The Red Queen_ Sex and the Evolution of Human Nature - Matt Ridley [78]
The implication for sexual selection is that it does not pay to lie. Having sex-hormone levels that are too high for your status increases the size of your ornaments but makes you more vulnerable to parasites, which are revealed in the state of those ornaments. It is possible that it works in the other direction: the immune system suppresses the production of testosterone. In Zuk’s words, ‘Males are thus necessarily more vulnerable to disease as they acquire the accoutrements of maleness.’59
The best proof of these conjectures comes from a study of roach, which are small fish with reddish fins, in the Bielersee in Switzerland. Male roach grow little tubercles all over their bodies during the breeding season, which seem to stimulate females during courtship as the fish rub against each other. The more parasites a male has, the fewer tubercles he grows. It is possible for a zoologist to judge, just from a male’s tubercles, whether he is infested with a roundworm or a flatworm. The implication follows: if a zoologist can deduce which parasite is present, a female roach probably can as well. This pattern results from different kinds of sex hormone; one can be raised in concentration only at the expense of leaving the roach vulnerable to one kind of parasite, and the other can be raised only at the expense of lowering defences against another kind of parasite.60
If cockerels’ wattles and roach tubercles are honest signals, so presumably are songs. A nightingale that can sing loud and long must be in vigorous health and one that has a large repertoire of different melodies must be experienced or ingenious, or both. An energetic display like the pas de deux of a pair of male manakins may also be an honest signal. A bird that merely shows its feathers, like a peacock or a bird of paradise, might be a cheat whose strength has been sapped by bad habits since he grew the plumes: after all, peacock feathers still shine brightly when their owner is dead and stuffed. Perhaps, then, it is no surprise that most male birds do not moult just before the breeding season, but adopt their spring plumage the autumn before. They have to keep it tidy all winter. The very fact that a male has looked after his plumes for six months tells a female something about his enduring vigour. Bill Hamilton points out that white fluffy feathers around a bird’s rear end, which are common in grouse of various kinds, must be especially hard to keep clean if the bird has parasite-induced diarrhoea.61
Zahavi certainly believed that honesty was a prerequisite of handicaps – and vice versa. To be honest, he thought, an ornament must be costly; otherwise it could be used to cheat. A deer cannot grow large antlers without consuming five times its normal daily intake of calcium; a pupfish cannot be iridescent blue unless it is genuinely in good condition, a fact that will be tested by other male fish in fights. On the assumption that anybody who refuses to play the game and use an honest signal must have something to hide, males are likely to find themselves dragged into honest displays. Therefore, display ornaments are examples of the notion of ‘truth in advertising’.62
All this is very logical, but in about 1990 it started to make one group of biologists uneasy. They had an instinctive aversion to the idea that sexual advertising is about the truth, because they knew that television advertising is not about passing on information; it is about manipulating the viewer. In the same way, they argued, all animal communication is about manipulating the receiver.
The first and most eloquent (manipulative?) champions of this view were the two Oxford biologists, Richard Dawkins and John Krebs. According to them, a nightingale does not sing to inform potential mates all about himself; he sings to seduce them. If that means lying about his true prowess, so be it.63
An ice-cream advertisement is honest,