having it both ways by conducting affairs with genetically more valuable men while not leaving their husbands:
What about the men? Baker and Bellis did an experiment on rats and discovered that a male rat ejaculates twice as much sperm when he knows that the female he is mating with has been near another male recently. The intrepid scientists promptly set out to test whether human beings do the same: Sure enough, they do. Men whose wives have been with them all day ejaculate much smaller amounts than men whose wives have been absent all day. It is as if the males are subconsciously compensating for any opportunities for female infidelity that might be present. But in this particular battle of the sexes, the women have the upper hand because even if a man—again unconsciously—begins to associate his wife 's lack of late orgasms with a desire not to conceive his child, she can always respond by faking them. 2S
CUCKOLDRY PARANOIA
The cuckold, however, does not stand by and accept his evolutionary lot even unto the extinction of his genes: Birkhead and Moller MONOGAMY AND THE NATURE OF WOMEN
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think that much of the behavior of male birds can be explained by the assumption that they are in constant terror of their wives ' infidelity. Their first strategy is to guard the wife during the period when she is fertile (a day or so before each egg is laid): Many male birds do this: They follow their mates everywhere, so that a female bird who is building a nest is often accompanied on every trip by a male who never lends a hand; he just watches. The moment she is finished laying the clutch of eggs, he relaxes his vigil and begins to seek adulterous opportunities himself.
If a male swallow cannot find his mate, he often gives a loud alarm call, which causes all the swallows to fly into the air, effectively interrupting any adulterous act in progress. If the pair has just been reunited after a separation or if a strange male intrudes into the territory and is chased out, the husband will often copulate with the wife immediately afterward, as if to ensure that his sperm are there to compete with the intruder 's.
Generally it works. Species that practice effective mate guarding keep the adultery rate low. But some species cannot guard their mates. In herons and birds of prey, for example, husband and wife spend much of the day apart, one guarding the nest while the other collects food. These species are characterized by extremely frequent copulation.
Goshawks may have sex several hundred times for every clutch of eggs.
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This does not prevent adultery, but at least it dilutes it: Just like herons and swallows, people live in monogamous pairs within large colonies. Fathers help to rear the young if only by bringing food or money: And crucially, because of the sexual division of labor that characterized early human hunter-gathering societies (broadly speaking, men hunt, women gather), the sexes spend much time apart. So women have ample opportunities for adultery, and men have ample incentives to guard their mates or, failing that, to copulate frequently with them.
To demonstrate that adultery is a chronic problem throughout human society, rather than an aberration of modern apartment blocks in Britain, is paradoxically difficult: first, because the answer is so stunningly obvious that nobody has studied it, and second, because it is a universally kept secret and therefore almost impossible to study. It is easier to watch birds:
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The Red Queen
Nonetheless, attempts have been made. The 570 or so Ache people of Paraguay were hunter-gatherers until 1971, living in twelve bands. They then gradually came into contact with the outside world and were lured onto government reservations run by missionaries: Today, they no longer depend on hunted meat and gathered fruit but grow most of their own food in gardens. But when they still depended on men's hunting skills for much of their food, Kim Hill of the University of New Mexico found an intriguing pattern: Ache men would donate any spare meat they had caught to women with whom they wanted to have sex. They were not doing so to feed children they might have fathered but as direct payment for an affair: It was not easy to discover. Hill found that he was gradually forced to drop questions about adultery from his studies because the Ache, under missionary influence, became increasingly squeamish about discussing the subject. The chiefs and the head men were especially reluctant to talk about it, which is hardly surprising in view of the fact that they were the ones having the most affairs. Nonetheless, by relying on gossip Hill was able to piece together the pattern of adultery in the Ache: As expected, he found that high-ranking men were involved most, which is consistent with the idea of having your paternal-genetic cake and eating it: However, unlike in birds, it was not just the wives of low-ranking men who indulged. It is true that Ache adulterers frequently ply their mistresses with gifts of meat, but Hill thinks the most important motive is that Ache women are constantly preparing for the possibility that they will be deserted by their husbands. They are building up alternative relationships and are more likely to be unfaithful if the marriage is going badly. That is, of course, a double-edged sword: The marriage could break up because the affair is discovered:''
Whatever the motive for women, Hill and others believe that adultery has been much underemphasized as an influence in the evolution of the human mating system. In hunter-gatherer societies the male opportunist streak would have been far more easily satisfied by adultery than by polygamy. In only two known hunter-gatherer societies is polygamy either common or extreme. In the rest it is rare to find a man with more than one wife and very rare MONOGAMY AND THE NATURE OF WOMEN
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to find a man with more than two. The two exceptions prove the rule. One is among the Indians of the Pacific Northwest of America, who depended on abundant and reliable supplies of salmon and were more like farmers than hunter-gatherers in their ability to stockpile surpluses: The other is certain tribes of Australian aborigines, which practice gerontocratic polygamy: Men do not marry until they are forty, and by the age of sixty-five they have usually accumulated up to thirty wives: But this peculiar system is far from what it seems. Each old man has younger assistant men whose help, protection, and economic support he purchases by, among other things, turning a blind eye to their affairs with his wives. The old man looks the other way when the helpful nephew carries on with one of his junior wives.28
Polygamy is rare in hunter-gatherer societies, but adultery is common wherever it has been looked for. By analogy with monogamous colonial birds, therefore, one would expect to find human beings practicing either mate guarding or frequent copulation: Richard Wrangham has speculated that human beings practice mate guarding in absentia: Men keep an eye on their wives by proxy.
If the husband is away hunting all day in the forest, he can ask his mother or his neighbor whether his wife was up to anything during the day. In the African pygmies Wrangham studied, gossip was rife and a husband 's best chance of deterring his wife 's affairs was to let her know that he kept abreast of the gossip. Wrangham went on to observe that this was impossible without language, so he speculated that the sexual division of labor, the institution of child-rearing marriages, and the invention of language—three of the most fundamental human characteristics shared with no other ape—all depend on another. 29