivores evolved, around 95 million years ago. Our research also shows that the common ancestor of primates and carnivores had a baculum. This means that any species in these groups without a baculum, such as humans, must have lost it over the course of evolution.
So, why on Earth would an animal need a bone in its penis in the first place? Scientists have come up with a few theories as to why a baculum might be handy. In certain species, such as cats, a female’s body doesn’t release its eggs until she mates, and some argue that the baculum may help to stimulate females and trigger ovulation. Another, somewhat colorfully named, theory is the vaginal friction hypothesis. This essentially argues that the baculum acts as a shoehorn, enabling a male to overcome any friction and squeeze himself into a female.
Finally, it has been proposed that the baculum helps prolong intromission, otherwise known as vaginal penetration. Far from simply being a nice way to spend an afternoon, prolonging intromission like this is a way for a male to prevent a female from sneaking off and mating with anyone else before his sperm have had a chance to work their magic. This theory brings a whole new meaning to the term “cock-blocking”.
We found that, over the entire course of primate evolution, having a baculum was linked to longer intromission durations (anything over three minutes). On top of this, males of primate species with longer intromission durations tend to have far longer bacula than males of species where intromission is short.
Another interesting discovery was that males of species facing high levels of sexual competition for females have longer bacula than those facing lower levels of sexual competition.
But what about humans? If the penis bone is so important in competing for a mate and prolonging copulation, then why don’t we have one? Well, the short answer to that is that humans don’t quite make it into the “prolonged intromission” category. The average duration from penetration to ejaculation for human males is less than two minutes.
But bonobos only copulate for about 15 seconds at a time and they still have a baculum, even if it is very small (about 8mm). So what makes us different? It’s possible that this comes down to our mating strategies. Human males (generally) have minimal sexual competition as females typically only mate with one male at a time. Perhaps the adoption of this mating pattern, in addition to our short intromission duration, was the last straw for the baculum.
Scientists are only just beginning to piece together the function of this most unusual bone. What seems to be clear is that changes in the primate baculum are driven, at least partly, by a species’ mating strategy. The picture that seems to be emerging is that, under high levels of sexual competition, bigger is better when it comes to the penis bone.