SciAm Mind - Secrets of the Phallus: Why is the Penis Shaped Like That?

OK, a little biology lesson today on most men's favorite appendage. Seriously, who hasn't thought about this at least once?

Secrets of the Phallus: Why is the Penis Shaped Like That?

Evolutionary psychologists decipher the "Rosetta Stone" of human sexuality

By Jesse Bering

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 such a peculiarly shaped device. Let’s face it—it’s not the most intuitively shaped appendage in all of evolution. But according to evolutionary psychologist Gordon Gallup of the State University of New York at Albany, 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 begun to study it in any detail. The reason for this neglect isn’t clear, though the most probable reason is because of its intrinsic snicker factor or, related to this, the likelihood of its stirring up uncomfortable puritanical sentiments. 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. But I think labeling it as a “crude” or “disgusting” area of study reveals more about the critic than it does the researcher. And 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 of “reverse-engineering” as it’s used in the field of evolutionary psychology. 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 “byproduct” 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, it appears there’s a genuine adaptive reason that it looks the way it does.

If one 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 to the same organ in other species, they’d notice the following uniquely human characteristics. First, despite variation in size between individuals, the erect human penis is especially large compared to that of other primates, measuring on average between 5-6 inches in length and averaging about 5 inches in circumference. (Often in this column I’ll relate the science at hand to my own experiences, but perhaps this particular piece is best written without my normally generous use of anecdotes.) 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, their 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 baby-sat 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 our 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, but rather 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 in spite of the fact that 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. Thus, in a theoretical paper published in the journal Evolutionary Psychology in 2004, Gallup and coauthor, Rebecca Burch, conjecture 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.”

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Tags: masculinity, penis, evolution, biology, Secrets of the Phallus, Why is the Penis Shaped Like That?, Scientific American, Mind, Evolutionary psychology