Dr. Tatiana's Sex Advice to All Creation by Olivia Judson Read Free Book Online Page A

the obviously greater risk of being eaten, Darwin came up with an idea even more radical than evolution by natural selection. He called it “sexual selection.” That is, males have these ornaments because females prefer to mate with the most beautiful males. Since beautiful males would therefore go on to have more children, enhanced sex appeal could more than make up for the risks involved in having a huge tail.
    Darwin was ridiculed. Females have a sense of beauty? Ludicrous. Females the ones who choose? Absurd. But as usual, he was right. These days, no one would dispute that females in many species actively decide whom to have sex with or that their preferences can drive the evolution of extravagant ornaments. What is still fiercely argued, though, is what they get by choosing the longest eyestalks or the biggest tail. Is it just sex appeal? Or is it something more?
    In theory, it could be either. Sheer sex appeal can evolve through something known as “Fisher’s runaway process,” or the advantage of having “sexy sons.” Who’s Fisher? Ronald Fisher was one of the great mathematical geneticists of the twentieth century. According to his model, female taste is initially arbitrary: girls like long tails, say, just because they do. But because of this, males with the longest tails mate the most. Females who
prefer the males with the longest tails have sons with the longest tails (the sexy sons)—who also mate the most. And so on. The result? Tails get longer and longer. When does it stop? One way it could stop is when the disadvantage of having a slightly longer tail—like becoming a predator’s lunch—outweighs the advantage of being just that little bit sexier.
    Alternatively, beauty might be more than skin deep. According to the “good genes” hypothesis, ridiculously long tails, fancy headdresses, or absurdly long eyestalks tell a girl not only that a boy has genes for long tails, fancy headdresses, and so on but that he has good genes in general. In other words, males with the longest tails are simply the best: they’re in the best shape because they have the best genes—and their offspring will be more likely to survive.
    Both the runaway process and the good genes theory make sense in principle. As usual, though, it’s difficult to work out what Mother Nature’s been up to in any particular case. Let me give you a couple of examples.
    When biologists study wild birds, they often put little colored bracelets on the birds’ legs so that they can tell who’s who from far away. Evidently, bracelets are not a characteristic that can be inherited. Yet it turns out that female zebra finches find a male sporting red bracelets to be extremely sexy. So sexy, indeed, that they will lay him an extra clutch of eggs. Green bracelets, however, have no such charm. Perhaps females find that green clashes with a fellow’s orange legs; in any case, males wearing green bracelets have no luck with the girls.
    But although this preference shows females can pick a mate based on sex appeal alone—the basic requirement for a runaway process—in more natural situations it is often exceedingly hard to tell whether females are mating because they find the males
sexy or because they’re hoping for good genes. An example of the difficulties comes from peacocks. Peachicks sired by males with fancier tails survive at a higher rate than those fathered by more humdrum-looking males—which at first glance supports the good genes hypothesis. The trouble is, there’s always the possibility that the effect appears simply because females who mate with sexy males take more trouble over their offspring. Among mallards, a duck who mates with the sexiest drake lays larger eggs than she does for an ugly fellow. Big eggs are a crucial factor in a chick’s early survival. But it turns out that a drake’s genes have no bearing on the size of the eggs.