The monogamous species lie in between. Some have fairly small testicles, implying little sperm competition; others have huge testicles, as big as those of polyandrous birds. Birkhead and Moller noticed that the ones with large testicles were mostly birds that lived in colonies: seabirds, swallows, bee eaters, herons, sparrows. Such colonies give females ample opportunity for adultery with the male from the nest next door, an opportunity that is not passed up."
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Bill Hamilton believes that adultery may explain why in so many " monogamous " birds the male is gaudier than the female: The traditional explanation, suggested by Darwin, is that the gaudiest males or the best songsters get the first females to arrive, and an early nest is a successful nest. That is certainly true, but it does not explain why song continues in many species long after a male has found a wife. Hamilton 's suggestion is that the gaudy male is not trying to get more wives but more lovers. As Hamilton put it,
"Why did Beau Brummel in Regency England dress up as he did?
Was it to find a wife or to find an ' affair ' ?"2o EMMA BOVARY AND FEMALE SWALLOWS
What 's in it for the birds? For the males it is obvious enough: Adulterers father more young. But it is not at all clear why the female is so often unfaithfuclass="underline" Birkhead and Moller .rejected several suggestions: that she is adulterous because of a genetic side effect of the male adulterous urge, that she is ensuring some of the sperm she gets is fertile, that she is bribed by the philandering males (as seems to be the case in some human and ape societies). None of these fit the exact facts. Nor did it quite work to blame her infidelity on a desire for genetic variety. There seems to be little point in having more varied children than she would have anyway: Birkhead and Moller were left with the belief that female birds benefit from being promiscuous because it enables them to have their genetic cake and eat it—to follow the Emma Bovary strategy. A female swallow needs a husband who will help look after her young, but by the time she arrives at the breeding site, she might find all the best husbands taken: Her best tactic is therefore to mate with a mediocre husband or a husband with a good territory and have an affair with a genetically superior neighbor. This theory is supported by the facts: Females always choose more dominant, older, or more "attractive" (that is, ornamented) lovers than their husbands; they do not have affairs with bachelors (presumably rejects) but with other females ' husbands; and they sometimes incite competition between potential lovers and choose the winners. Male swallows
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with artificially lengthened tails acquired a mate ten days sooner, were eight times as likely to have a second brood, and had twice as high a chance of seducing a neighbor 's wife as ordinary swallows.'
(Intriguingly, when female mice choose to mate with males other than those they "live with, " they usually choose ones whose disease-resistance genes are different from their own.)''
In short, the reason adultery is so common in colonial birds is that it enables a male bird to have more young and enables a female bird to have better young:
One of the most curious results to come out of bird studies in recent years has been the discovery that "attractive" males make inattentive fathers: Nancy Burley, whose zebra finches consider one another more or less attractive according to the color of their leg bands, first noticed this,' and Anders Moller has since found it to be true of swallows as welclass="underline" When a female mates with an attractive male, he works less hard and she works harder at bringing up the young. It is as if he feels that he has done her a favor by providing superior genes and therefore expects her to repay him with harder work around the nest. This, of course, increases her incentive to find a mediocre but hardworking husband and cuckold him by having an affair with a superstud next door.'
In any case, the principle—marry a nice guy but have an affair with your boss or marry a rich but ugly man and take a handsome lover—is not unknown among female human beings: It is called having your cake and eating it, too: Flaubert's Emma Bovary wanted to keep both her handsome lover and her wealthy husband: The work on birds has been conducted by people who knew little of human anthropology: Iri just the same way, a pair of British zoologists had been studying human beings in the late 1980s, largely in isolation from the bird work: Robin Baker and Mark Bellis of Liverpool University were curious to know if sperm competition happened inside women, and if it did, whether women had any control over it. Their results have led to a bizarre and astonishing explanation of the female orgasm.
What follows is the only part of this book in which the details of sexual intercourse itself are relevant to an evolutionary MONOGAMY AND THE NATURE OF WOMEN
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argument: Baker and Bellis discovered that the amount of sperm that is retained in a woman 's vagina after sex varies according to whether she had an orgasm and when. It also depends on how long it was since she last had sex: The longer the period, the more sperm stays in, unless she has what the scientists call "a noncopulatory orgasm " in between:
So far none of this contained great surprises; these facts were unknown before Baker and Bellis did their work (which consisted of samples collected by selected couples and of a survey of four thousand people who replied to a questionnaire in a magazine), but they did not necessarily mean very much: But Baker and Bellis also did something rather brave: They asked their subjects about their extramarital affairs: They found that in faithful women about 55 percent of the orgasms were of the high-retention (that is, the most fertile) type: In unfaithful women, only 40 percent of the copulations with the partner were of this kind, but 70 percent of the copulations with the lover were of this fertile type: Moreover, whether deliberately or not, the unfaithful women were having sex with their lovers at times of the month when they were most fertile: These two effects combined meant that an unfaithful woman in their sample could have sex twice as often with her husband as with her lover but was still slightly more likely to conceive a child by the lover than the husband.
Baker and Bellis interpret their results as evidence of an evolutionary arms race between males and females, a Red Queen game, but one in which the female sex is one evolutionary step ahead: The male is trying to increase his chances of being the father in every way. Many of his sperm do not even try to fertilize her eggs but instead either attack other sperm or block their passage.
But the female has evolved a sophisticated set of techniques for preventing conception except on her own terms. Of course, women did not know this before now and therefore did not set out to achieve it, but the astonishing thing is that if the study by Baker and Bellis proves to be right, they are doing it anyway, perhaps quite unconsciously. This, of course, is typical of evolutionary explanations. Why do women have sex at all? Because they con-
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sciously want to. But why do they consciously want to? Because sex leads to reproduction, and being the descendants of those who reproduced, they are selected from among those who want things that lead to reproduction. This is merely a form of the same argument: The typical woman's pattern of infidelity and orgasm is exactly what you would expect to find if she were unconsciously trying to get pregnant from a lover while not leaving a husband: Baker and Bellis do not claim to have found more than a tantalizing hint that this is so, but they have tried to measure the extent of cuckoldry in human beings. In a block of flats in Liverpool, they found by genetic tests that fewer than four in every five people were the sons of their ostensible fathers. In case this had something to do with Liverpool, they did the same tests in southern England and got the same result. We know from their earlier work that a small degree of adultery can lead to a larger degree of cuckoldry through the orgasm effect. Like birds, women may be—quite unconsciously—