Why Is the Penis Shaped Like That?

The Extended Cut

Here’s a piece from my 2012 essay collection Why Is the Penis Shaped Like That? And Other Reflections on Being Human (Scientific American/Farrar, Straus and Giroux). It’s one I still get asked about—and penises haven’t done much evolving since then—so I thought I’d share it here for new readers. *Image from a Pompeii wall fresco of Mercury depicted with an enormous erect penis, painted between 89 B.C. and 79 A.D., now in the National Archaeological Museum of Naples.

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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.

The curious thing about the evolution of the human penis is that for something that differs so obviously in shape and size from that of our closest living relatives, only in the past few years have researchers began studying its natural history in any detail. The reason for this neglect isn’t clear. It’s hard to imagine that hard-nosed scientists would be worried about the subject stirring up uncomfortable puritanical sentiments. The issue does have an intrinsic snicker factor, so I realize it takes a special type of psychological scientist to tell the little old lady sitting next to him on a flight to Denver that he studies how people use their penises when she asks what he does for a living. In any event, 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 it’s used in the field of evolutionary psychology, and reverse engineering is an often unspoken concept that you’ll find me using repeatedly throughout my writings. This is a logico-deductive investigative technique for uncovering the adaptive purpose or function of existing (or extant) physical traits, psychological processes, or cognitive biases. 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. In the present case, we’re talking about penises, but the logic of reverse engineering can be applied to just about anything organic, from the shape of our incisors, to the opposability of our thumbs, to the arch of our eyebrows.

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. I’m afraid that I’m a more reliable source on this than most. Having spent the first five years of my academic life studying great ape social cognition, I’ve seen more simian penises than I care to mention. I once spent a summer with a 450-pound silverback gorilla that was hung like a wasp (great guy, though) and babysat a lascivious young orangutan that liked to insert his penis in just about anything with a hole, which unfortunately one day included my ear.

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. The diameter of the glans where it meets the shaft is wider than the shaft itself. This results in the coronal ridge that runs around the circumference of the shaft—something Gallup, by using the logic of reverse engineering, believed might be an important evolutionary clue to the origins of the strange sight of the human penis.

Now, the irony doesn’t escape me. But even though this particular evolutionary psychologist (yours truly) is gay, for the purposes of research we must consider the evolution of the human penis in relation to the human vagina. 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 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. We may prefer to regard our species as being blissfully monogamous, but at least some degree of fooling around has been our modus operandi for at least as long as we’ve been on two legs. 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. According to Gallup and Burch, “Examples include, group sex, gang rape, promiscuity, prostitution, and resident male insistence on sex in response to suspected infidelity.” And although semen displacement is the competitor male’s goal, even nicely evolved penises aren’t perfect. In fact, the authors cite the well-documented cases of human heteroparity, where “fraternal twins” are in fact sired by two different fathers who had sex with the mother within close succession of each other, as evidence of our species’ natural sexual inclinations.

So how did natural selection equip men to solve the adaptive problem of other men impregnating their sexual partners? The answer, according to Gallup, is 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 that’s fine and dandy but one couldn’t possibly prove such a thing. You’d be underestimating Gallup, however, who just so happens to be a very talented experimental researcher (among other things, he’s also well-known for developing the famous mirror self-recognition test for use with chimpanzees back in the early 1970s). 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. They even concocted several batches of realistic seminal fluid.

Findings from the study may not have “proved” the semen displacement hypothesis, but they certainly confirmed its principal points. Here’s how the basic study design worked. (And perhaps I ought to preempt the usual refrain by pointing out that yes, Gallup and his co-authors did receive full ethical approval from their university to conduct this study.) 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 tied off at one end to prevent leakage, and three artificial phalluses. The first latex phallus was 6.1 inches long and 1.3 inches in diameter with a coronal ridge extending approximately 0.20 inch from the shaft. The second phallus was the same length, but its coronal ridge extended only 0.12 inch from the shaft. Finally, the third phallus matched the other two in length but lacked a coronal ridge entirely. In other words, 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 from another like-minded evolutionary psychologist, Todd Shackelford, and created several batches of seminal fluid. 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 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). Additionally, the farther the phalluses were inserted—that is to say, the deeper the thrust—the more semen was displaced. When the phallus with the more impressive coronal ridge was inserted three-fourths of the way into the vagina, it removed only a third of the semen, whereas it removed nearly all of the semen when inserted completely. Shallow thrusting, simulated by the researchers inserting the artificial phallus halfway or less into the artificial vagina, failed to displace any semen at all. So if you want advice that’ll give you a leg up in the evolutionary arms race, don’t go west, young man—go deep.

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) “mate-guarding” 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—namely, 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.

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.

