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Gangestad to claim that women do in fact experience genuine estrus like other mammals. 9 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.
Sexual Liberation?
     
    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 thathave 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