July 3 (Bloomberg) -- Just as wool clothing shrinks in a hot wash, sheep on a remote Scottish island are getting smaller from one generation to the next as temperatures rise.

The average Soay sheep on Hirta island off western Scotland decreased in weight by 81 grams (3 ounces) a year for more than two decades, researchers said in the journal Science. That’s because they have to eat less in their early months to survive winters that became shorter and milder, they wrote.

“Climate change has led to a change in the growth rate which has fed through to the fact that the average sheep is now smaller,” study co-author Tim Coulson, a biologist at Imperial College London, said in an interview. “Whether in the future we’re going to get miniature bonsai sheep, I have no idea.”

Animal size can adjust to changing climate and geography, examples of which include extinct Mediterranean dwarf elephants that were a quarter of their ancestors’ size after being isolated for generations on Cyprus and Crete, according to Yale University’s Institute for Biospheric Studies. Many mammals evolve following “Bergmann’s rule,” which says animals are smaller in warmer parts of their geographic range.

The research is the most comprehensive yet linking animal size to global warming, Coulson said. His team’s results tally with studies of crustaceans called ostracodes from as far back as 40 million years and red-billed gulls in New Zealand. Studies have also shown gnats are emerging sooner, birds are laying eggs earlier and plants are moving uphill as temperatures warm.

Species Getting Smaller

The study of ostracodes showed they increased in length by about 5 percent per degree of Celsius as the sea cooled, Gene Hunt, a paleobiologist at the Smithsonian Institution in Washington, said in an e-mailed reply to questions. At the same time, modern-day ostracodes follow Bergmann’s rule, he said.

“Given our ostracode study, I would expect similar species to get smaller as temperatures warmed, by about the same 5 percent per degree Celsius,” Hunt said. “Mammals usually follow Bergmann’s rule too so I would expect them to follow the same general pattern, all else being equal.”

The weight of red-billed gulls in New Zealand fell over a 47-year period as temperatures rose, a University of Helsinki- led team wrote in 2008 in the journal Proceedings of the National Academy of Sciences. A Danish study about Arctic species found some seabirds in 2005 laid eggs 10 days sooner than in 1996 and that gnats took flight a month earlier.

Maternity vs. Paternity

For the sheep study, the researchers from Imperial College, the universities of Leeds, Edinburgh and Cambridge in the U.K. and Stanford University in California used body weight and leg- length measurements of an average 140 individual sheep per year from 1986 to 2007. They used data only from female sheep because maternity was known whereas paternity wasn’t.

The researchers found that due to climate change, survival conditions on Hirta, which they likened to an outdoor laboratory, had become less challenging. Slower-growing, smaller sheep were more likely to survive winter than before, they said.

With longer springs and milder winters, the scientists said lambs didn’t need to put on as much weight in their early months of life to survive as when temperatures were worse.

The findings were correlated with data from the North Atlantic Oscillation Index, a climatic measure based on differences in atmospheric pressure over the ocean. That measure indicated St. Kilda-area winters by Hirta were arriving later and spring was coming sooner, Coulson said.

The data showed that while normal evolutionary selection is still occurring, favoring larger sheep, two other effects were causing the farm animals on balance to become smaller from one generation to another: climate change and a tendency for younger mothers to have smaller lambs, a phenomenon that’s probably always occurred but hasn’t been fully documented, Coulson said.

“The classic theory would have been that sheep should be getting bigger,” he said. “They’re not getting bigger and the reason is this ‘young mum’ effect. The reason they’re getting smaller is that on top of that, there’s an effect of climate change which is impacting the growth rate of young sheep.”

To contact the reporters on this story: Alex Morales in London at amorales2@bloomberg.netJeremy van Loon in Berlin at jvanloon@bloomberg.net

Last Updated: July 3, 2009 06:56 EDT