Why the naked ape really has pubic hair

Author: Matthew Stevens

The evolutionary origin of pubic hair in humans remains a diverting puzzle. Why, uniquely among the apes—and all other mammals—do we have a patch of hair in the pubic region but little elsewhere? The other great apes have less hair there than we have, so our evolution of hairlessness, evidently in parallel with our evolution of upright bipedalism, calls attention to the counterintuitive gain of pubic hair.

Various reasons have been proposed. The most obvious one—that it protects against rubbing during sex—is dismissed by the evidence of successful procreation in the age of genital gardening, and the fact that the other animals don’t suffer in that respect for want of pubic hair.

In 2009, Robin Weiss, of University College London, proposed that pubic hair allows the presentation of pheromones, although there is no evidence that humans can even detect pheromones, having a non-functional vomeronasal organ. Still, our ability to detect potential compatible mates through smell thanks to its association with the major histocompatibility complex gives pubic hair a potential role in advertising the products of the apocrine glands concentrated in that region.

Other explanations include a role in improving evaporative cooling, which conflicts with the fact that hairlessness serves that role; a social display of sexual maturity, which is unlikely to have been visible among our dark-skinned hominin ancestors in Africa; and protection of the reproductive organs from dirt, which doesn’t work for men.

In fact, this last explanation comes closest to what I think is the real reason. Our ancestors evolved their upright posture, bipedalism, hairlessness and capacity to run long distances to wear down prey on the savannahs of East Africa. The savannahs are the key. As nomads who had to move with the seasons and who ran down their prey, our ancestors had to contend with terrain vegetated mostly by shrubs and countless tough, fibrous grasses. The males could cope with the assaults to the glans thanks to the prepuce, or foreskin. The females, however, had no such protection. Moreso, the vertical orientation of the vulva and its placement at the base of the torso exposed the vagina to direct threat of injury by the entry of vertical grass stems or, worse, grass seeds with sharp awns. If any of these entered the vagina, they would have caused injury to the sensitive membranes, with possible infection. Thatchless females would have struggled to reproduce. On the other hand, a patch of pubic hair, with its distinctive wiry structure, would have deflected grass stems, allowing women who possessed it to reproduce and pass the trait on. Because all genes present in females are also present in males, the trait of pubic hair is expressed in men also, though with no selection pressure to either retain it or lose it, all because it was necessary in our female ancestors. Thus, pubic hair was selected at the same time as hairlessness.

This hypothesis is easily tested, if only by mannequins fitted with tensiometers. A simple thought experiment can show that it’s likely, or at least plausible.

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