Homo Mysterious: Evolutionary Puzzles of Human Nature (5 page)

The assumption here is that attractiveness, which is very much in the eye of the beholder, is likely to be determined by whether the person in question has traits that signal higher fitness to be experienced by his potential offspring: either unusually healthy, for example, or simply possessing traits that—once manifested in their sons—would likely be found attractive by the next generation of women. The possibility therefore exists that by concealing their ovulation, ancestral women were able to obtain desirable genes—as well as perhaps other immediate material resources—from desirable men with whom they mated adulterously, while also retaining paternal assistance from their socially designated but cuckolded partner.

Another way of saying this: By dispensing with estrus (which is essentially equivalent to concealing one’s ovulation), early hominid women may have been able to exercise greater control over their choice of a sexual partner. This could have worked in either of two ways. For one, insofar as concealed ovulation facilitated clandestine matings with men other than their designated mate, this needn’t only have involved taking out “infanticide insurance.” It would also have provided the opportunity for women to engage sexually with men of their choice. And for another, consider that female mammals in “heat” (which is to say, in estrus because they are ovulating) are typically no more rational than their male counterparts. Lacking estrus—that is, having concealed their ovulation—women can remain comparatively cool and in sexual control … at least, compared to other mammals.

By foregoing estrus, this particular argument goes, women have become masters of their genetic fate, empowered to pick and choose, deciding (maybe not consciously, but by exercising a degree of judgment nonetheless) among potential suitors. After all, the word
estrus
comes from a Greek term for a parasitic fly that pursues cattle and drives them crazy; a female animal in estrus seems more than a little crazy. By the same token, females who are not in estrus—which is to say, all women, all the time—are more likely to be sane, sober, and capable of better judgment.

In his poem “If,” Rudyard Kipling wrote about the merits of being able to keep your head while all those around you are losing
theirs. If you can do this, according to his famous poem, “then you will be a man, my son.” Maybe by favoring those women able to “just say no” to the tyranny of estrus, natural selection endowed our female ancestors with the ability to reap the benefits of being more discriminating than any animal (and probably more so than most men, to boot). “If you can keep your secret,” evolution might have been betting, “while all those around you are divulging theirs, then you will be a reproductively successful woman, my daughter.”

Concealed ovulation could also have adaptively paved the way for women to enjoy greater mating control by reducing the intensity of male–male competition. Think of it this way: If women were only sexually receptive for a day or so each month, at which time they broadcast their availability by sudden, seductive signals of sight, smell, and sexual interest, men might respond by huffing and puffing and blowing each other away—even more than they do now—leaving women little choice, perhaps, but to accept the victor. Dominant males may be desirable sperm donors, if only because their sons may also turn out to be dominant and/or desirable males: This is the so-called sexy son hypothesis (about which more later). But such males may also be terrible fathers, more interested in beating other males over the head than in caring for their own children. By keeping their ovulation secret, and thereby dampening the competitive ardor of men, women might have given themselves the opportunity of choosing men who may be less pushy but more paternal.

At this point it is worth noting that even though women neither broadcast dramatic ovulatory messages like chimpanzees nor behave in a manner commensurate with estrus in other animals, research has shown that women’s behavior does indeed undergo subtle but substantial changes during their menstrual cycle. When they are ovulating, women are especially likely to wear clothing that is comparatively sexy and reveals more skin, to speak with greater fluency and creativity, to prefer images of men who are especially “manly” as well as favoring male voices that are lower pitched, to be perceived by both men and women as more facially attractive, to have a heightened sense of smell, and literally to move around more.
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Moreover, evidence is also accumulating that men are able to perceive at least some of these changes, albeit unconsciously. Thus, in a
series of renowned “dirty T-shirt” experiments, researchers got female college students to sleep alone wearing the same T-shirts for several days during their fertile phase, and again wearing different shirts when postovulatory. Young men were then presented with these pairs of shirts, each worn by the same woman but at different reproductive phases. Sure enough, they preferred the ones worn during the follicular phase (just prior to ovulation) in 75% of the cases.
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In addition, when the women had been taking hormonal contraceptives, which suppress ovulation, this preference disappeared.
⁷

Most dramatic are some findings involving lap dancers. A number of these hard-working women were asked to keep track of two things: their ovulatory cycles and how much money they made on tips while dancing on any given night. The results were staggering, even for people who might have anticipated an effect. During a 5-hour shift while maximally fertile, the women averaged $355; when in the luteal phase (so-so fertility), they brought in $260; and while menstruating, they made a paltry $185.
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Perhaps these women were sending pheromonal signals to which their male customers were unconsciously responding, or perhaps they unintentionally behaved more sexily as a function of their fertility; either way, they were doing something different and men were responding.

Evidence of this sort has led biologist Randy Thornhill and psychologist Steven Gangestad to claim that women do in fact experience genuine estrus like other mammals.
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I am dubious, if only because typical mammalian estrus is so obvious that it calls to mind the comment made to someone considering purchase of a Rolls Royce: If you have to ask how much it costs, you can’t afford it. If you have to debate whether estrus is occurring, it isn’t. On the other hand, although human ovulation is concealed from our conscious awareness, this doesn’t mean that it is totally, 100% hidden. Perhaps we should talk about an oxymoronic “cryptic estrus” or some neologism.

In any event, recent findings that women show a sexually and evolutionarily consistent suite of behavior when they are most fertile comports nicely with the “keeping control” hypothesis, described earlier, in that ironically, by concealing their ovulation—even as fertility is influencing their behavior—women might succeed in keeping control without being aware of the control that they are exercising!

