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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 he wants to. He can feed his wife while she is pregnant and thereby contribute to the growth of the foetus, and he can carry the baby about when it is born or bring it food when it is weaned, but he cannot carry a foetus 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 if 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, 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 father’s sons. Male birds were cuckolding each other at a tremendous rate. In the indigo bunting, a pretty little blue bird from North America, which seemed to be faithfully monogamous, about forty per cent of the babies that the average male feeds in his nest are bastards.18

The 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 Møller 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 are fertilizing one female, and it is not hard to see why. The husband who ejaculates the most sperm will presumably fertilize the most eggs.

That came as no surprise. But the testicles of lekking birds like 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 only with 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 is competing 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 Møller noticed that the ones with large testicles were mostly birds that lived in colonies: sea birds, 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.19

Bill Hamilton believes that adultery may explain why in so many ‘monogamous’ birds the male is more gaudy 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 long after a male has found a wife in many species. Hamilton’s suggestion is that the gaudy male is not, like a peacock, trying to get more wives, but to get more lovers. He is advertising his availability for an