"The expense of spirit in a waste of shame" is how William Shakespeare described lust, but he was speaking as a poet, not a pragmatist. True, copulation is not cheap--it always exacts a toll, spiritual or otherwise, from its participants. But "a waste"? Hardly. Sex perpetuates the species, and lust--shameful though some human primates choose to make it--is the overnight express to sex.

For men and women alike the objective of lust is orgasm. It's hard to imagine a more powerful inducement to sexual activity. Indeed, orgasm is the kind of experience that could have been invented by gametes (reproductive cells). Imagine being stuck in somebody's gonads, where your goal in life is to form a union with someone else's gamete. The objective? To produce an organism that makes more gametes. What possible incentive could you offer your host to bring about that union? Try a somatic blitzkrieg of ecstasy, courtesy of the limbic system, the pleasure (as well as the pain) center of the brain. That's orgasm.

Today, when orgasm has been divorced almost entirely from reproduction, the "how" of sexual climax has been largely demystified. Masters and Johnson established the physical parameters of orgasm in their landmark 1966 book, Human Sexual Response. These include, in men, contraction of the rectal sphincter at .8-second intervals, a reduction of voluntary muscle control, and involuntary muscle spasms throughout the body (one distinctive example being the so-called carpopedal spasm, in which the big toe is held straight out while the other toes bend back and the foot arches--a contortion most people couldn't pull off consciously if they were paid to). And of course, there is ejaculation, which is usually, but not always, a telltale sign. In women the telltale signs are similar: along with its chaotic effects on other muscles, orgasm causes contractions of the uterus, vagina, and rectal sphincter, again at .8-second intervals. For both sexes, the pudendal cataclysm typically lasts less than a minute.

Despite such insights on the mechanics of orgasm, the origins of the phenomenon are as mysterious as ever. When did orgasm evolve? Who (or what) had the first orgasm? What selective pressures shaped the redoubtable reflex? Without fossil evidence, scientists interested in those questions must examine in excruciating detail the sexual practices of contemporary human cultures as well as the sex lives of chimpanzees, gorillas, orangutans, and other uninhibited relatives of Homo sapiens.

The monumental problem confronting anyone who wants to explore the antiquity of orgasm is the difficulty of ascertaining, even in existing animals, who has orgasms and who does not. Most experts believe that most male mammals do--which would make the experience at least 65 million years old, since that's when modern mammals began to evolve. But the logical pitfalls of such speculation are manifold. While the mechanics of an orgasm may be known, it is still a sensation, and like all sensations it's subjective. As a result, its existence can't be incontrovertibly demonstrated or disproved by empirical measures. That unhappy truth becomes especially clear when evolutionary biologists turn their attention to the female orgasm--which they do with unseemly fascination. Even human lovers have to take their lady's word for it. How much more ineffable, then, must be the coital consciousness of Madame Marmoset.

To complicate matters, some critics charge that personal experience often shapes a theorist's conclusions. "Sex is one of those subjects that everybody thinks they know about," says Elisabeth Lloyd, a philosopher at the University of California at Berkeley who's writing a book about bias in evolutionary theories about female sexuality. "People just assume that what they know is right, even if they learned it in the locker room." That said, the students of orgasm's evolution know a few things you wouldn't expect to hear at halftime.

Researchers generally use Masters and Johnson's description of human orgasm as evidence of orgasm in other species as well. In every species the apparent symptoms of satiety are more conspicuous among males than females. Ejaculation is of course the least subtle index, but there are others. For example, at the pinnacle of the sex act, a male rat gives a final thrust, straightens his front legs, and abandons his erstwhile desperate grasp on the female; then, with an abstracted gaze one researcher describes as "starry-eyed," the male slowly rises up on his rear legs and relinquishes the mount.

He saw; he conquered; he came. But did he feel the earth move? Does the rat's behavior necessarily correspond to the very particular neural experience we call orgasm? For that matter, does the salmon that spills its seed over a bed of eggs feel a frisson of pleasure? In short, does ejaculation equal orgasm?

There's reason to believe it doesn't. Whereas ejaculation is a somatic phenomenon, orgasm (as Shakespeare implies) is a phenomenon of the spirit, and even in human males a strict correlation between mind and matter hasn't been established. Ejaculation in men is not always coincident with orgasm: paralysis victims bereft of feeling below the waist often get erections and ejaculate without having a climax, and prepubescent boys can achieve orgasm, even multiple ones, without ejaculating. But if ejaculation doesn't equal orgasm, how can you tell whether an erupting male is enjoying himself?

Trouble is, you can't. "I observe insects mating all the time," says John Alcock, a zoologist at Arizona State University. "But I see no way of answering the question of whether male carpenter bees experience anything remotely similar to pleasure when they succeed in copulating with a female."

For that very reason, Alcock says, evolutionary biologists have shied away from tracing orgasm's lineage: "People don't see the point of getting worked up about a question they can't answer." Says Donald Symons, an anthropologist at the University of California at Santa Barbara, "I can't remember any serious discussion of male orgasm at all." Yet scientific debate has raged for years over an even more elusive phenomenon: the female orgasm. And the issue being debated isn't who, how, when, or where, but why. Why should women have the capacity for climax when they can make babies perfectly well without it? In Darwinian terms, what is the adaptive value of the female orgasm?

