The Red Queen_ Sex and the Evolution of Human Nature - Matt Ridley [75]
The evidence has been pouring in since Hamilton and Zuk first published their theory. Some of it supports them, some does not. None quite meets all the criteria set out above. Just as the theory predicts that the more flamboyant species should be the ones most troubled by parasites, so it predicts that within a species, the more flamboyant a male’s ornament is, the lower will be his parasite burden. This proves to be true in diverse cases; it is also true that females generally favour males with fewer parasites. This holds for sage grouse, bowerbirds, frogs, guppies, even crickets.44 In swallows, females prefer males with longer tails, those males have fewer lice, and their offspring inherit louse resistance even when reared by foster swallow parents.45 Something similar is suspected in pheasants and junglefowl (the wild species to which domestic chickens belong).46 Yet these results are not shocking in the least. It would have been far more surprising to find females being seduced by sick, scrawny males than to find them succumbing to the charms of the healthiest. After all, they might be avoiding a sick male for no better reason than that they do not wish to catch his bug.47
Experiments done on sage grouse have begun to satisfy some of the sceptics. Mark Boyce and his colleagues at the University of Wyoming found that male grouse sick with malaria do poorly and so do males covered with lice. They noticed, too, that the lice were easy to notice because they left spots on the males’ inflated air sacs. By painting such spots on a healthy male’s sac, Boyce and his colleagues were able to reduce males’ mating success.48 If they could go on to show cycles from one resistance gene to another mediated by female choice, they would have given the Good-gene theory a significant boost.
The Symmetry of Beauty
In 1991 Anders Møller and Andrew Pomiankowski stumbled upon a possible way of settling the civil war between Fisher and Good-geners. It is symmetry. It is a well-known developmental accident that animals’ bodies are more symmetrical if they were in good condition when growing up, and less symmetrical if they were stressed while growing. For example, scorpionflies develop more symmetrically when fathered by well-fed fathers that could afford to feed their wives. The reason for this is simply the old spanner-in-the-works argument: making something symmetrical is not easy. If things go wrong, the chances are it will come out asymmetrical.49
Most body parts, like wings or beaks, should therefore be most symmetrical when just the right size and least symmetrical when stress has left them too small or too large. If Good-geners are right, ornaments should be most symmetrical when largest, because large ornaments indicate best genes and least stress. If Fisherians are right, you would expect no relationship between ornament size and symmetry; if anything, the largest ornaments should be least symmetrical. This is because the ornament’s size reflects nothing about its owner other than the fact that it can grow the largest ornament.
Møller noticed that, among the swallows he studied, the males with the longest tails also had the most symmetrical tails. This was quite unlike the pattern of other feathers, like wings, which obeyed the usual rule: the most symmetrical were the ones closest to the average length. In other words, whereas most feathers show a U-shaped curve of symmetry against length, tail streamers show a steady upward progression. Since the swallows with longest tails are the most successful in securing mates, it follows that the most symmetrical tails are also