The handicap hypothesis is a special variation of the good-genes hypothesis. Under the handicap hypothesis, the elaborate male trait indicates the presence of other good genes, specifically because the male trait is so costly that only males with especially good genes could produce the elaborate trait and still survive. According to this hypothesis, a peacock that can survive with an enormous tail must have some other really good genes to balance out the obviously bad tail. Maybe it's especially good at evading predators and at finding food, for example, if it can manage to do both while still dragging around that beautiful but cumbersome tail.
¿jtJiABEft An important component of the handicap hypothesis is that the elaborate male trait must actually be a handicap. In the example of the peacock, researchers have been taking for granted that the peacock's enormous tail hinders its ability to do all the sorts of things that birds need to do to survive.
No studies have been done with actual peacocks (mainly because they don't make good laboratory critters — they're big, they're expensive, and they bite) — but studies of other birds have provided evidence that elaborate tails are a handicap. For their study, Andres Moller and Florentino de Lope used barn swallows. Male barn swallows have long tails, and the females preferentially mate with the males that have the longest tails.
eSs-^JSi' Moller and de Lope artificially increased or decreased the length of male barn ¡¡ff swallows' tails and then measured the effects of different tail lengths on feeding ability and mating success. They found that giving males longer tails increased their attractiveness to females and at the same time decreased their efficiency in catching food. These experimental results show that in this bird species, the elaborate trait that is attractive to females is indeed a handicap for the male birds.
As explained in the preceding section, female choice is one type of sexual section. The other type is male-male competition, when males compete with each other for access to females. Competition among males for access to females can take several forms. In each case, natural selection favors characters that increase success in those contests.
¿jtiM^ In the case of male-male competition, as in female choice, selection favors characters that increase male mating success, which can lead to physical differences between the two sexes. Unlike the case of female choice, however, in which the differences result in males having some increasingly elaborate attractive trait, selection resulting from male-male competition favor traits that facilitate winning contests with other males.
Finally, there may be cases when female choice and male-male competition both operate. For example, male deer use their antlers in contests with other males. Yet it is also possible that large antlers could be appealing to female deer. Female preference could drive the evolution of antler size past the optimum needed to bash other male deer. Or visa versa. Remember that in either case, selection for being able to do all the other more mundane things that deer do (like find food and run away from predators) will place some upper limit on antler size.
The largest deer antlers on record are from the extinct Giant Deer which had antlers 12 feet across! Sexual selection has been suggested as a reason for the evolution of these impressive antlers, but we can't know for sure. If you've heard about this beast, you might have also heard the idea that the reason they went extinct was that their antlers got too big for their own good. But of course, evolution doesn't work that way. Any deer with antlers that were simply too big to allow him to survive wouldn't be passing those super duper antler genes to the next generation.
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