No one questions the Darwinian wisdom of the mammalian male orgasm. The first male to demonstrate such an aptitude would be inclined to indulge it so ferociously that other males' anorgasmic sex drives would pale in comparison. Adaptation can also explain the relative speed with which males achieve their bliss. Since the limiting factor in male reproductive success is usually the availability of receptive females, when said females make themselves available, a guy who can work fast has a distinct advantage over the slow male. He begets more kids, for one, and he is also less likely to fall prey to enemies that might attack while a couple is flagrante delicto. Furthermore, the less time it takes to philander, the more time there is to eat--another essential component of survival.

Female reproductive success, in contrast, is usually limited by the availability of resources to sustain mother and child through pregnancy, labor, and nursing, rather than an availability of mating partners. Because most females must gestate and raise their offspring, repeated bouts of lovemaking yield diminishing returns where conception is concerned and interfere considerably with the duties of motherhood. There is no strong evidence that orgasm in females directly contributes to fertility or fecundity. So why is it there at all?

Some evolutionary theorists, including Desmond Morris, maintain that human female orgasm was adaptive because it helped cement the "pair- bond" between ancestral parents that was necessary to ensure the survival of vulnerable infants. If the woman is rewarded during sex as much as (or more than) the man, goes the theory, then she will remain perpetually eager for intercourse and retain her allure with her mate, who will be less likely to stray. Scientists had also speculated that female orgasm aided conception by keeping a woman on her back long enough for her to ensure insemination, or by actively sucking sperm up into the uterus (an idea whose time has gone, since the contractions that accompany climax turn out to be expulsive).

Such theories assume that female orgasm is a reliable companion of copulation. But as Alfred Kinsey announced in his landmark 1953 report, it's not. Kinsey's interviews with nearly 6,000 women, as well as subsequent surveys, revealed that the vast majority of women do not climax during sexual intercourse without direct stimulation of the clitoris. Even in today's era of relative enlightenment, the most recent statistics from the Kinsey Institute for Research in Sex, Gender, and Reproduction show that fewer than half the women surveyed achieve orgasm through intercourse. The disparity between the reproductive act and the orgasmic reflex prompted Shere Hite, in her controversial 1976 Hite Report, to call sexual intercourse the Rube Goldberg method of female sexual fulfillment.

Such data present a conundrum to anyone who wants to argue that female orgasm is a sexually selected trait. John Alcock suggests that the legerdemain required to bring about the effect may have helped our foremothers distinguish between the sensitive guys who would make good parents and the love-'em-and-leave-'em louts. But the bitter truth, as Margaret Mead pointed out, is that many, many human cultures donÕt even recognize that women can enjoy sex, let alone climax the way men do. If female orgasm is an adaptive behavior, then there are plenty of women whose genes are headed the way of the dinosaurs.

The tenuous link between orgasm and intercourse in women has led other theorists, including Donald Symons, to conclude that the female climax is more accident than adaptation. "Saying that a trait is adaptive is different from saying a trait has an evolutionary history," says Symons. "Everything has an evolutionary history. To show that something is an adaptation, you have to be able to explain the how and the why, show that it has some kind of special design, a design that solved a specific problem.

"In the case of males, the design argument makes good sense. Male orgasm doesn't occur at just any time--there's this intense burst of pleasure that accompanies ejaculation, which is of extreme reproductive significance," he says. "That's an obvious adaptation."

Symons says that the most parsimonious interpretation of the evidence is that the female orgasm is a by-product, like male nipples, that exists merely because the same trait in the opposite sex confers a selective advantage. In other words, male orgasm by way of the penis is a smashing success, and since the clitoris is made from the same fetal tissue as the penis, it can't help but precipitate orgasms too. That's not to say that female arousal is superfluous, or that it doesn't have a function. Symons believes arousal is an adaptive mechanism in female sexuality, and that female genitals have indeed been "designed" to provide pleasurable stimulation during intercourse. But the particular experience of orgasm, he says, is neither a necessary nor sufficient condition for reproductive fitness in females. "It's simply a by-product of the ability of males to have orgasms."

Such a view, however reasonable, cannot go uncontested, and the contest comes primarily from Sarah Blaffer Hrdy, an anthropologist at the University of California at Davis. Hrdy has marshaled evidence from studies of nonhuman primates to make hypotheses about human sexuality. "There is just no reason to think that orgasm is nonadaptive," she says. Her research suggests that among our closest relatives, promiscuous rather than connubial behavior greatly behooves females who want their progeny to survive.

She points out that in virtually all primates male behavior has an important effect on the survival of infants. A male can lavish a youngster with care and protection or he can kill it, depending on how confident he is that he's sired the youngster in question. A female that can persuade the community's males to be nice to her kids stands a better chance of passing on her genes. And she can do so, says Hrdy, by pulling a few males into "the web of possible paternity"; that is, by sleeping around.

