We have seen that several classes of human sexual signals— men’s muscles, facial beauty, and women’s body fat concentrated in certain places— apparently conform to the truth-in-advertising model. However, as I mentioned in discussing animals’ signals, different signals may conform to different models. That’s also true of humans. For example, the pubic and axillary hair that both men and women have evolved to grow in adolescence is a reliable but wholly arbitrary signal of attainment of reproductive maturity. Hair in those locations differs from muscles, beautiful faces, and body fat in that it carries no deeper message. It costs little to grow, and it makes no direct contribution to survival or to nursing babies. Poor nutrition may leave you with a scrawny body and disfigured face, but it rarely causes your pubic hair to fall out. Even weak ugly men and skinny ugly women sport axillary hair. Men’s beards, body hair, and low-pitched voices as signals of adolescence, and men’s and women’s hair whitening as a signal of age, seem equally devoid of inner meaning. Like the red spot on a gull’s bill and many other animal signals, these human signals are cheap and wholly arbitrary— many other signals can be imagined that would serve equally well.
Is there any human signal that exemplifies the operation of Fisher’s runaway selection model or Zahavi’s handicap principle? At first, we seem devoid of exaggerated signaling structures comparable to a widowbird’s sixteen-inch tail. On reflection, however, I wonder whether we actually do sport one such structure: a man’s penis. One might object that it serves a nonsignaling function and is nothing more than well-designed reproductive machinery. However, that is not a serious objection to my speculation: we have already seen that women’s breasts simultaneously constitute signals and reproductive machinery. Comparisons with our ape relatives hint that the size of the human penis similarly exceeds bare functional requirements, and that that excess size may serve as a signal. The length of the erect penis is only about 1 ¼ inches in gorillas and 1 ½ inches in orangutans but 5 inches in humans, even though males of the two apes have much bigger bodies than men.
Are those extra couple of inches of the human penis a functionally unnecessary luxury? One counterinterpretation is that a large penis might somehow be useful in the wide variety of our copulatory positions compared to many other mammals. However, the 1 ½-inch penis of the male orangutan permits it to perform in a variety of positions that rival ours, and to outperform us by executing all those positions while hanging from a tree. As for the possible utility of a large penis in sustaining prolonged intercourse, orangutans top us in that regard too (mean duration fifteen minutes, versus a mere four minutes for the average American man).
A hint that the large human penis serves as some sort of signal may be gained by watching what happens when men take the opportunity to design their own penises, rather than remaining content with their evolutionary legacy. Men in the highlands of New Guinea do that by enclosing the penis in a decorative sheath called a phallocarp. The sheath is up to two feet long and four inches in diameter, often bright red or yellow in color, and variously decorated at the tip with fur, leaves, or a forked ornament. When I first encountered New Guinea men with phallocarps, among the Ketengban tribe in the Star Mountains last year, I had already heard a lot about them and was curious to see how they were used and how people explained them. It turned out that men wore their phallocarps constantly, at least whenever I encountered them. Each man owns several models, varying in size, ornamentation, and angle of erection, and each day he selects a model to wear according to his mood, much as each morning we select a shirt to wear. In response to my question as to why they wore phallocarps, the Ketengbans replied that they felt naked and immodest without them. That answer surprised me, with my Western perspective, because the Ketengbans were otherwise completely naked and left even their testes exposed.
In effect, the phallocarp is a conspicuous erect pseudo penis representing what a man would like to be endowed with. The size of the penis that we evolved was unfortunately limited by the length of a woman’s ######. A phallocarp shows us what the human penis would look like if it were not subject to that practical constraint. It is a signal even bolder than the widowbird’s tail. The actual penis, while more modest than a phallocarp, is immodestly large by the standards of our ape ancestors, although the chimpanzee penis has also become enlarged over the inferred ancestral state and rivals men’s penises in size. Penis evolution evidently illustrates the operation of runaway selection just as Fisher postulated. Starting from a 1 ½-inch ancestral ape penis similar to the penis of a modern gorilla or orangutan, the human penis increased in length by a runaway process, conveying an advantage to its owner as an increasingly conspicuous signal of virility, until its length became limited by counterselection as difficulties fitting into a woman’s ###### became imminent.
The human penis may also illustrate Zahavi’s handicap model as a structure costly and detrimental to its owner. Granted, it is smaller and probably less costly than a peacock’s tail. However, it is large enough that if the same quantity of tissue were instead devoted to extra cerebral cortex, that brainy redesigned man would gain a big advantage. Hence a large penis’s cost should be regarded as a lost-opportunity cost: because any man’s available biosynthetic energy is finite, the energy squandered on one structure comes at the expense of energy potentially available for another structure. In effect, a man is boasting, “I’m already so smart and superior that I don’t need to devote more ounces of protoplasm to my brain, but I can instead afford the handicap of packing the ounces uselessly into my penis.”
What remains debatable is the intended audience at which the penis’s proclamation of virility is directed. Most men would assume that the ones who are impressed are women. However, women tend to report that they are more turned on by other features of a man, and that the sight of a penis is, if anything, unattractive. Instead, the ones really fascinated by the penis and its dimensions are men. In the showers in men’s locker rooms, men routinely size up each other’s endowment.
Even if some women are also impressed by the sight of a large penis or are satisfied by its stimulation of the ######## and ###### during intercourse (as is very likely), it is not necessary for our discussion to degenerate into an either/ or argument that assumes the signal to be directed at only one sex. Zoologists studying animals regularly discover that sexual ornaments serve a dual function: to attract potential mates of the opposite sex, and to establish dominance over rivals of the same sex. In that respect, as in many others, we humans still carry the legacy of hundreds of millions of years of vertebrate evolution engraved deeply into our sexuality. Over that legacy, our art, language, and culture have only recently added a veneer.
The possible signal function of the human penis, and the target of that signal (if there is one), thus remain unresolved questions. Hence this subject constitutes an appropriate ending to this book because it illustrates so well the book’s main themes: the importance, fascination, and difficulties of an evolutionary approach to human sexuality. Penis function is not merely a physiological problem that can be straightforwardly cleared up by biomechanical experiments on hydraulic models, but an evolutionary problem as well. That evolutionary problem is posed by the fourfold expansion in human penis size beyond its inferred ancestral size over the course of the last 7 to 9 million years. Such an expansion cries out for a historical, functional interpretation. Just as we have seen with strictly female lactation, concealed ovulation, men’s roles in society, and menopause, we have to ask what selective forces drove the historical expansion of the human penis and maintain its large size today.
Penis function is also an especially appropriate concluding subject because it seems at first so nonmysterious. Almost anyone would assert that the functions of the penis are to eject urine, inject sperm, and stimulate women physically during intercourse. But the comparative approach teaches us that those functions are accomplished elsewhere in the animal world by a relatively much smaller structure than the one with which we encumber ourselves. It also teaches us that such oversized structures evolve in several alternative ways that biologists are still struggling to understand. Thus, even the most familiar and seemingly most transparent piece of human sexual equipment surprises us with unsolved evolutionary questions.
Diamond, Jared. Why Is Sex Fun?: The Evolution of Human Sexuality (Science Masters) (p. 146). Basic Books. Kindle Edition.
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