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By Root 238 0

The woman gains by recruiting an active coparent. But the man also gains, provided that he cooperates and plays by the rules of his wife’s body. By staying home, he acquires confidence that the child whom he is helping to rear really does carry his genes. He needn’t be fearful that, while he is off hunting, his wife (like a female baboon) may start flashing a bright red derrière as an advertisement for her imminent ovulation, thereby attracting swarms of suitors and publicly mating with every man around. Men accept these ground rules to such a degree that they continue to have sex with their wives during pregnancy and after menopause, when even men know that fertilization is impossible. Thus, in Alexander and Noonan’s view, women’s concealed ovulations and constant receptivity evolved in order to promote monogamy, paternal care, and fathers’ confidence in their paternity.

Competing with this view is the many-fathers theory developed by the anthropologist Sarah Hrdy of the University of California at Davis. Anthropologists have long recognized that infanticide used to be common in many traditional human societies, although modern states now have laws against it. Until recent field studies by Hrdy and others, though, zoologists had no appreciation for how often infanticide occurs among animals as well. The species in which it has been documented now include our closest animal relatives, chimpanzees and gorillas, in addition to a wide range of other species from lions to African hunting dogs. Infanticide is especially likely to be committed by adult males against infants of females with whom they have never copulated—for example, when intruding males try to supplant resident males and acquire their harem of females. The usurper thus “knows” that the infants killed are not his own.

Naturally, infanticide horrifies us and makes us ask why animals (and formerly humans) do it so often. On reflection, one can see that the murderer gains a grisly genetic advantage. A female is unlikely to ovulate as long as she is nursing an infant. But a murderous intruder is genetically unrelated to the infants of a troop that he has just taken over. By killing such an infant, he terminates its mother’s lactation and stimulates her to resume estrus cycles. In many or most cases of animal infanticide and takeovers, the murderer proceeds to fertilize the bereaved mother, who bears an infant carrying the murderer’s own genes.

As a major cause of infant death, infanticide is a serious evolutionary problem for animal mothers, who thereby lose their genetic investment in murdered offspring. For instance, a typical female gorilla over the course of her lifetime loses at least one of her offspring to infanticidal intruding male gorillas attempting to take over the harem to which she belongs. Indeed, over one-third of all infant gorilla deaths are due to infanticide. If a female has only a brief, conspicuously advertised estrus, a dominant male can easily monopolize her during that time. All other males consequently “know” that the resulting infant was sired by their rival, and they have no compunctions about killing the infant.

Suppose, though, that the female has concealed ovulations and constant sexual receptivity. She can exploit those advantages to copulate with many males—even if she has to do it sneakily, when her consort isn’t looking. While no male can then be confident of his paternity, many males recognize that they might have sired the mother’s eventual infant. If such a male later succeeds in driving out the mother’s consort and taking her over, he avoids killing her infant because it could be his own. He might even help the infant with protection and other forms of paternal care. The mother’s concealed ovulation will also serve to decrease fighting between adult males within her troop because any single copulation is unlikely to result in conception and hence is no longer worth fighting over.

As an example of how widely females may thus use concealed ovulation to confuse paternity, consider the African monkeys called vervets, familiar