"For a number of years," Hrdy says, "I've been arguing that for nonhuman female primates, the goal is not simply to be inseminated by the single best male--that's an old Darwinian and Victorian notion. What really happens is that a female primate tries to mate with a number of males to establish a network that will preclude males from attacking her offspring. It will also increase the amount of resources the males are likely to provide the infants. If a male isn't certain that he's not the father, he cannot afford to kill the infants--it's too big a risk. This margin of error that males must allow is one of the few advantages females have.

"If you look at primate breeding systems the way I do, where the problem that the female faces is how to draw multiple partners into this net, then you see the need for a psychophysiological phenomenon that keeps her motivated to solicit and mate with a number of male partners"--in short, she needs an incentive for promiscuity. And Hrdy believes the most expedient route to promiscuity is via the clitoral orgasm. "The very fact," she says, "that it is erratic in its relationship with intercourse means it works as a more powerful conditioning mechanism." A female that does not climax during her first coital encounter has the incentive to demand another round--immediately. Chances are her newly sated mate won't be able to gratify her, so she dumps him for a fresh partner. The principle is a familiar one: if at first you don't succeed, try, try again. And again. And again, each time expanding the web of possible paternity.

This is something that B. F. Skinner pointed out years ago, Hrdy says--the principle of intermittent reinforcement. In the behavioral psychologist's scenario, a rat that is rewarded a pellet when it presses a bar in its cage will press the bar only when it's hungry. If, however, the rat gets a pellet every 10 or 20 times it presses a bar, it will spend every waking hour pumping away at the bar. The rat learns that a great deal of effort is required for any return at all. Hrdy suggests that our female ancestors may have been subject to similar conditioning.

Hrdy's notion that female primates must be ever-ready lovers also jibes with Masters and Johnson's finding that women remain sexually excitable after orgasm. Whereas men's anticlimactic physiology quickly returns to its baseline level, women, before they cool down, retrace their steps, so to speak, to the plateau of arousal that immediately preceded their climax. It's now widely accepted that women can experience more prolonged arousal and more orgasms per unit of time than men--which, without benefit of Hrdy's interpretation, would seem to run counter to what you'd expect in a Darwinian world.

In her 1991 book, Mystery Dance, biologist Lynn Margulis of the University of Massachusetts tries to walk the center line of the design argument, claiming that while early on "the clitoris had no evolutionary significance ... the opportunities for evolutionary invention were so rich that such a useful little mechanism was eventually put to use within the complex framework of human evolution."

Trouble is, according to Symons and other critics, "adaptationists" such as Hrdy have precious little evidence to appeal to. "It's pure imagination about some polygynous past we supposedly had," carps Symons. "You find no evidence that women anywhere are behaving in ways to confuse the issue of paternity."

That's because human males, Hrdy argues, developed institutions such as marriage and clitoridectomy (the amputation of the clitoris) to repress women's indiscriminate sexuality, so that paternity could not be so easily confused. She points to the behavior of female chimpanzees, gorillas, and macaques as an indication of a past in which human females flaunted their desire. She notes too that if the evolution of the clitoris were dependent on the simultaneous development of the penis, you'd expect to see some correspondence between penis size and clitoris size in primates--which you often don't.

"What you do see," she says, "is variation across primate species. If you look at species where you have multiple-mate breeding systems, the clitoris is more developed. So it is not tracking the penis." The chimpanzee penis, for example, is "very small and pencillike" in comparison with the human penis, while the clitoris is among the more pendulous of its class.

Hrdy's argument would be buttressed by observations showing that orgasm among nonhuman female primates is correlated with promiscuous behavior. But again, while a physical reaction that is probably an orgasm can be noted, unequivocal evidence of the sensation of orgasm among species less articulate than our own is impossible to come by. There's the clutching reaction of the rhesus monkey, in which the female generally reaches back with one hand and grasps the male, sometimes turning and looking over her shoulder at the presumed moment of his climax--which, if it does indicate a female eruption as well, would leave people in the dust when it comes to simultaneous orgasms.

Then there's the disturbingly familiar "ejaculation face" of the stump-tailed macaque: a round-mouthed expression composed of one part surprise, one part epiphany, and one part catatonia. Males and females alike commonly adopt such expressions in the throes of passion. The unanswerable question is, of course, does the "face" correspond to a seismic tweak of ecstasy?

In fact, there is ample evidence of masturbation among captive animals, and contractions la Masters and Johnson have been recorded in artificially stimulated female primates of several species. "It's much more logical to assume that they do have orgasms than that they don't," says Helen Fisher, a research associate of the American Museum of Natural History. "We're not talking about a complicated physiological response here."

But primate researchers say the animals don"t have much spare time in the wild for mating, let alone for playing with themselves, and it's in the wild that the evolutionary pressures Hrdy talks about are exerted. So in lieu of sworn testimonials from the animal kingdom, the "proof" of female orgasm's adaptive value still redounds to the person who can tell the most convincing story about its history of selection. "The beauty of this topic is that you, too, can make up a theory," says Fisher. The truth probably lies somewhere between Shakespeare's poetry and Darwin's pragmatism.