Why Is Sex Fun__ The Evolution of Human Sexuality - Jared M. Diamond [35]
But we can still resort to another powerful technique preferred by evolutionary biologists for solving such problems. It’s termed the comparative method. We humans, it turns out, aren’t unique in our concealment of ovulation. While it’s exceptional among mammals in general, it’s fairly common among higher primates (monkeys and apes), the group of mammals to which we belong. Dozens of primate species show no externally visible signs of ovulation; many others do show signs, albeit slight ones; and still others advertise it flagrantly. The reproductive biology of each species represents the outcome of an experiment, performed by nature, on the benefits and drawbacks of concealing ovulation. By comparing primate species, we can learn which features are shared by those species with concealed ovulation but are absent from those species with advertised ovulation.
That comparison throws new light on our sexual habits. It was the subject of an important study by the Swedish biologists Birgitta Sillén-Tullberg and Anders Møller. Their analysis proceeded in four steps.
Step 1. For as many higher primate species as possible (sixty-eight in all), Sillén-Tullberg and Møller tabulated visible signs of ovulation. Aha!—you may object immediately—visible to whom? A monkey may give signals invisible to us humans but obvious to another monkey, such as odors (pheromones). For example, cattle breeders trying to perform artificial insemination on a prize dairy cow have big problems figuring out when the cow is ovulating. Bulls, though, can tell easily by the cow’s smell and behavior.
Yes, that problem can’t be ignored, but it’s more serious for cows than for higher primates. Most primates resemble us in being active by day, sleeping at night, and depending heavily on their eyes. A male rhesus monkey whose nose isn’t working can still recognize an ovulating female monkey by the slight reddening around her vagina, even though her reddening is not nearly so obvious as in a female baboon. For those monkey species that we humans classify as having no visible signs of ovulation, it’s often clear that the male monkeys are equally confused, because they copulate at totally inappropriate times, such as with non-estrous or pregnant females. Hence our own ratings of “visible signs” aren’t worthless.
The result of this first step of the analysis was that nearly half of the primates studied—thirty-two out of sixtyeight—resemble humans in lacking visible signs of ovulation. Those thirty-two species include vervets, marmosets, and spider monkeys, as well as one ape, the orangutan. Another eighteen species, including our close relative the gorilla, exhibit slight signs. The remaining eighteen species, including baboons and our close relatives the chimpanzees, advertise ovulation conspicuously.
Step 2. Next, Sillén-Tullberg and Møller categorized the same sixty-eight species according to their mating system. Eleven species—including marmosets, gibbons, and many human societies—turn out to be monogamous. Twenty-three species—including other human societies, plus gorillas—have harems of females controlled by a single adult male. But the largest number of primate species—thirtyfour, including vervets, bonobos, and chimpanzees—have a promiscuous system in which females routinely associate and copulate with multiple males.
Again I hear cries of Aha!—Why aren’t humans also classified as promiscuous? Because I was careful to specify routinely. Yes, most woman have multiple sex partners in sequence over their lifetimes, and many women are at times involved with multiple men simultaneously. However, within any given estrus cycle the norm is for a woman to be involved with a single man, but the norm for a female vervet or