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preference for the chuck just happens to exist in the fact that the female 's ear (to be precise, the basilar papillae of the inner ear) is tuned to the chuck 's frequency; the male has, in evolutionary terms, discovered and exploited this. In Ryan 's mind this deals a blow to the whole house of female-choice theory: That theory, whether in Fisher 's sexy-son form or the Good-genes form, predicts that the male's ornament and the female 's preference for such an ornament will evolve together: Ryan's result seems to suggest that the preference existed fully formed before the male ever had the ornament: Peahens preferred eyed tails a million years ago when peacocks still looked like big chickens. 66
Lest the tungara frog be thought a fluke, a colleague of Ryan's, Alexandra Basolo, has found exactly the same thing in a fish called the platyfish. Females prefer males who have had long sword-shaped extensions stuck onto their tales: Males of a different species called the swordtail have such swords on their tails, yet none of the platyfish 's other relatives have swords, and it stretches belief to argue that they all got rid of the sword rather than that the swordtail acquired it: The preference for sworded tails was there, latent, in platyfish before there were swords."
In one sense what Ryan is saying is unremarkable. That male displays should be suited to the sensory systems of females is only to be expected: Monkeys and apes are the only mammals with good color vision: Therefore, it is not surprising that they are the only mammals decorated with bright colors such as blue and pink.
Likewise, it is hardly remarkable that snakes, which are deaf, do not sing to each other. (They hiss to scare hearing creatures:) Indeed, one could list a whole panoply of "peacocks ' tails" for each of the five senses and more: the peacock 's tail for vision, the nightingale's song for hearing, the scent of the musk deer for smell;" the pheromones of the moth for taste; the "morphological exuberance "
of some insect "penises " for touch; 69 even the elaborate electrical courtship signals of some electric fish" for a sixth sense. Each species chooses to exploit the senses that its females are best at detecting. This is, in a sense, to return to Darwin 's original idea: that females have aesthetic senses, for whatever reason, and that those senses shape male ornaments."
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Moreover, you would expect the males to pick the method of display that is least dangerous or costly. Those that did so would last longer and leave more descendants than those that did not. As every bird-watcher knows, the beauty of a bird 's song is inversely correlated with the colorfulness of its plumage. The oper-atic male nightingales, warblers, and larks are brown and usually almost indistinguishable from their females. Birds of paradise and pheasants (in which the males are gorgeous, the females dull) are monotonous, simple songsters given to uninspired squawks.
Intriguingly, the same pattern holds among the bowerbirds of New Guinea and Australia: The duller the bird, the more elaborate and decorated its bower. What this suggests is that nightingales and bowerbirds have transferred their color to their songs and bowers.
There are clear advantages to doing so. A songster can switch his ornament off when danger threatens. A bower builder can leave his behind."
More direct evidence of this pattern comes from fish. John Endler of the University of California at Santa Barbara studies the courtship of guppies and is especially interested in the colors adopted by male guppies: Fish have magnificent color vision; whereas we use three different types of color-detecting cells in the eye (red, blue, and green), fish have four, and birds have up to seven. Compared to the way birds see the world, our lives are mono-chrome. But fish : also have a very different experience from us because their world filters out light of different colors in all sorts of variable ways. The deeper they live, the less red light penetrates compared with blue. The browner the water, the less blue light penetrates. The greener the water, the less red or blue light penetrates.
And so on. Endler 's guppies live in South American rivers; when courting, they are usually in clear water where orange, red, and blue are the colors that show up best. Their enemies, however, are fish that live in water where yellow light penetrates best. Not surprisingly, male guppies are never yellow.
The males use two kinds of color, one red-orange, which is produced by a carotenoid pigment that the guppy must acquire from its food, and the other blue-green, which is caused by guanine crystals in the skin that are laid down when the guppy reaches
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maturity. Female guppies that live in tea-colored water, where red-orange is more easily seen, are more sensitive to red-orange light than to blue, which makes sense. The brains of such guppies are tuned to exactly the wavelength of the red-orange carotenoid pigment the male uses in display—and perhaps vice versa."
OF MOZART AND GRACKLE SONG
Down the corridor from Ryan at the University of Texas is Mark Kirkpatrick, who is prepared to upset even more apple carts. Kirkpatrick is acknowledged as one of those who understands sexual selection theory most thoroughly; indeed, he was one of those who made Fisher's idea mathematically respectable in the early 1980s.
But he now refuses to accept that we must choose between Fisher and Zahavi. He does so partly because of what Ryan has discovered: This does not mean Kirkpatrick rejects female choice, as Julian Huxley did. Whereas Huxley thought males did the choosing by fighting among themselves, Kirkpatrick prefers to believe that in many species the females do choose, but their preferences do not evolve. They merely saddle the males with their own idiosyncratic tastes.
Both Good-genes and Fisher theories are obsessed with trying to find a reason for exuberant display that benefits the male.
Kirkpatrick looks at it from the female 's point of view: Suppose, he says, that peahens' preferences have indeed saddled peacocks with their tails. Why must we explain these female preferences only in terms of the effects on their sons and daughters? Might the peahens not have perfectly good direct reasons for choosing as they do? Might their preferences not be determined by something else entirely? He thinks "other evolutionary forces acting on the preferences will overwhelm the Good-genes factor and often establish female preferences for traits that decrease male survival.'
Two recent experiments support the idea that females simply have idiosyncratic tastes that have not evolved. Male grackles—
blackish birds of medium size—sing only one kind of song. Female
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grackles prefer to mate with males that sing more than one kind of song: William Searcy of the University of Pittsburgh discovered why. He made use of the fact that a female grackle will go up to singing loudspeakers and adopt a soliciting posture as if waiting to be mated. Her tendency to do so declines, however, as she gets bored with the song: Only if the loudspeaker starts singing a new song will her soliciting start afresh. Such " habituation" is just a property of the way brains work; our senses, and those of grackles, notice novelty and change, not steady states: The female preference did not evolve; it just is that way."
Perhaps the most startling discovery in sexual selection theory was Nancy Burley 's work on zebra finches in the early 1980s.
She was studying how these small Australian finches choose their mates, and to make it easier she kept them in aviaries and marked each one with a colored ring on its leg. After a while she noticed something odd: The males with red rings seemed to be preferred by the females. Further experiments proved that the rings were drastically affecting the "attractiveness " of both males and females.