The Incredible Shrinking Sheep of Scotland

News alert: the sheep of Scotland are shrinking! On Soay Island, off the western coast of Scotland, wild sheep are apparently defying the theory of evolution and progressively getting smaller.

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Why? In short, because of climate change. Generally, the sheep's life cycle goes like this: they fatten up on grass during the fertile, sunny summer; then the harsh winter comes, the grass disappears and the smallest, scrawniest sheep die off, while their bigger cousins survive. That's how you end up with big sheep, which — according to Darwin's laws of natural selection — will pass on their big genes to the next generation. (See pictures of sheep and other animals.)

But over the past 25 years, the average Soay Island wild sheep has decreased in size, according to a report in the July 2 issue of Science by a team of researchers led by Tim Coulson of Imperial College London. Thanks largely to global warming, the winters on Soay Island are becoming shorter and milder. That makes food more abundant and allows some of the smaller, more vulnerable and younger sheep to survive. Then they go on to have offspring that tend to be small themselves — and have a better chance of survival because of the increasingly mild winters. "The environmental and evolutionary processes are intertwined," says Coulson "There's still natural selection, but it's not leaving as big a signature as it used to. There's still a disadvantage to being small, but not as much." (Read "The Fine Art of Sheep-Shearing — for Fun and Profit")

And while being big is still an advantage — size offers a better survival cushion if food proves hard to find — there are other factors that limit how easily that trait is passed down. Coulson and his colleagues identified what they call the "young mum" factor. Sheep, unlike many other mammals, tend to have offspring quite early in life — mothers can have lambs at one-year-old, before they're fully grown. Since the size of the lambs is limited by the size of the mothers, younger mothers have smaller babies. Thanks to the milder winters, more sheep are able to survive that difficult first year and have lambs, which means there are more small lambs being born, bringing down the flock's average size — and causing the shrinking of the Soay sheep. "Those lambs had been born with a slight disadvantage, but that's been erased as the climate has warmed," says Coulson. (Read "Why Some Animals (and People) Are Gay")

The Science paper explores a fascinating topic: how species will adjust to a warmer world. The environment has always driven natural selection — successful species adapt to their surroundings, or they die — and while the environment has also always changed, never has it done so as quickly as it does today, thanks to the billions of tons of CO2 we're shoveling into the atmosphere. (Watch TIME's video "How to Shear a Sheep")

As the Soay sheep show, animals can respond to climate change, but not in the ways we're accustomed to. "They can do so in two ways," says Coulson. "They can do so through the evolutionary process, which is a little slower, but they can also adapt by changing their growth rate in response to their environment." Scientists like Coulson can then separate the effects of evolution from the effects of the environment.

That doesn't mean that animals will adapt and thrive in a warmer world. Far from it — by some estimates, rapid climate change could drive as many as one-third of the species on the planet out of existence by mid-century. Though warmer winters in blustery Scotland might sound nice — especially if you're a sheep on the small side — the changes due to global warming are likely to be far from positive in most parts of the world. Evolution will help species adapt, but there's a term for what happens when the pace of evolution can't match the pace of climate change: extinction.

See pictures of sheep on LIFE.com.

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