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Bye Bye Black Sheep: Warming Dooms Dark Wool

Michael Reilly, Discovery News

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July 22, 2009 -- Winters in the windswept Scottish St. Kilda islands have gotten steadily warmer over the past 20 years and the rising temperatures are whittling away the islands' number of black sheep, replacing them with lighter ones, according to a new study.

As a symbol of the unique, the misunderstood, and the outcast, the dark-colored sheep of St. Kilda are a prime example. And their chilly home in the North Atlantic Ocean is a picture of exile.

But the hideaway may not support the black sheep much longer.

St. Kilda's Soay sheep (Ovis aries) are the feral leftovers of herds that were once domestic -- the islands' last few dozen human settlers left for good in 1930. Small and hardy, the animals have largely adopted a dark brown color.

The sheep also appear to be a color-indicator for climate change. The number of dark-colored sheep declined steadily, from nearly 80 percent of the herds to less than 70 percent, according to a new analysis. The decline followed in near lock step with a 1.2 degree Centigrade (2.2 degrees Fahrenheit) rise in winter temperatures from 1985 until 2005.

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The finding is peculiar because dark Soay sheep also tend to be bigger, and bigger animals tend to be better at competing for resources and reproduction. Their dark wool also soaks up sunlight, which may make them better at staying warm in St. Kilda's bone-chilling cold weather.

"The effect is small, but you'd expect coat color effects on energetics to be subtle," said Shane Maloney of the University of Western Australia, lead author on the study that was published yesterday in Biology Letters. "In cold weather, a dark coat could provide that edge that allows them to get through tough winters."

As winters have gotten more mild, big, dark animals lost their advantage, Maloney reasons. If so, black sheep will keep dwindling as Earth continues its warming trend.

"Population crashes every few years in the winter, when there are a lot of sheep and there's not a lot of fodder around," Jake Gratten of the University of Sheffield in the United Kingdom. "It's possible that environmental factors are contributing to the trend."

However, Gratten believes the decline in dark sheep has nothing to do with their color. In a study published last year, he and a team of researchers found that dark sheep with only one gene for dark color were better survivors than those that had two genes for dark color, even though both had the same color wool.

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