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Peacock Feathers: That's So Last Year

31 March 2008 (All day)
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Mariko Takahashi

What females don't want.
Peahens in Japan don't always choose the male with the most elaborate train.

It's been a truism since Darwin's day: Female peahens prefer a male peacock with a gorgeous train--the fancy feathered fan he unfurls to wow the gals. But a new 7-year study questions this long-held notion, reporting that females in a feral population of Indian peafowl (Pavo cristatus) showed no such preference. The controversial paper contradicts previous, lauded studies that did reveal a link and that are part of the canon of evolutionary biology.

Because natural selection cannot explain the evolution of seemingly useless male ornaments, such as elaborate feathers, Charles Darwin proposed that they arise through sexual selection. In most species, females choose the male they want to mate with, presumably by evaluating traits that give clues to genetic health. For example, the peacock's train is longer than his body and decorated with gaudy eyespots. The number of eyespots may correlate with the quality of the male's genes, so a female peahen should pick the fellow with the highest count. In the most cited study of the peacock's train, evolutionary biologist Marion Petrie of Newcastle University in the U. K. snipped off the eyespot portion of some males' tail feathers; the females snubbed these males. Furthermore, chicks fathered by more ornamented males had higher long-term survival than other chicks.

Mariko Takahashi's team planned to confirm these results. But despite observing 268 matings, the team was unable to pinpoint any single male trait that females preferred, they report in April's issue of Animal Behaviour. The train might originally have served as a sign to females, hypothesizes Takahashi, an animal behaviorist at the University of Tokyo in Japan. But like last year's fashions, it has now gone out of style.

If the females ignore the feathers, why do the males spend the energy to make and haul them around? Michael Ryan, an animal behaviorist at the University of Texas, Austin, says that male peacocks may keep these trains because after a female shows interest, the peacock shakes the feathers to make a rustling noise that's necessary for mating. "Without a train, they can't make that music," he says.

Evolutionary biologist Marlene Zuk of the University of California, Riverside, doesn't think the study disproves Petrie's studies because it didn't involve experiments on tail gaudiness. Stein Are Sæther, an evolutionary biologist at the University of Oslo in Norway, says the new study shows that "we really do not understand exactly what females ... are doing when they evaluate males."

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