together, and parade their wares for visiting females: The characteristic of the lek is that one or a few males, usually those that display near its center, achieve most of the matings: But the central position of a successful male is not the cause of his success so much as the consequence: Other males gather around him: The sage grouse of the American West has been the best studied of lekking birds: It is an extraordinary experience to drive out to the middle of Wyoming before dawn, stop the car on a fea-tureless plain that looks like every other one, and see it come alive with dancing grouse. Each knows his place; each runs through his routinof inflating the air sacs in his breast and strutting forward, bouncing the fleshy sacs through his feathers for all the world like a dancer at the Folies Bergere. The females wander through this market, and after several days of contemplating the goods on offer, they mate with one of the males: That they are choosing, not being forced to choose, seems obvious: The male does not mount the female until she squats in front of him. Minutes later his job is done, and her long and lonely parenthood is beginning. She has received only one thing from her mate—genes—and it looks as if she has tried hard to get the best there were to be had.
Yet the problem of greatest choosiness in the species where choice least matters reappears: A single sage grouse cock may perform half of all the matings at one lek; it is not unknown for this top male to mate thirty or more times in a morning. 22 The result is that in the first generation the genetic cream is skimmed from the surface of the population, in the second the cream of the cream, in the third the cream of the cream of the cream, and so on. As any dairy farmer can attest, this is a procedure that quickly becomes pointless. There is just not enough separability in cream to keep taking the thickest layer. It is the same for sage grouse. If 10 percent of the males father the next generation, pretty soon all the
females and all the males will be genetically identical, and there will be no point in selecting one male over another because they are all the same: This is known as the ' lek paradox, ' and it is the hurdle that all modern theories of sexual selection attempt to leap. How they do so is the subject of the rest of this chapter.
MONTAGUES AND CAPULETS
It is time to introduce the great dichotomy. Sexual selection theory is split into two warring factions. There is no accepted name for each party; most people call them ' Fisher ' and 'Good-genes. '
Helena Cronin, who has written a masterful history of the sexual selection debate, 2' prefers 'good-taste ' and 'good-sense. ' They are sometimes also known as the 'sexy-son ' versus the 'healthy-offspring' theories.
The Fisher (sexy-son, good-taste) advocates are those who insist that the reason peahens prefer beautiful males is that they seek heritable beauty itself to pass on to their sons, so that those sons may in turn attract females. The Good-geners (healthy-offspring, good-sense) are those who believe that peahens prefer beautiful males because beauty is a sign of good genetic qualities—
disease resistance, vigor, strength—and that the females seek to pass these qualities on to their offspring: Not all biologists admit to being members of one school or the other: Some insist there can be a reconciliation; others would like to form a third party and cry with Mercutio, 'A plague on both your houses. ' But nonetheless the distinction is as real as the enduring feud between Capulets and Montagues in Romeo
The Fisherians derive their ideas mostly from Sir Ronald Fisher ' s great insight about despotic fashion, and they follow Darwin in thinking the female 's preference for gaudiness is arbitrary and without purpose. Their position is that females choose males according to the gaudiness of their colors, the length of their plumes, the virtuosity of their songs, or whatever, because the
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THE PEACOCK S TALE
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species is ruled by an arbitrary fashion for preferring beauty that none dares buck. The Good-gene people follow Alfred Russel Wallace (though they do not know it) in arguing that arbitrary and foolish as it may seem for a female to choose a male because his tail is long or his song loud, there is method in her madness. The tail or the song tells each female exactly how good the genes are of each male. The fact that he can sing loudly or grow and look after a long tail proves that he can father healthy and vigorous daughters and sons just as surely as the fishing ability of a tern tells his mate that he can feed a growing family. Ornaments and displays are designed to reveal the quality of genes.
The split between Fisher and Good-genes began to emerge in the 1970s once the fact of female choice had been established to the satisfaction of most. Those of a theoretical or mathematical bent—the pale, eccentric types umbilically attached to their computers—became Fisherians. Field biologists and naturalists—
bearded, besweatered, and booted—gradually found themselves Good-geners.'
1S CHOOSING CHEAP?
The first round went to the Fisherians. Fisher 's intuition was fed into mathematical models and emerged intact. In the early 1980s three scientists programmed their computers to play an imaginary game of females choosing long-tailed males and bearing sons that had the long tails and daughters that shared the preference of their mothers. The longer the male 's tail, the greater his mating success but the smaller his chances of surviving to mate at all: The scientists' key discovery was that there exists a 'line of equilibrium ' on which the game can stop at any point. On that line the handicap to a female ' s sons of having a long tail is exactly balanced by the advantage those sons have in attracting a mate.''
In other words, the choosier the females, the brighter and more elaborate the male ornaments will be, which is exactly what you find in nature. Sage grouse are elaborately ornamented, and
only a few males get chosen; terns are unornamented, and most males win mates.
The models also showed that the process could run away from the line of equilibrium with Fisher 's 'ever- increasing speed '
but only if females vary in their (heritable) preference and if the male 's ornament is not much of an encumbrance to him. These are fairly unlikely conditions except early in the process when a new preference and a new trait have just emerged.
But the mathematicians discovered more. It mattered greatly if the process of choosing was costly to females. If in deciding which male to mate with a female wastes time that could be more profitably spent incubating eggs or she exposes herself to the risk of being caught by an eagle, then the line no longer stands: As soon as the species reaches it, and the advantages of long tails are balanced by their disadvantages, there is no net benefit to being choosy, so the costs of choice will drive females into indifference.
