The Red Queen_ Sex and the Evolution of Human Nature - Matt Ridley [81]
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 colour vision. Therefore, it is not surprising that they are the only mammals decorated with bright colours such as blue or 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;69 the pheromones of a moth for taste; the ‘morphological exuberance’ of some insect ‘penises’ for touch;70 even the elaborate electrical courtship signals of some electric fishes71 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.72
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 colourfulness of its plumage. The operatic male nightingales, warblers and larks are brown and usually almost indistinguishable from their females. Birds of paradise, 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 is its bower. What this suggests is that nightingales and bowerbirds have transferred their colour 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.73
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 colours male guppies adopt. Fish have magnificent colour vision; whereas we use three different types of colour-detecting cell 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 monochrome. But fish also have a very different experience from us because their world filters out light of different colours 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 Trinidad’s rivers; when courting, they are usually in clear water, where orange, red and blue are the colours 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 colour, 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 maturity. Female guppies that live in tea-coloured 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