First scene: a dimly lit bedroom, with a handsome man lying in bed. A beautiful young woman in a nightgown runs to the bed. A diamond wedding ring flashes virtuously on her left hand, while her right hand clutches a small blue strip of paper. She bends down and kisses the man's ear.

She: “Darling! It's exactly the right time!”

Next scene: same bedroom, same couple, evidently making love, but details tastefully obscured by the dim lighting. Then the camera shifts to a calendar slowly being flipped (to indicate the passage of time) by a graceful hand wearing the same diamond wedding ring.

Next scene: the same beautiful couple, blissfully holding a clean smiling baby.

He: “Darling! I'm so glad that Ovu-stick told us when it was exactly the right time!”

Last frame: close-up of the same graceful hand, clutching the small blue strip of paper. Caption reads: “Ovu-stick. Home urine test to detect ovulation.”

If baboons could understand our TV ads, they'd find that one especially hilarious. Neither a male nor female baboon needs a hormonal test kit to detect the female's ovulation, the sole time when her ovary releases an egg and when she can be fertilized. Instead, the skin around the female's vagina swells and turns a bright red color visible at a distance. She also gives off a distinctive smell. In case a dumb male still misses the point, she crouches in front of him and presents her hindquarters. Most other female animals are equally aware of their own ovulation and advertise it to males with equally bold visual signals, odors, or behaviors.

We consider female baboons with bright red hindquarters bizarre. In fact, we humans are the ones whose scarcely detectable ovulations make us members of a small minority in the animal world. Men have no reliable means of detecting when their partners can be fertilized, nor did women in traditional societies. I grant that many women experience headaches or other sensations around the midpoint of a menstrual cycle. However, they wouldn't know that these are signs of ovulation if they hadn't been told so by scientists-and even scientists didn't figure that out until around 1930. Similarly, women can be taught to detect ovulation by monitoring their body temperature or mucus, but that's very different from the instinctive knowledge possessed by female animals. If we too had such instinctive knowledge, manufacturers of ovulation test kits and contraceptives wouldn't be doing such a booming business.

We're also bizarre in our nearly continuous practice of sex, a behavior that is a direct consequence of our concealed ovulations. Most other animal species confine sex to a brief estrous period around the advertised time of ovulation. (The noun estrus and adjective estrous are derived from the Greek word for “gadfly,” an insect that pursues cattle and drives them into a frenzy.) At estrus, a female baboon emerges from a month of sexual abstinence to copulate up to one hundred times, while a female Barbary macaque does it on the average every seventeen minutes, distributing her favors at least once to every adult male in her troop. Monogamous gibbon couples go several years without sex, until the female weans her most recent infant and comes into estrus again. The gibbons relapse once more into abstinence as soon as the female becomes pregnant.

We humans, though, practice sex on any day of the estrus cycle. Women solicit it on any day, and men perform without being choosy about whether their partner is fertile or ovulating. After decades of scientific inquiry, it isn't even certain at what stage in the cycle a woman is most interested in men's sexual advances-if indeed her interest shows any cyclical variation. Hence most human copulations involve women who are unable to conceive at that moment. Not only do we have sex at the “wrong” time of the cycle, but we continue to have sex during pregnancy and after menopause, when we know for sure that fertilization is impossible. Many of my New Guinea friends feel obliged to have regular sex right up to the end of pregnancy, because they believe that repeated infusions of semen furnish the material to build the fetus's body.

Human sex does seem a monumental waste of effort from a “biological” point of view-if one follows Catholic dogma in equating sex's biological function with fertilization. Why don't women give clear ovulatory signals, like most other female animals, so that we can restrict sex to moments when it could do us some good? This chapter seeks to understand the evolution of concealed ovulation, nearly constant female sexual receptivity, and recreational sex-a trinity of bizarre reproductive behaviors that is central to human sexuality.

By now, you may have decided that I'm the prime example of an ivory tower scientist searching unnecessarily for problems to explain. I can hear several billion of the world's people protesting, “There's no problem to explain, except why Jared Diamond is such an idiot. You don't understand why we have sex all the time? Because it's fun, of course!”

Unfortunately, that answer doesn't satisfy scientists. While animals are engaged in sex, they too look as if they're having fun, to judge by their intense involvement. Marsupial mice even seem to be having lots more fun than we do, if the duration of their copulations (up to twelve hours) is any indication. Then why do most animals consider sex fun only when the female can be fertilized? Behavior evolves through natural selection, just as anatomy does. Hence if sex is enjoyable, natural selection must have been responsible for that outcome. Yes, sex is fun for dogs too, but only at the right time: dogs, like most other animals, have evolved the good sense to enjoy sex when it can do some good. Natural selection favors those individuals whose behavior lets them pass their genes to the most babies. How does it help you make more babies if you are crazy enough to enjoy sex at a time when you couldn't possibly make a baby?

A simple example illustrating the goal-directed nature of sexual activity in most animal species is provided by Pied Flycatchers, the bird species I discussed in chapter 2. Normally, a female Pied Flycatcher solicits copulation only when her eggs are ready to be fertilized, a few days before laying. Once she begins egg laying, her interest in sex vanishes and she resists propositions from males or behaves indifferently toward them. But in an experiment in which a team of ornithologists made twenty female Pied Flycatchers into widows after completion of egg laying by removing their mates, six of the twenty experimental widows were seen to solicit copulation from new males within two days, three were seen actually to copulate, and more may have done so unobserved. Evidently, the females were attempting to trick the males into believing them to be fertile and available. When the eggs eventually hatched, the males would have no way of realizing that some other male had actually fathered the clutches. In at least a few cases, the trick worked, and the males proceeded to feed the hatch-lings as a biological father would have. There was thus not the slightest indication that any of the females was a merry widow, pursuing sex for mere pleasure.

Since we humans are exceptional in our concealed ovu-lations, unceasing receptivity, and recreational sex, it can only be because we evolved to be that way. It's especially paradoxical that in Homo sapiens, the species unique in its self-consciousness, females should be unconscious of their own ovulation, when female animals as dumb as cows are aware of it. Something special was required to conceal ovulation from a female as smart and aware as a woman. As we'll discover, it has proven unexpectedly difficult for scientists to figure out what that special something was.