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So how didwomen get their curves? Why do they have breasts, while other mammals only develop breast tissue while lactating, and why do women menstruate, when virtually no other beings do so? What are the reasons for female orgasm? Why are human females kept in the dark about their own time of ovulation and maximum fertility, and why are they the only animals to experience menopause?David P. Barash and Judith Eve Lipton, coauthors of acclaimed books on human sexuality and gender, discuss the theories scientists have advanced to explain these evolutionary enigmas (sometimes called "Just-So stories" by their detractors) and present hypotheses of their own. Some scientific theories are based on legitimate empirical data, while others are pure speculation. Barash and Lipton distinguish between what is solid and what remains uncertain, skillfully incorporating their expert knowledge of biology, psychology, animal behavior, anthropology, and human sexuality into their entertaining critiques. Inviting readers to examine the evidence and draw their own conclusions, Barash and Lipton tell an evolutionary suspense story that captures the excitement and thrill of true scientific detection.

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Excerpt fromChapter 5: The Enigmatic Orgasm

It used to be conventional wisdom among biologists that human beings are unique in experiencing female orgasm, but no longer. Nonetheless, female orgasm remains both a marvelous phenomenon and a contentious, unsolved mystery among evolutionary biologists. Given the longstanding and widespread sexual repression of women in both Western and Eastern societies, it is not surprising that only recently has anorgasmia (failure to experience orgasm) been identified and treated. Nonetheless, the real biological mystery isn't why some women don't climax, but why some do....

Anthropologist-primatologist Sarah Blaffer Hrdy suggests that female orgasm evolved as a spur to having sex with many different males. "Based on both clinical observations and interviews with women," writes Hrdy, "there is a disconcerting mismatch between a female capable of multiple sequential orgasms and a male partner typically capable of one climax per copulatory bout." A potential consequence of this "mismatch" is that females would be inclined to seek multiple partners in order to achieve their orgasmic potential. As for why this potential exists at all, Hrdy suggests that it is ultimately driven by the fitness benefit of taking out an anti-infanticide insurance policy, as proposed earlier for the evolution of concealed ovulation. Thus, female orgasm and its requirement of sustained stimulation may have provided the proximate mechanism underpinning the ultimate payoff deriving from having sex with multiple partners. Here are Hrdy's own words: "It is possible that as in baboons and chimps the pleasurable sensations of sexual climax once functioned to condition females to seek sustained clitoral stimulation by mating with successive partners, one right after the other, and that orgasms have since become secondarily enlisted by humans to serve other ends (such as enhancing pair-bonds)."

Picture humanity's mother studiously going from one sexual partner to the next, maintaining and motivated by unsatisfied sexual tension while transitioning among males, egged on in her search for "sustained clitoral stimulation" by the hope that the next guy will finish what the previous one hadn't quite managed. Or maybe if she had already climaxed, she might nonetheless be inspired to encounter multiple males by the simple fact that female orgasm is rekindlable (whether this is the right word for the experience is not certain, but it clearly is a bona fide phenomenon). If so, then the ultimate motivation for such behavior would have been the fitness bonus of taking out an anti-infanticide insurance policy, proximately motivated by the prospect of an orgasm. Or several.

Nonhuman female primates—notably macaques, baboons, and chimpanzees—do in fact mate sequentially with a number of different partners, and if stimulation from these encounters is cumulative, orgasm might be a proximal payoff. It might accordingly be part of a complex reward system among many animals, not just human beings, that induces females to mate with many different males. In addition, it might have evolved in nonhuman primates for one reason (e.g., infanticide insurance), then been maintained among modern human beings for another.

For a distinctly nonsexual example of a trait's adaptive significance changing over time, consider feathers. It is clear from the structure of fossil dinosaurs and birds that the earliest feathers did not evolve in the service of flight because the earliest feathered reptiles lacked hollow bones and were for the most part earthbound. Far more likely, feathers helped primitive "birds" to keep warm and only subsequently became elaborated as aeronautic devices. Among modern birds—especially males—feathers are further adapted as sexually selected traits (remember the peacock). A trait's current adaptive significance can therefore be quite different from its original function.

Even if female orgasm evolved as a mechanism that induced women to mate with multiple men—a questionable if intriguing hypothesis—it doesn't mean that human cultural traditions would necessarily welcome it. Thus, the hideous practice of "female circumcision," still widespread in much of northern and eastern Africa, may owe its existence to a recognition that female sexual desire can lead to multiple partners: in order for a woman to be considered marriageable, it is necessary to guarantee her fidelity by curtailing her orgasmic potential, if not eliminating it altogether. Hrdy has made the interesting proposal that female orgasm may thus be a relic, adaptive among our primate ancestors but potentially disadvantageous—even dangerous—to some women today. Thus, insofar as orgasm might even occasionally induce women to seek out additional sex partners beyond their designated husband, this consequence in itself might have serious (and certainly fitness-reducing) results. In much of the world, the penalty for a woman's having sex with more than one man (especially if she is married) is quite severe, sometimes including death.

As with menstruation, concealed ovulation, the existence of nonlactating breasts, and the other evolutionary enigmas yet to be explored, the conceptual waters surrounding female orgasm are muddy indeed. Thus, another potential evolutionary payoff of orgasm would seem to push in precisely the other direction from Hrdy's hypothesis, toward monogamy rather than multiple partners. Let's return to her suggestion, with the intervening phrases deleted: "It is possible that . . . orgasms have since become secondarily enlisted by humans to serve other ends (such as enhancing pair-bonds)."

There is some evidence that women are more likely to climax with familiar partners because they are more likely to feel (and to be) safe and thus comfortable and relaxed, to be able to make their needs and preferences clear, and more likely to have them met. Put this all together, and a case might be made that rather than being an inducement for polyandry, as Hrdy proposes, female orgasm is an evolutionary sweetener for its opposite, monogamy, as Hrdy also proposes! In this regard, it is altogether consistent to have it both ways because, as already noted, a trait can evolve for one reason, then be employed for another. (In any event, if there were evidence that chimpanzee females are more likely to achieve orgasm by mating sequentially with multiple males, this would strengthen Hrdy's infanticide insurance hypothesis for female orgasm, but diminish confidence in the monogamy hypothesis; at present, there are no data either way.)

A variant on the monogamy hypothesis is also worth considering. Aside from its possible role in promoting pair bonding via the female's inclination, it is not unreasonable to suppose that female orgasm might promote monogamy by acting on the male as follows: if female orgasm serves as a sexual reward, inducing ancestral women to engage in a higher frequency of copulations than would otherwise occur, and if their mates respond positively to an enhanced sex life vis-à-vis that experienced by partners whose females are nonorgasmic, and if part of this response involves a greater commitment to a monogamous pair bond with the women in question, and if this commitment results in greater evolutionary success for these women, then natural selection may have favored female orgasm. That's a lot of "ifs," but—pardon the expression—if they all fall into place, so would a case for the validity of this particular hypothesis.

The Monogamy Hypothesis also leads to the prediction that monogamous species should be heavily represented among animals that experience female orgasm, but this is not the case. The best-documented nonhuman examples of female orgasm—bonobos, chimpanzees, certain species of macaque monkeys—are notoriously nonmonogamous.