PENILE stiffness is the stuff of smutty jokes. In Darwinian terms, though, it is no laughing matter. Intromission, the meeting of penis and vagina, is crucial to reproduction. With insufficient stiffness, intromission will not happen and the genes of the male will fail to make it into the next generation.
It is no surprise, therefore, that many male mammals have a bone, known as a baculum, in their penises to add to stiffness. What is surprising is that many others—men included—do not. What causes a baculum to evolve is not clear. But a study just published in the Proceedings of the Royal Society, by Matilda Brindle and Christopher Opie of University College London, has shed some light on the matter.
Ms Brindle and Dr Opie have reviewed what data there are about mammalian bacula, particularly those of primates and carnivores, and compared these with what is known about different species’ sex lives. They picked the primates and the carnivores in particular because both groups contain some species whose males have a baculum and others whose males do not. (The picture above is of a skeleton of an extinct wolf species, in which the bone is particularly prominent.)
The researchers’ prediction was that species with a baculum will be those in which male-male competition is worked out more at the level of the sperm, in the female’s reproductive tract, than it is at the level of the individual, by fighting and fancy display. There is a precedent here. In primates, testis size is inversely correlated with harem formation. If you, as a male, have fought off the competition and established reasonably exclusive access to a group of females, then your sperm are unlikely to be competing directly with those of other males. You therefore need to generate fewer sperm, and so can get away with smaller testes. This, the story goes, is why gorillas, which form harems, have much smaller testes, relative to their body sizes, than do chimpanzees, which are promiscuous. (Men’s testis size lies between these two extremes.)
It might therefore be expected that baculum size correlates with testis size. Surprisingly, Ms Brindle and Dr Opie found that it does not. They did, however, find three different but pertinent correlations. First, despite the lack of a relationship between baculum size and testis size, there was a clear one between the bone’s length and a species’ promiscuity: more promiscuous species had longer bacula. Second, species with specific mating seasons, rather than all-year-round mating, had longer bacula. Third, there was a strong correlation between the length of the bone in a species (scaled for the size of the animal in question), and the average length of time intromission lasted in that species.
All of these observations make sense if the baculum’s purpose is to compete with the mating efforts of other males. Promiscuity increases the risk that a female will be inseminated by another male before the first male’s spermatozoa have had a chance to fertilise the female’s eggs. Seasonal breeding similarly piles on the pressure, by concentrating mating attempts into a small period of time. And increasing the length of coitus, which a baculum’s stiffening presence permits, reduces the time available for competitors to engage in a successful mating of their own.
Ms Brindle’s and Dr Opie’s prediction thus turns out to be correct—and it applies to people, too. The lack of a baculum in humans is of a piece with the lack of a mating season and with the existence of a pair-bonded mating system that has, by comparison with many other species, only limited levels of promiscuity. As for the length of time that sexual congress lasts in Homo sapiens, the adequacy of that is, perhaps, not a matter into which science should dare to trespass.