Excerpted from "WHY IS THE PENIS SHAPED LIKE THAT? …And Other Reflections on Being Human" by Jesse Bering. Published July 7th by Scientific American/Farrar, Straus and Giroux, LLC. Copyright © 2012 by Jesse Bering. All rights reserved.

If you’ve ever had a good long look at the human phallus, whether yours or someone else’s, you’ve probably scratched your head over its peculiar shape.

Let’s face it: it’s not the most intuitively configured appendage in all of evolution. But according to the evolutionary psychologist Gordon Gallup, the human penis is actually an impressive “tool” in the truest sense of the word, one manufactured by nature over hundreds of thousands of years of human evolution.

You may be surprised to discover just how highly specialized a tool it is. Furthermore, you’d be amazed at what its appearance can tell us about the nature of our sexuality. If you think there’s only one way to use your penis, that it’s merely an instrument of internal fertilization that doesn’t require further thought, or that size doesn’t matter, well, that just goes to show how much you can learn from Gallup’s research findings.

Gallup’s approach to studying the design of the human penis is a perfect example of reverse engineering as the term is used in the field of evolutionary psychology. That is to say, if you start with what you see today—in this case, the oddly shaped penis, with its bulbous glans (the “head,” in common parlance), its long, rigid shaft, and the coronal ridge that forms a sort of umbrella-lip between these two parts—and work your way backward regarding how it came to look like that, the reverse engineer is able to posit a set of function based hypotheses derived from evolutionary theory.

For the evolutionary psychologist, the pressing questions are, essentially, Why is it like that? and What is that for? The answer isn’t always that it’s a biological adaptation—that it solved some evolutionary problem and therefore gave our ancestors a competitive edge in terms of their reproductive success. Sometimes a trait is just a “by- product” of other adaptations.

Blood isn’t red, for example, because red worked better than green or yellow or blue, but only because it contains the red hemoglobin protein, which happens to be an excellent transporter of oxygen and carbon dioxide. But in the case of the human penis, all signs point to a genuine adaptive reason that it has come to look the way it does.

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.

Magnetic imaging studies of heterosexual couples having sex reveal that during coitus, the typical penis completely expands and occupies the vaginal tract and with full penetration can even reach the woman’s cervix and lift her uterus. This, combined with the fact that human ejaculate is expelled with great force and over considerable distance (up to two feet if not contained), suggests that men are designed to release sperm into the uppermost portion of the vagina possible.

In an article published in the journal Evolutionary Psychology, Gallup and Rebecca Burch argue that “a longer penis would not only have been an advantage for leaving semen in a less accessible part of the vagina, but by filling and expanding the vagina it also would aid and abet the displacement of semen left by other males as a means of maximizing the likelihood of paternity.”

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.

You might think this is all fine and dandy, but one can’t possibly prove such a thing. You’d be underestimating Gallup. In a series of studies published in Evolution and Human Behavior, Gallup and a team of his students put the semen displacement hypothesis to the test using artificial human genitalia of different shapes and sizes. Findings from the study may not have “proved” the semen displacement hypothesis, but they certainly confirmed its principal points.

The researchers selected several sets of prosthetic genitals from erotic novelty stores, including a realistic latex vagina, sold as a masturbation pal for lonely straight men, and three artificial phalluses. Whereas the first two phalluses closely resembled an actual human penis, varying only in the coronal ridge properties, the third (the control phallus) was the bland and headless horseman of the bunch.

Next, the researchers borrowed a recipe for simulated semen. The recipe “consisted of 0.08 cups of sifted, white, unbleached flour mixed with 1.06 cups of water. This mixture was brought to a boil, simmered for 15 minutes while being stirred, and allowed to cool.”

In a controlled series of “displacement trials,” the vagina was loaded with this fake semen, and the phalluses were inserted at varying depths (to simulate thrusting) and removed, whereupon the latex orifice was examined to determine how much semen had been displaced from it. As predicted, the two phalluses with the coronal ridges displaced significantly more semen from the vagina (each removed 91 percent) than the “headless” control (35.3 percent).

For the second part of the study, Gallup administered a series of survey questions to college-age students about their sexual history. Drawing from previous studies that showed how sexual jealousy inspires predictable (and biologically adaptive) “mateguarding” responses in human males, these questions were meant to determine whether certain “penile behavior” (my term, not theirs) could be expected based on the men’s suspicion of infidelity in their partners.

In the first of these anonymous questionnaires, heterosexual men and women reported that in the wake of allegations of female cheating, men thrust deeper and faster. Results from a second questionnaire revealed that upon first being sexually reunited after time apart, couples engaged in more vigorous sex— amely, compared with baseline sexual activity where couples see each other more regularly, vaginal intercourse following periods of separation involved deeper and quicker thrusting.

Hopefully, you’re thinking as an evolutionary psychologist at this point and can infer what these survey data mean: by using their penises proficiently as a semen displacement device, men are subconsciously (in some cases consciously) combating the possibility that their partners have had sex with another man in their absence.

Once ejaculation has occurred, men typically become flaccid fairly quickly, and further stimulation of the penis is even uncomfortable. This is important because continued thrusting would be self-defeating: the man would essentially be removing his own sperm at that point.

Doubtful about this interpretation? The really beautiful thing about evolutionary psychology—or the most frustrating, if you’re one of its many critics—is that you don’t have to believe it’s true for it to work precisely this way. Natural selection doesn’t much mind if you favor an alternative explanation for why you get so randy upon being reunited with your partner. Your penis will go about its business of displacing sperm regardless.

It’s perhaps useful to reflect in closing on cat penises. Like human males, male cats possess remarkably specialized penises. They come equipped with a band of about 150 sharp, backward-pointing spines that, literally, rake the internal walls of the female cat’s vagina (hence the deafening yowl that often accompanies feline sex). This both triggers ovulation and displaces the sperm of prior males that may have recently mounted her.

We should give thanks—and I say this as a gay man, and one not without some stakes in this whole painful affair—that evolution took a somewhat gentler course in our species.

