Proceeding now to the genitalia themselves, the combined weight of the testes in the average man is about \Vi ounces. This may boost the macho man's ego when he reflects on the slightly lower testis weight in a 450-pound male gorilla. But wait—our testes are dwarfed by the 4-ounce testes of a 100-pound male chimpanzee. Why is the gorilla so economical, and the chimp so well-endowed, compared to us?
The Theory of Testis Size is one of the triumphs of modern physical anthropology. By weighing the testes of thirty-three primate species, British scientists identified two trends: species that copulate more often need bigger testes; and promiscuous species in which several males routinely copulate in quick sequence with one female need especially big testes (because the male that injects the most semen has the best chance of being the one to fertilize the egg). When fertilization is a competitive lottery, large testes enable a male to enter more sperm-tickets in the lottery.
Here is how these considerations account for the differences in testis size among the great apes and humans. A female gorilla does not resume sexual activity until three or four years after giving birth, and she is receptive for only a couple of days a month until she becomes pregnant again. So even the successful male gorilla with a harem of several females experiences sex as a rare treat—if he is lucky, a few times a year. His relatively tiny testes are quite adequate for those modest demands. The sex life of a male orangutan may be somewhat more demanding, but not much. However, each male chimp in a promiscuous troop of many females lives in sexual nirvana, with nearly daily opportunities to copulate for a common chimp and several daily copulations for the average pygmy chimp. That, plus his need to outdo other male chimps in semen output if he is to fertilize the promiscuous female, explains his need for gigantic testes. We humans make do with medium-sized testes because the average man copulates more often than gorillas or orangutans but less often than chimps. In addition, the typical woman in a typical menstrual cycle does not force several men into sperm competition to fertilize her.
Thus, primate testis design well illustrates the principles of trade-offs and evolutionary cost/benefit analyses explained on page 52. Each species has testes big enough to do their job, but not unnecessarily larger ones. Bigger testes would just entail more costs without proportional benefits, by diverting space and energy from other tissues and increasing the risk of testicular cancer.
From this triumph of scientific explanation we descend to a glaring failure: the inability of twentieth-century science to formulate an adequate Theory of Penis Length. The length of the erect penis averages 1 Vi inches in a gorilla, 1 Vz inches in an orangutan, 3 inches in a chimp, and 5 inches in a man. Visual conspicuousness varies in the same sequence: a gorilla's penis is inconspicuous even when erect because of its black colour, while the chimp's pink erect penis stands out against the bare white skin behind it. The flaccid penis is not even visible in apes. Why does the human male need his relatively enormous, attention-getting penis, which is larger than that of any other primate? Since the male ape successfully propagates his kind with much less, does not the human penis represent largely wasted protoplasm that would be more valuable if devoted, say, to cerebral cortex or improved fingers?
Biologist friends to whom I pose this conundrum usually think of distinctive features of human coitus where they suppose a long penis might somehow be usefuclass="underline" our frequent use of the face-to-face position, our acrobatic variety of coital positions, and the supposedly long duration of our coital bouts. None of these explanations survives close scrutiny. The face-to-face position is also a preferred one for orangutans and pygmy chimps, and is used occasionally by gorillas. Orangutans vary face-to-face copulation with dorso-ventral and sideways positions, and do it while hanging from branches of trees—surely that demands more penile acrobatics than our comfortable boudoir exercises. Our mean duration of coitus (about four minutes for Americans) is much longer than for gorillas (one minute), pygmy chimps (fifteen seconds), or common chimps (seven seconds), but shorter than for orangutans (fifteen minutes) and lightning-fast compared to the twelve-hour-long copulations of marsupial mice. (Are you listening, ghosts of Errol Flynn and Don Juan?)
Since it thus seems unlikely that special features of human coitus demand a large penis, a popular alternative theory is that the human penis has also become an organ of display, like a peacock's tail or a lion's mane. This theory is reasonable but begs the question, what type of display, and to whom?
Proud male anthropologists unhesitatingly answer, an attractive display, to women, but this represents mere wishful thinking. Many women say that they are turned on by a man's voice, legs, and shoulders more than by the sight of his penis. A telling point is that the women's magazine Viva initially published photos of nude men but dropped them after surveys showed lack of female interest. When Vivas nude men disappeared, the number of female readers increased, and the number of male readers decreased. Evidently, the male readers were the ones buying Viva for its nude photos. While we can agree that the human penis is an organ of display, the display is intended not for women but for fellow men. icn.
Other facts confirm the role of a large penis as a threat or status display towards other men. Recall all the phallic art created by men for men, and the widespread obsession of men with their penis size. Evolution of the human penis was effectively limited by the length of the female vagina: a man's penis would damage a woman if it were significantly larger. However, I can guess what the penis would look like if this practical constraint were removed and if men could design it themselves. It would resemble the penis sheaths (phallocarps) used as male attire in some areas of New Guinea where I do field work. Phallocarps vary in length (up to 2 feet), diameter (up to 4 inches), shape (curved or straight), angle made with the wearer's body, colour (yellow or red), and decoration (such as a tuft of fur at the end). Each man has a wardrobe of several sizes and shapes from which to choose each day, depending on his mood that morning. Embarrassed male anthropologists interpret the phallocarp as something used for modesty or concealment, to which my wife had a succinct answer on seeing a phallocarp: 'The most immodest display of modesty I've ever seen!
Thus, astonishing as it seems, important functions of the human penis remain obscure. Here is a rich field for research.
Passing now from anatomy to physiology, we are immediately confronted by our sexual activity pattern, which must be considered freakish by the standards of other mammal species. Most mammals are sexually inactive most of the time. They copulate only when the female is in oestrus—that is, when she is ovulating and capable of being fertilized. Female mammals apparently 'know' when they are ovulating, for they solicit copulation then by presenting their genitals towards males. Lest a male miss the point, many female primates go further; the area around the vagina, plus in some species the buttocks and breasts, swells up and turns red, pink, or blue. This visual advertisement of female availability affects male monkeys in the same way that the sight of a seductively dressed woman affects male humans. In the presence of females with brightly swollen genitals, male monkeys stare much more often at the female's genitals, develop higher testosterone levels, attempt to copulate more often, and penetrate more quickly and after fewer pelvic thrusts than in the presence of females not displaying their wares. Human sexual cycles are quite different. The human female maintains her sexual receptivity more or less constantly, instead of having it sharply confined to a short oestrus phase. Indeed, despite numerous studies aimed at settling whether a woman's receptivity varies at all through her cycle, there is still no agreement about the answer—nor about the cycle phase when receptivity is maximal if it does vary.