Jesse Bering explains why your dick—or your partner's dick, or the silicone dick you and your partner keep in your nightstand—looks like that:
If you were to examine the penis objectively—please don’t do this in a public place or without the other person’s permission—and compare the shape of this organ with the design of the same organ in other species, you’d notice the following uniquely human characteristics. First, despite variation in size between individuals, the human penis is especially large compared with that of other primates. When erect, it measures on average between five and six inches in length and about five inches in circumference. Even the most well-endowed chimpanzee, the species that is our closest living relative, doesn’t come anywhere near this. Rather, even after correcting for overall mass and body size, chimp penises are about half the size of human penises in both length and circumference. In addition, only the human species has such a distinctive mushroom-capped glans, which is connected to the shaft by a thin tissue of frenulum (the delicate tab of skin just beneath the urethra). Chimpanzees, gorillas, and orangutans have a much less extravagant phallic design—more or less all shaft. It turns out that one of the most significant features of the human penis isn’t so much the glans per se as the coronal ridge it forms underneath.
This “semen displacement theory” is the most intriguing part of Gallup’s story. Since sperm cells can survive in a woman’s cervical mucus for up to several days, if she has more than one male sexual partner over this period of time, say within forty-eight hours, then the sperm of these two men are competing for reproductive access to her ovum. So how did nature equip men to solve the adaptive problem of other men impregnating their sexual partners? The answer, according to Gallup, is that their penises were sculpted in such a way that the organ would effectively displace the semen of competitors from their partner’s vagina, a well-synchronized effect facilitated by the “upsuck” of thrusting during intercourse. Specifically, the coronal ridge offers a special removal service by expunging foreign sperm. According to this analysis, the effect of thrusting would be to draw other men’s sperm away from the cervix and back around the glans, thus scooping out the semen deposited by a sexual rival.
Click through to read the rest of the excerpt from the brilliant Jesse Bering's new book—Why Is the Penis Shaped Like That: ...And Other Reflections On Being Human—over at HuffPo.