Why is Sex Fun?: the evolution of human sexuality (10 page)

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 1/2-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 phallo-carp. 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 phallo-carps, 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 vagina. 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 1/4-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 vagina 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 biosyn-thetic 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 clitoris and vagina 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 <>x-periments on hydraulic models, but an evolutionary problem as well. That evolutionary problem is posod 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.

For readers whose interest has been sufficiently aroused to read further, here are some suggestions. The first list consists of books on sexuality, behavior, primates, evolutionary reasoning, and related subjects. Many of them are written so as to be understandable to laypeople with no scientific training. They are available in large libraries, and many are still in print and available in bookstores. The second list consists of a dozen examples of technical articles, written for scientists and describing some of the specific studies that I discuss.

Alcock, John. Animal Behavior: An Evolutionary Approach. 5th ed. Sunderland, Mass.: Sinauer Associates, 1993.

Austin, C. R., and R. V. Short. Reproduction in Mammals. 2d ed., vols. 1–5. Cambridge: Cambridge University Press, 1982-86.

Chagnon, Napoleon A., and William Irons, eds. Evolutionary Biology and Human Social Behavior: An Anthropological Perspective. North Scituate, Mass.: Duxbury Press, 1979.

Cronin, Helena. The Ant and the Peacock: Altruism and Sexual Selection from Darwin to Today. Cambridge: Cambridge University Press, 1991.

Daly, Martin, and Margo Wilson. Sex, Evolution, and HahnV' ior. 2d ed. Boston: Willard Grant Press, 1983.

Darwin, Charles. The Descent of Man, and Selection in Holdtion to Sex. London: Murray, 1871. Paperback reprint, Princeton, N.J.: Princeton University Press, 1981.

Diamond, Jared. The Third Chimpanzee: The Evolution and Future of the Human Animal. New York: HarperCollins, 1992.

Fedigan, Linda Marie. Primate Paradigms: Sex Roles and Social Bonds. Chicago: University of Chicago Press, 1992.

Goodall, Jane. The Chimpanzees of Gombe: Patterns of Behavior. Cambridge, Mass.: Harvard University Press, 1986.

Halliday, Tim. Sexual Strategy. Chicago: University of Chicago Press, 1980.

Hrdy, Sarah Blaffer. The Woman That Never Evolved. Cambridge, Mass.: Harvard University Press, 1981.

Kano, T. Takayoshi. The Last Ape: Pygmy Chimpanzee Behavior and Ecology. Stanford, Calif.: Stanford University Press, 1992.

Kevles, Bettyann. Females of the Species: Sex and Survival in the Animal Kingdom. Cambridge, Mass.: Harvard University Press, 1986.

Krebs, J. R., and N. B. Davies. Behavioural Ecology: An Evolutionary Approach. 3d ed. Oxford: Blackwell Scientific Publications, 1991.

y Ricklefs, Robert E., and Caleb E. Finch. Aging: A Natural History. New York: Scientific American Library, 1995.

Rose, Michael R. Evolutionary Biology of Aging. New York: Oxford University Press, 1991.

Small, Meredith F. Female Choices: Sexual Behavior of Female Primates. Ithaca, N.Y.: Cornell University Press, 1993.

Smuts, Barbara B., Dorothy L. Cheney, Robert M. Seyfarth, Richard W. Wrangham, and Thomas T. Struhsaker, eds. Primate Societies. Chicago: University of Chicago Press, 1986.

Symons, Donald. The Evolution of Human Sexuality. New York: Oxford University Press, 1979.

Wilson, Edward O. Sociobiology: The New Synthesis. Cambridge, Mass.: Harvard University Press, 1975.

Alexander, Richard D. “How Did Humans Evolve?” Special publication no. 1. University of Michigan Museum of Zoology, Ann Arbor, 1990.

Emlen, Stephen T., Natalie J. Demong, and Douglas J. Emlen., “Experimental Induction of Infanticide in Female Wattled Jacanas.” Auk 106 (1989): 1–7.

Francis, Charles M., Edythe L. P. Anthony, Jennifer A. Bran-ton, and Thomas H. Kunz. “Lactation in Male Fruit Until” Nature 367 (1994): 691-92.

Gjershaug, Jan Ove, Torbjorn Jarvi, and Elvin Roskaft. “Mar-riage Entrapment by 'Solitary' Mothers: A Study on Male Deception by Female Pied Flycatchers.” American Naturalist 133 (1989): 273-76.

Greenblatt, Robert B. “Inappropriate Lactation in Men and Women.” Medical Aspects of Human Sexuality 6, no. 6 (1972): 25–33.

Hawkes, Kristen. “Why Do Men Hunt? Benefits for Risky Choices.” In Risk and Uncertainty in Tribal and Peasant Economies, edited by Elizabeth Cashdan (pp. 145-66). Boulder, Colo.: Westview Press, 1990.

Hawkes, Kristen, James F. O'Connell, and Nicholas G. Blurton Jones. “Hardworking Hadza Grandmothers.” In Comparative Socioecology: The Behavioral Ecology of Humans and Other Mammals, edited by V. Standen and R. A. Foley (pp. 341-66). Oxford: Blackwell Scientific Publications, 1989.

Hill, Kim, and A. Magdalena Hurtado. “The Evolution of Premature Reproductive Senescence and Menopause in Human Females: An Evaluation of the Grandmother Hypothesis.” Human Nature 2 (1991): 313-50.

Kodric-Brown, Astrid, and James H. Brown. “Truth in Advertising: The Kinds of Traits Favored by Sexual Selection,” American Naturalist 124 (1984): 309-23.

Oring, Lewis W., David B. Lank, and Stephen J. Maxson. “Population Studies of the Polyandrous Spotted Sandpiper.” Auk 100 (1983): 272-85.

Sillen-Tulberg, Birgitta, and Anders P. Mailer. “The Relationship Between Concealed Ovulation and Mating Systems in Anthropoid Primates: A Phylogenetic Analysis.” American Naturalist 141 (1993): 1-25.