The Queen - Matt Ridley

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But human testicles are not nearly as large as those of chimps, and there is some tentative evidence that they are not operating on full power (that is, they might once have been bigger in our ancestors): Sperm production per gram of tissue is unusually low in man. All in all, it seems fair to conclude that women are not highly promiscuous, which is what we expected to find:"

It is not just monkeys, apes, and dolphins that have large testicles when faced with sperm competition. Birds do, too. And it is from birds that the clinching clue comes about the human mating system. Zoologists have long known that most mammals are polygamous and most birds are monogamous. They put this down to the fact that the laying of eggs gives male birds a much earlier opportunity to help rear his children than a male mammal ever has: A male bird can busy himself with building the nest, with sharing the duties of incubation, with bringing food for the young; the only thing he cannot do is lay the eggs: This opportunity allows junior male birds to offer females a more paternal alternative than merely inseminating them, an offer that is accepted in species that have to feed their young, such as sparrows, and rejected in those that do not feed their young, such as pheasants.

Indeed, in some birds, as we have seen, the male does all these things alone, leaving his mate with the single duty of egg laying for her many husbands. In a mammal, by contrast, there is not much he can do to help even if wants to: He can feed his wife while she is pregnant and thereby contribute to the growth of the fetus, and he can carry the baby about when it is born or bring it food when it is weaned, but he cannot carry a fetus in his belly or feed it milk when it is born. The female mammal is left literally holding the baby, and with few opportunities to help her, the male is often better off expending his energy on an attempt to be a polygamist.

Only when opportunities for further mating are few and his presence increases the baby 's safety—as in gibbons—will he stay.

This kind of game-theory argument was commonplace by the mid-1970s, but in the 1980s when it became possible for the first time to do genetic blood testing of birds, an enormous surprise was in store for zoologists: They discovered that many of the baby

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birds in the average nest were not their ostensible father 's offspring.

Male birds were cuckolding one another at a tremendous rate. In the indigo bunting, a pretty little blue bird from North America that seemed to be faithfully monogamous, about 40 percent of the babies the average male feeds in his nest are bastards. 1eThe zoologists had entirely underestimated an important part of the life of birds. They knew it happened, but not on such a scale. It goes under the abbreviation EPC, for extra-pair copulation, but I will call it adultery, for that is what it is. Most birds are indeed monogamous, but they are not by any means faithful.

Anders Moller is a Danish zoologist of legendary energy whom we have already met in the context of sexual selection. He and Tim Birkhead from Sheffield University have written a book that summarizes what is now known about avian adultery, and it reveals a pattern of great relevance to human beings. The first thing they proved is that the size of a bird 's testicles varies according to the bird 's mating system. They are largest in polyandrous birds, where several males fertilize one female, and it is not hard to see why. The male who ejaculates the most sperm will presumably fertilize the most eggs.

That came as no surprise. But the testicles of lekking birds, such as sage grouse, where each male may have to inseminate fifty females in a few weeks, are unusually small: This puzzle is resolved by the fact that a female sage grouse will mate only once or twice and usually with only one male. That, remember, is the whole point of female choosiness at leks. So although the master cock may need to mate with many hens, he need not waste much sperm on each because those sperm will have no competitors. It is not how often a male bird copulates that determines the size of his testicles but how many other males he competes with.

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."

MONOGAMY AND THE NATURE OF WOMEN

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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 unfaithful: 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 well: 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.'