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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