There are many other related hypotheses that are based on the central logic of the semen displacement theory. In their Evolutionary Psychology piece, for example, Gallup and Burch expound on a number of fascinating spin-off ideas in a follow-up article to their earlier work on the natural history of the penis. For example, one obvious criticism of the theory is that men would essentially disadvantage their own reproductive success by removing their own sperm cells from their sexual partner. However, in your own sex life, you’ve probably noticed the “refractory period” immediately following ejaculation, during which males almost instantly lose their tumescence (the erection deflates to half its full size within one minute of ejaculating), their penises become rather hypersensitive, and further thrusting even turns somewhat unpleasant. In fact, for anywhere between thirty minutes and twenty-four hours, most men are rendered temporarily impotent following ejaculation. According to Gallup and Burch, these post-ejaculatory features, in addition to the common “sedation” effect of orgasm, may be adaptations to the problem of “self-semen displacement,” meaning that the odds of removing your own sperm are reduced considerably when your penis is sore or flaccid or while you’re soundly asleep.

Gallup and Burch also leave us with a very intriguing hypothetical question in their article. “Is it possible (short of artificial insemination),” they ask, “for a woman to become pregnant by a man she never had sex with? We think the answer is ‘yes.’” It’s a tricky one to wrap your head around, but basically Gallup and Burch say that semen displacement theory predicts that something like the following example would be possible (note that I’ve modified this from the original article for your reading pleasure; also, observe how the scenario is especially relevant to uncircumcised men): If “Josh” were to have sex with “Kate,” who recently had sex with “Mike,” in the process of Josh’s thrusting his penis back and forth in her vagina, some of Mike’s semen would be forced under Josh’s frenulum, would collect behind his coronal ridge, and would be displaced from the area proximate to the cervix. After Josh ejaculates and substitutes his semen for that of the other male, as he withdraws from the vagina, some of Mike’s semen will still be present on the shaft of his penis and behind his coronal ridge. As his erection subsides, the glans will withdraw under the foreskin, raising the possibility that some of Mike’s semen could be captured underneath the foreskin and behind the coronal ridge in the process. Were Josh to then have sex with “Amy” several hours later, it is possible that some of the displaced semen from Mike would still be present under his foreskin and thus may be unwittingly transmitted to Amy, who, in turn, could then be impregnated by Mike’s sperm . . . It’s not exactly an immaculate conception. But just imagine the gasps from your average Maury Povich show audience.

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People have some pretty strong feelings about penises. Initial reactions to the essay you’ve just read ranged from the incredulous (“are you seriously suggesting that chimpanzees aren’t promiscuous?”), to the imaginative (“penises! they’re so cute, you just want to pinch their cheeks and give them cookies”), to the rather irritable (“stupid, biased thinking again from an ‘evolutionary psychologist’”). So I decided to speak directly with Gordon Gallup, whose controversial semen displacement theory, after all, was the one that struck up such a brouhaha regarding the adaptive functioning of this enigmatic organ. Perhaps, I appealed to him, he might offer us a few more clarifying details regarding the theory. I took many of the “core” questions to heart and asked Gordon to respond by e-mail to several of them:

READERS: The latex genitalia study wasn’t terribly convincing because the models were circumcised, and in real life the foreskin would interfere with the semen-displacing functions of the coronal ridge. So, does the foreskin pose a problem for the semen displacement theory?

GALLUP: The length of the foreskin is one of the most variable features of the human penis. When most uncircumcised males achieve an erection it pulls the foreskin back over the glans and back down the shaft of the penis, enabling the coronal ridge to do its business and scoop rival males’ semen away from the woman’s cervix. Because circumcision reduces the diameter of the shaft immediately behind the glans and accentuates the coronal ridge, we’ve speculated that the practice of circumcision may have unwittingly modified the penis in ways that enable it to function as a more effective semen displacement device. Armchair speculation? No. The idea could be tested by comparing the incidence of non-paternity between circumcised and intact males. My prediction would be that circumcised males ought to experience a lower incidence of being cuckolded.

READERS: So why did human penises evolve to have foreskin at all then?

GALLUP: Evolution does not occur by design. The best way to think about most adaptations is in terms of cost/benefit ratios. I suspect that the foreskin provided protection of the glans and what you see is the result of a statistical compromise of sorts.

READERS: If the penis really evolved to displace semen, then why wouldn’t other promiscuous primate species, namely chimpanzees, have evolved similarly designed penises with the coronal ridge?

GALLUP: Again, evolution doesn’t occur by design. It occurs by selection, and the raw material for such selection consists of nothing more than random genetic accidents (mutations). Embedded in the evolutionary history of human genital design were some penis shape mutations, not present in other species, that led to a device that could be used to compete with other males for paternity. Other promiscuous primates such as chimpanzees have solved the problem through sperm competition. Male chimpanzees have testicles that are three times the size of humans and differences in sperm count are on the same order of magnitude. Chimpanzees compete among one another for paternity by leaving the largest and most potent volume of semen in the female reproductive tract. When it comes to selection based on genetic accidents, there are a number of ways to skin the adaptive cat.

Bering here. And speaking of cats, and penises, 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.