COPYRIGHT Statesman and Nation Publishing Company Ltd. (UK) 1994

In the 1980s, a number of women scientists, led by Sarah Hrdy, now of the University of California at Davis, began to notice that the promiscuous behaviour of female chimpanzees and monkeys sat awkwardly alongside theories that heavily female-biased parental investment leads directly to female sexual choosiness. Far from being choosy, female primates seemed to be initiators of much promiscuity. Hrdy began to suggest that there was something wrong with the theories.

The solution to Hrdy's concern lay in her own work. In her study of the langurs of Abu, in Rajasthan, Hrdy had discovered a grisly fact; the murder of baby monkeys by adult male monkeys was routine. Every time a male takes over a troop of females, he kills all the infants in the group. Exactly the same phenomenon had been discovered in lions and is common in rodents, carnivores and primates. Even our closest relatives, the chimpanzees, are guilty. Infanticide, said Hrdy and her colleagues, was an "adaptation"--an evolved strategy. By killing their stepchildren, the males would halt the females' milk production and so bring forward the date on which the mother could conceive again. Hrdy suggested that female promiscuity in monkeys and apes can be explained by the need to share paternity among many males to prevent infanticide. But does it apply to humanity?

The short answer is no. It is a fact that stepchildren are 65 times more likely to die than children living with their true parents, and it is inescapable that young children often have a terror of new stepfathers that is hard to overcome. But neither of these facts is of much relevance, for both apply to older children, not to suckling infants. Their deaths do not free the mother to breed again.

Moreover, the fact that we are apes can be misleading. Our sex lives are very different from those of our cousins. Compared to our ape cousins, we, the commonest of the great apes, have pulled off a surprising trick. We have somehow reinvented monogamy and paternal care without losing the habit of living in large multi-male groups. Like gibbons, men marry women singly and help them to rear their young, confident of paternity, but, like chimpanzees, those women live in societies where they have continual contact with other men. There is no parallel for this among apes. However, there is a close parallel among birds. Many birds live in colonies, but mate monogamously within the colony. And the bird parallel brings an altogether different explanation for females to be interested in sexual variety.

A female human being does not have to share her sexual favours with many males to prevent infanticide, but she may have a good reason to share them with one well-chosen male apart from her husband. This is because her husband is, almost by definition, usually not the best male there is---else how would he have ended up married to her? His value is that he is monogamous and will not divide his child-rearing effort among several families. But why accept his genes? Why not have his parental care and another's genes?

In the 1980s, it became possible for the first time to do genetic blood-testing of birds, and an enormous surprise was in store for zoologists. They discovered that many of the baby birds in the average nest were not their ostensible fathers' sons. Male birds were cuckolding each other at a tremendous rate.

What's in it for the birds? For the males, it is obvious enough: adulterers father more young. But for the female, it is not at all clear why she is so often unfaithful. Zoologists Tim Birkhead and Anders Moller have suggested that female birds benefit by being promiscuous because it enables them to have their genetic cake and eat it--to follow the Emma Bovary strategy of adultery from within marriage. A female bird needs a husband who will look after her young, but by the time she arrives at the breeding site she might find all the best husbands taken. Her best lactic is therefore to mate with a mediocre husband or a husband with a good nest site and have an affair with a genetically superior neighbour.

This theory is supported by the facts: females always choose males that are more dominant, older, or more "attractive" (for example, ornamented with longer tail feathers) lovers than their husbands; they do not have affairs with bachelors (meaning, presumably, those rejected by other females), but with other females' husbands; and they sometimes incite competitions between potential lovers and choose the winners. In Moller's study, male swallows 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 neighbour's wife as ordinary male swallows.

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.

The work on birds has been conducted by people who knew little of human anthropology. In 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 an astonishing explanation for female orgasm.

They discovered that the amount of sperm retained in the vagina varies according to the woman's tendency to have an orgasm. If she has no orgasm, or if she has an orgasm more than a minute before the male ejaculates, very little sperm remains in the vagina at all. If she has an orgasm less than a minute before him or up to 45 minutes after him, then most of the sperm stays in.

Baker and Bellis also asked their subjects about their extramarital affairs. They found that in faithful women about 55 per cent of the orgasms were of the high-retention (most fertile) type. In unfaithful women, only 40 per cent of the copulations with the partner were of this kind, but 70 per cent 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. This 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, 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 fertilise her eggs, but instead either attack other sperm or block their passage. By these, and other means, the male's sexual behaviour is designed to maximise his chances of fertilising an egg. But the female has evolved a sophisticated set of techniques for preventing conception except on her own terms. In particular, by judicious orgasm she can virtually decide by which of two lovers she chooses to be impregnated. Of course, women did not know this before now and do 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.

Baker and Bellis do not claim to have found more than a tantalising 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. The rest were apparently fathered by somebody else. In case this was 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. Just like birds,. women may be--quite unconsciously--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.

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