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.