Published online 18 April 2011 | Nature | doi:10.1038/news.2011.245

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Peahens don't necessarily choose the males with the biggest tails — but small tails are right out.

She might not be as interested in his tail as he hopes. Ashley Cooper/Visuals Unlimited/Corbis

"The sight of a feather in a peacock's tail," Charles Darwin wrote in 1860, "makes me sick."

The seemingly useless, even cumbersome, gaudy plumage did not fit with his theory of natural selection, in which traits that help to secure survival are passed on. But Darwin eventually made peace with the peacock's train, and its plumage has become the poster child for his theory of sexual selection, in which ostensibly useless traits can evolve when they are preferred by choosy females.

In recent years, however, a furious debate has emerged among behavioural ecologists over whether the train of the male peafowl, Pavo cristatus, still woos peahens. Research in which peacocks' tails were experimentally plucked, published online this month in Animal Behaviour1, now suggests that the answer is yes — but only sometimes.

"There are other things that we think are going into that decision," says Roslyn Dakin, a PhD student in behavioural ecology at Queen's University in Kingston, Canada.

Dakin and a colleague, Robert Montgomerie, tracked three populations of feral peacocks and peahens during the spring breeding season, when hopeful males stage elaborately choreographed routines for picky females. They found that males with very few eyespots in their tail feathers — a measure of the size of the tail — were unattractive to females, but males with more spots than average had no advantage.

Eyeing the evidence

Beginning in the 1980s, Marion Petrie, a behavioural ecologist at Newcastle University, UK, examined the role of the peacock's tail in mating rituals. " I started to work on peacocks because Darwin had suggested it, and nobody had gone out and tested the idea," she says.

As she expected, Petrie found that males with the most eyespots were also the most successful with females. Plucking feathers from a male's train ruined his chances2. Later, French scientists found that males with lots of eyespots had stronger immune systems than less showy males, suggesting that the trait is an indicator of a male's fitness3.

"I think there's clear evidence that peahens use a peacock's tail in their mate choice," says Petrie.

However, in 2008, a team of Japanese ecologists studying the same group of feral peafowl over seven years reported that, overall, females didn't seem to favour males with the largest, most symmetrical tails4.

"We propose that the peacock's train is an obsolete signal for which female preference has already been lost or weakened," wrote Mariko Takahashi at the University of Tokyo and her colleagues.

That paper garnered widespread media coverage, including attention from Creationists who were delighted to see Darwin questioned. Petrie and the French scientists published a rebuttal5.

"I think everyone would agree that things are more complex than Takahashi et al. conclude," says Dakin, whose own studies were under way when that paper came out.

Dakin repeated Petrie's experimental work by plucking the feathers of peacocks. She noticed a drop in their success with peahens. However, she also found that, before plucking, males typically had between 165 and 170 eyespots on their trains, and on average, those with the most eyespots didn't mate any more than males with less extravagant tails.

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Dakin concludes that under most situations, females don't pick mates on the basis of the number of eyespots on their trains, but that the trait could help to weed out particularly unfit males that are missing lots of feathers. Other characteristics, including the colour and pattern of a train, may still entice females, she says.

Petrie is glad to see her experiments repeated, but is not convinced that the natural variation in the number of eyespots on a tail is so small. She also says that males do not shed feathers at random, and peacocks that manage to hold onto their plumes are likely to be the healthiest and fittest.

Still, Petrie admits that traits such as the number of eyespots are only rough measures of tail quality, and probably mean more to scientists than to peahens. "At the end of the day, we will never know what peahens are looking at and how they select their mates. You can't ask them."