As a general rule, male animals trade resources for sex, whereas females trade sex for resources. Imagine a prehistoric hominid woman who, like females of many different species, is prepared to exchange sex in return, let’s say, for food. If, chimplike, she made it anatomically obvious when she was fertile, it would also be clear when she wasn’t, and as a result, she might have lost some leverage otherwise available. The male might well be uninterested unless her anatomy or pheromones made her interesting. Such a female might then find herself forced to have sex with him, thereby possibly getting his genes, when all she really wanted was his banana.

On the other hand, if she didn’t give obvious cues as to whether or not she was fertile, our same great-, great-, great-grandmother could get the banana and eat it, too, and even if she had to comply sexually as part of the deal, she wouldn’t necessarily be committing her precious eggs into the bargain.

It may seem paradoxical, but by losing full-blown estrus, women may also have set the stage for having more sex, not less. Estrus-based sex is largely, if not entirely, reproductive. By severing the link between sexual intercourse and fertilization (or at least obscuring that link), concealed ovulation provides opportunities for nonreproductive sex.

This, in turn, might be part of the biological reason for concealed ovulation, if it enables couples to bond together especially tightly, to express and enhance their love via “liberated sex.” But, we might ask,
why
are sex and love so tightly connected? Why don’t romantic partners express and enhance their love by picking each other’s lice (many primates do just that), or via beautifully coordinated, mutually satisfying bouts of highly choreographed hiccupping? One guess is that people find shared sex reinforcing to the “pair bond” for a particular reason. Hints of why can be glimpsed in other species.

Look at it this way: Why do people—and certain other animals—copulate so often? The answer may seem distressingly practical, even cynical. When we look at those other species that
have frequent sex, some even more often than do human beings, we find that many (bonobos, chimpanzees, some species of dolphins, and lions excepted) are “socially monogamous” but prone to being sexually
un
faithful. Certain birds—for example, goshawks, osprey, white ibises, and acorn woodpeckers—copulate hundreds of times for every clutch of eggs. And they don’t limit those copulations to their designated partners.
¹⁰

The possible connection is as follows. When males are separated from their mates for a substantial part of each day, they risk having their female partners copulate with someone else, after which they could end up rearing another male’s offspring. The sexually adventurous female, by contrast, would be risking little. After all, she is guaranteed to be the mother of her offspring, no matter how many partners she may have. But the male, by contrast, has no such automatic confidence: “Mommy’s babies, Daddy’s maybes.” Accordingly, males who spend substantial time away from their female partners may be especially inclined to copulate frequently when they are at home, to increase the chances that their offspring are in fact theirs. We might conclude that they “love” their mates all the more when given the opportunity to make love with them. Moreover, the more loving they do, the more love they feel, with “love” in such cases defined as a powerful inclination to remain with and be devoted to one’s partner. In biological terms, the greater is the confidence of shared genetic investment, the greater is the love.

Now, let’s take the female’s perspective, and say that the lady osprey, goshawk, or white ibis is occasionally inclined to have sex with males in addition to her mate, perhaps because her extracurricular partner is particularly able or inclined to invest in her offspring or because his genes are especially fitness enhancing. At the same time, however, she dearly wants to retain the parental assistance of her social mate. It would make sense in such cases for the female to indulge her social partner’s sexual inclinations and to copulate often, if only because by assuaging his unconscious anxieties, she is more likely to obtain his continuing assistance and commitment while still remaining free to indulge her own extra-pair inclinations when her mate isn’t around.

Maybe all this has nothing to do with human beings. But it is probably no coincidence that other species among which pairs
copulate frequently are particularly likely to do so outside the pair bond as well. The prospect looms, therefore, that the remarkably high frequency of in-pair human sex isn’t so remarkable after all, considering that human beings are more than a little prone to sexual dalliance.

So perhaps we don’t advertise our ovulation, at least in part, because to do so would be to invite an unacceptably high level of sexual jealousy and obsessive mate guarding on the part of the male at these times. This would be all the more biologically awkward for females in proportion if they are somewhat inclined to infidelity … which they are. Women would accordingly be more fit if they didn’t incite their social mates to keep too-close tabs on their sexual activities. Lacking a powerful peak of sexual desire—and of desirability—women would also have been liberated to have sex with other males, and not just their designated mates. This possibility is a variant on the earlier notion of keeping control by keeping him guessing, and it emphasizes the sexual liberation of women, but it differs in paying attention to each woman’s payoff, benefits that needn’t involve purchasing infanticide insurance or even sire choice, but rather, covering her tracks. Not only that, but such liberation might also free a woman to have abundant sex with her designated partner as well, in the process keeping him sexually satisfied and also less worried about his paternity—perhaps less worried than he ought to be.

Had enough? For better or worse, the above suggestions do not exhaust the many possible explanations for concealed ovulation. It has been suggested, for example, that concealed ovulation has been selected for as a means of keeping the peace. Imagine, if you will, the situation in an otherwise staid commercial office, university, or bank—or Pleistocene campfire or Ice Age cave, for that matter—if the women regularly underwent a dramatic and readily apparent transition each month when they became fertile. The resulting chaos and heightened competition might well be so disadvantageous as to give an advantage to those women whose reproductive status remained demurely incognito.

This may seem compelling, but actually it is not persuasive to most evolutionary biologists, for the simple reason that under such conditions, any benefit derived by a concealed ovulator would be shared by everyone, including those whose reproductive status
was public. On the other hand, if the payoff of concealing one’s ovulation rests upon keeping the peace, then presumably those who weren’t thus concealed would have gained something by going public—and yet, they would be the only ones to profit as a result, while the costs of chaos would largely extend to everyone. Therefore, although concealing one’s ovulation might have benefited society as a whole, it is difficult to see how it would have been selected for at the level of individuals and their genes, which is pretty much the only way natural selection works.
^v