Warmer, shorter winters along with giving birth at a younger age are helping make sheep on the Scottish island of Hirta smaller, scientists have determined.

The average Soay sheep on Hirta, western Scotland, decreased in weight by 81 grams a year for more than two decades, researchers said today 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.

Researchers from Imperial College London, the universities of Leeds, Edinburgh and Cambridge in the UK 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 wasnt.The data was correlated with the North Atlantic Oscillation Index, a climatic measure based on differences in atmospheric pressure over the ocean.

That measure indicated St. Kilda-area winters near Hirta are arriving later and spring is coming sooner than in the past, Tim Coulson,a biologist at Imperial College London, said in an interview.

The measurements also 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 thats probably always occurred but hasnt been fully documented, Coulson said.

The sheep, usually with short brown or tan fleece, are descended from feral animals on the 244-acre Soay island off northwest Scotland and thrive on marginal land, according to the Southern Oregon Soay Sheep Farms Web site.

The species is considered one of the oldest livestock breeds in the U.K. and related to some of the first domesticated ungulates, or hoofed mammals including deer, cattle and horses.

BLOOMBERG LONDON