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Consider, though, the case of Archbold’s bowerbird, which lives in New Guinea. As with other bowerbirds, the male builds an elaborate bower of twigs and ferns and therein tries to seduce females. The female inspects the bower and mates with the male if she likes the workmanship and the decorations, which are usually objects of one unusual colour. What is peculiar about Archbold’s bowerbird is that the best decorations consist of feathers from one particular kind of bird of paradise, known as the King of Saxony. These feathers, which are several times longer than the original owner’s body and stem from just above his eye, are like a car’s antenna sporting dozens of square blue pennants. Because they are moulted once a year, do not grow until the bird of paradise is four years old and are much in demand among local tribesmen, the plumes must be very hard for the bowerbird to acquire. Once acquired they must be guarded against other jealous male bowerbirds anxious to steal them for their own bowers. So, in the words of Jared Diamond, a female bowerbird who finds a male that has decorated his bower with King of Saxony plumes knows ‘that she has located a dominant male who is terrific at finding or stealing rare objects and defeating would-be thieves’.36

So much for the bowerbird. What about the bird of paradise itself, the rightful owner of the plumes? The fact that he survived long enough to grow plumes, grew longer ones than any other male nearby and kept them in good condition would be an equally reliable indicator of his genetic quality. But it reminds us of the thing that most puzzled Darwin and got the whole debate started. If the point of the plumes is to indicate his quality, might not the plumes themselves affect his quality? After all, every tribesman in New Guinea is out to get him, and every hawk will find him easier to spot. He may have indicated that he is good at surviving, but his chances of survival are now lower for having the plumes. They are a handicap. How can a system of females choosing males that are good at surviving encumber those males with handicaps to survival?

It is a good question with a paradoxical answer, for which we owe a debt of thanks to Amotz Zahavi, a mercurial Israeli scientist. In 1975 he saw that the more a peacock’s tail or a bird of paradise’s plumes handicapped the male, the more honest was the signal they sent to the female. She could be assured by the very fact of his survival that the long-tailed male in front of her had been through a trial and passed. He had survived despite being handicapped. The more costly the handicap, the better it would be as a signal of his genetic quality; therefore peacocks’ tails would evolve faster if they were handicaps than if they were not. This is the reverse of Fisher’s prediction that peacocks’ tails should gradually cease evolving once they become severe handicaps.37

It is an appealing – and familiar – thought. When a Maasai warrior killed a lion to prove himself to a potential wife, he was running a risk of being killed, but also showing that he had the necessary courage to defend a herd of cattle. Zahavi’s handicap was only a version of such initiation rituals. Yet it was attacked from all sides and the consensus was that he was wrong. The most telling argument against it was that the sons would inherit the handicap as well as the good genes. So they would be encumbered to exactly the same degree as they were endowed. They would be no better off than if they were unencumbered and unsexy.38

In recent years, however, Zahavi has been vindicated. Mathematical models proved that he might be right and his critics wrong.39 His vindicators have added to his theory two subtleties that lend it special relevance to the Good-gene theory of sexual selection. The first is that handicaps might (perhaps must) not only affect survival and reflect quality, but also do so in a graduated way: the weaker the male, the harder it would be to produce or maintain a tail of a given length. And, indeed, experiments on swallows have shown that