The proximate reason is “to attract mates”—peacocks use their vibrant colors and fan-shaped tails in elaborate courtship displays to woo their frumpy female love interests.
Upon closer examination, these tails seem awfully impractical. Natural selection is supposed to be the mortal enemy of such extravagances, weeding out maladaptive traits. Peacocks, with their “LOOK AT ME!” plumage and unwieldy tails, seem liable to attract the wrong kind of attention: it’s not hard to imagine that peacocks are more likely to be spotted by predators and, if spotted, to have a harder time escaping with their massive tails weighing them down.
“LOOK AT ME!”
It’s not just peacocks which have unnecessarily convoluted adaptations relating to courtship. Consider the elaborate tails of many birds of paradise, the impressive antlers on moose (or, better yet, extinct Irish elk, Megaloceros giganteus), the expandable, brightly-colored dewlaps on anole lizards, or the wicked-looking horns of male stag beetles. Yes, some of these traits may be useful for defense, but if defense were the driving force in their evolution, why wouldn’t females evolve these traits as well?
The brightly-colored dewlaps of anole lizards are likely products of sexual selection.
The explanation for these phenomena is sexual selection, a ‘special case’ of natural selection. The idea of sexual selection was proposed by Darwin himself, but biologists since have done much to flesh out his initial ideas on the topic.
Today, we divide sexual selection into two broad categories:
Intrasexual Selection: usually male-to-male combat, wherein males fight one another for access to females. The traits used in this combat (horns, antlers, etc.) are often called ‘weapons’.
Intersexual Selection: usually female choice of male mates, wherein females will pick certain males to mate with on the basis of certain characteristics (tail length, crest size, etc.)—these characteristics are often called ‘ornaments’.
There have been several competing theories proposed to explain sexual selection, but they all hinge on the fact that attracting a mate is a very important component of ‘fitness’ in most cases. To get your genes into the next generation, sexually reproducing species need to find a mate. For all of these sexually-selected traits, the benefits of these traits in terms of securing a chance to mate outweigh any costs in terms of individual survival chances.
Two of the hypotheses to better explain sexual selection are the sexy son hypothesis and the good genes hypothesis.
Sexy Son Hypothesis
This idea was put forth by P. J. Weatherhead and R. J. Robertson in 1979. As Dawkins points out in The Selfish Gene, “In a society where males compete with each other to be chosen as he-man by females, one of the best things a mother can do for her genes is make a son who will turn out in his turn to be an attractive he-man.” Once females start using a strait as a basis for selecting mates, a runaway process can begin: no matter whether the trait is helpful or harmful. All that’s required is that it be considered attractive.
Once ‘horns’ are attractive, for example, then it’s good for a female to have a son with large horns. A son with large horns will have better luck with the ladies, who prefer large horns, and will therefore have more sons—each of which will also be likely to have large horns. Furthermore, he will be getting genes to favor large horns from the ladies who choose him as a mate. The whole thing becomes a self-feeding process, and one feasible result is the gargantuan rack of the Irish elk mentioned earlier. Again, Dawkins sums it up nicely: “one of the most desirable qualities a male can have in the eyes of a female is, quite simply, sexual attractiveness itself.”
With any luck, this moose will pass on his impressive antlers to his own ‘sexy sons’.
Good Genes Hypothesis
This alternative, but not necessarily exclusive, theory suggests that sexually selected traits sure as honest indicators of the quality of the male’s genes. For instance, house finches (Carpodacus mexicanus) females prefer males with bright, colorful plumage. Studies have shown that these males are also more likely to survive winter.
One way to look at it is this: if males are healthy enough to produce big, showy ornaments, then they must be doing something right. They must have good parasite resistance, or good foraging skills. Ornaments provide a reliable indication of a healthy male.
These two theories are still being discussed and researched today. It’s also possible, of course, that natural systems follow some combination of these hypotheses. There are also other hypotheses, like the controversial-but-influential ‘handicap principle’ proposed by Amotz Zahavi in 1975.
(Standford.edu) Sexual Selection
(Wikipedia) Sexual Selection
(Wikipedia) Handicap Principle
(Wikipedia) Irish Elk
(Berkeley.edu) Sexual Selection
(Kimball’s BiologyPages) Sexual Selection
Sexual Selection in Bowerbirds (Borgia Lab)
Since Darwin put forth the theory, it has long been held that male peacocks have beautiful tails as part of courtship, to attract females (have you ever seen a female peacock—or peahen? not that pretty). The bigger the tail and the more eye spots, the better the chance the peahen will choose him. Not everyone agrees. Some scientists in Japan did studies and failed to find a link between the plumage and attraction, but that was one study that has been highly disputed. It seems that the tail, in its complexity, beauty and brilliant colors, is the reason that peahens choose their peacocks.
The peacock is not the only animal where the male displays eccentrically proud dances coupled with colorful arrays of plumage to empress the ladies. The Bird of Paradise also is known to use a one-two courship dance punch to attempt to attract the oposite sex.
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