The shape of the human penis is quite distinct from that of many other primate species. In particular, the glans (“head”) of the human penis is shaped like a wedge. The diameter of the posterior glans is larger than the penis shaft itself, and the coronal ridge, which rises at the interface between the glans and the shaft, is positioned perpendicular to the shaft, as the picture indicates.
In addition, the human male during copulation engages in repeated thrusting motions before he ejaculates. The combined effect of the particular shape of the penis glans and the repeated thrusting motions during intercourse is to draw foreign semen back away from the cervix. If a female copulated with more than one male within a short period of time, this would allow subsequent males to “scoop out” semen left by others before ejaculating. In other words, according to Gallup, the human penis is a “semen displacement device.” It is designed and used to remove other men’s semen from the cervix before the man ejaculates.
If women did not engage in extensive extra-pair copulations throughout human evolutionary history, then the human penis would not be shaped as it is (like a wedge or scoop), and the human male would not engage in repeated thrusting motions during intercourse before ejaculating. Clear evidence of women’s promiscuity throughout evolutionary history is in the size and shape of men’s genitals and what men do with them.
This figure schematically summarizes my last two posts and more. The main circles for both sexes in comparison to the black circle to the right indicate the degree of sexual dimorphism in size (the relative body size of the male and the female within each species). The main circles for males of all great ape species are larger than the female comparison figure, and the main circles for females of all species are smaller than the male comparison figure. This is because males of all great ape species are larger than their females.
The pair of smaller circles indicates the relative size of testes (for male) and breasts (for females) across species. The appendage indicates the relative size of the penis (for males) and ovaries (for females) across species. As I mention in my last post, men have testes which are larger than gorillas and orangutans (whose females are largely sexually exclusive to their mates) but smaller than bonobos and chimpanzees (whose females are highly promiscuous). Incidentally, the figure indicates that, relative to males of other great ape species, men have an unusually large penis. I’m not aware of any evolutionary explanation for this observation, but if this doesn’t shut up your girlfriend, I don’t know what does.
Of course, the figure also indicates that, relative to females of other great ape species, women have unusually large breasts. We explain why this might be in Chapter 3 of our book Why Beautiful People Have More Daughters (“Barbie -- Manufactured by Mattel, Designed by Evolution”) or more briefly here. If this doesn’t shut up your boyfriend, I don’t know what does.