Growing and displaying the spectacular tail feathers with their iridescent greens, blues and burnished gold is a huge burden for the male peacock. It is cumbersome, heavy and dangerous, making the bird much more liable to being seen, and seized, by predators. But without a magnificent fail there is absolutely not chance of getting any sex. The drab yet presumably seductive females, safe in their camouflaged fatigues of brown and cream, demand and receive a full display before they consent to mate. If the show fails to impress then the peahen turns and retreats into the undergrowth, leaving the poor male disappointed and, literally, crest-fallen. He packs away his finery and carries on with life until the next time.
The splendour of the peacock's tail is a direct result of what Darwin called sexual selection. As each new mutation arose to make the tail just that bit longer or the eye just that bit bluer it will have spread to succeeding generations through its ability to impress the females, who have also evolved a discerning eye that has a preference for such opulence. But what is it that the female really wants? The peacock isn't going to help raise the chicks and, after mating, the pair need never meet again. So quite why have generations after generations of peahens demanded to see the shimmering display? In a word, it's advertising. The peacock is signalling something else to the female—the quality of his genes. He is saying, in effect: I am so healthy and So strong that I can afford to waste all that energy on producing an intrinsically useless ornament—so my other genes must be absolutely sensational.
The world is full of other examples of sexual selection, where the preferences of one sex drive the evolution of features in the other which they find attractive in a mate. The supply and demand economics of sperm and egg production means that it is almost always the male who is trying to impress the female; the male who advertises and the female who chooses. As in any commercial campaign, only those males that do what the consumer wants reap the rewards. It is no good adding a new feature to the display that females don't appreciate. Peacocks may have beautiful tails but they can't sing, A peacock that could sing as sweetly as a nightingale would be wasting his time because peahens are not tuned in to song. Equally, a male nightingale with a brilliant blue-green tail would not make any impression on a female nightingale. Darwin realized that it was not just the features of the display itself that were evolving under the pressure of consumer choice, but the complementary ability to appreciate the product—and the desire for more of the same.
This was shown very nicely in an experiment with African widowbirds. The males have extremely long tail-feathers which they show off as they fly around their breeding territories. As you would expect, the males with the longest tails had most success in persuading females to mate with them. A team of biologists captured males and artificially shortened or lengthened their tails by cutting and grafting the central feathers with glue, then released them to see whether they did better or worse in attracting females than before the surgery. Sure enough, the males whose feathers had been artificially lengthened now did much better in that department, while the birds whose tails had been shortened suddenly found their seductive powers dramatically diminished. This straightforward test showed that the male's success in attracting females depended entirely on the length of his tail—not on his general vitality or on any other feature which the females on the ground were able to make out and factor in to their mating decisions. They were gauging these qualities indirectly by the extravagance of the tail. When the researchers released birds with tails surgically enhanced so that they were longer than any ever seen in the wild, these birds did best of all, irrespective of how puny their tails had been at the start. Clearly, the female widowbirds' appetite for longer and longer tails is still not satisfied, and the males will just have to try harder in the future.
There seems to be no consistency in which features are enhanced by sexual selection, and it may just have been chance which started the ball rolling in one particulars direction. The ancestor of the first peacock probably just happened to grow a slightly showy tail, which just happened to appeal to a female. It could well have been something else—a slightly different head shape or a new way of walking. But once the male and female, advertiser and consumer, were on the same wavelength, they were both locked into an evolutionary spiral which exaggerated that particular feature and not others.
Darwin realized two things about sexual selection that set it apart from his earlier and better known theory of evolution by natural selection. The first was the speedy
with which it could change a species. Evolution by natural selection is usually excruciatingly slow, but sexual selection can transform a species extremely quickly, and where there has been rapid change, it is worth considering whether sexual rather than natural selection is at work. In this respect, no species has changed more rapidly than our own. Our immediate ancestors have conquered the world in less than a quarter of a million years since our beginnings in Africa. The common ancestor we share with chimpanzees, our closest primate relative, lived only six million years ago. These are long periods of time by our day-to-day reckoning, to be sure, but extremely brief in evolutionary terms. We certainly do have a lot in common with chimpanzees and other apes, but there are also a hell of a lot of differences: our upright posture, a very large brain, language, reasoning, art, superb manual dexterity—all features that are almost invisible in our primate cousins. All these features developed extremely rapidly in our ancestors, but failed to materialize in pur close genetic relatives. Could this speedy transition have something to do with sexual selection? Did our male ancestors, with slightly larger brains and slightly better communication skills, slightly cleverer and slightly better with their hands, have the edge over their contemporaries, not so much in adapting to the external environment, but in getting more women to mate with them? Just as in the case of the peacock's tail, a successful campaign depends on a receptive and appreciative female audience who keep asking for more. But while the avian admirers could be appreciative without themselves growing gaudy plumage, our female ancestors would have needed to keep up, even keep one step ahead of the game. Eloquence is no use to a suitor whose object of desire doesn't speak a word.