Sheep in Scotland are shrinking, study finds

Los Angeles Times

Along with polar ice caps and sandy beaches, sheep on a remote Scottish island are gradually shrinking as a result of global warming, according to a study published today in the journal Science. The finding offers unusual proof that large animals already are having to adapt to changes wrought by climate change, experts said.

The average weight of sheep in the feral flock has been falling nearly 3 ounces per year since 1985, the researchers reported. The cumulative effect has been a 5 percent reduction in total body size.

That trend had puzzled scientists, because they knew that evolution clearly favored larger sheep that are better equipped to survive the harsh winters of Hirta, a rocky outpost more than 100 miles west of mainland Scotland.

Now, using a sophisticated mathematical model, British and American researchers have concluded that warming temperatures have made it easier for scrawnier sheep to survive, thus reducing the average size of animals in the herd.

“Environmental change is having a substantial influence on the population,” said Arpat Ozgul, a postdoctoral research associate at Imperial College London and lead author of the report.

That influence appears to have played out in a surprisingly intricate and counterintuitive manner, said University of California, San Diego, biologist Kaustuv Roy, who wasn’t involved in the study. For example, milder winters have helped the overall herd grow larger even as the average size of animals got smaller.

“Most of the thinking about how climate is going to affect species is fairly simplistic,” Roy said. “These dynamics are fairly complex. We’re going to have to sort out some of these details if we’re ever going to make predictions about how individual species are going to respond to warming.”

Scientists have long been interested in the relationship between climate and body size.

German biologist Christian Bergmann observed in 1847 that as members of a species migrate to higher latitudes with lower temperatures, their body size tends to increase. He speculated that larger bodies helped animals conserve heat by reducing their surface area relative to their volume.

More recently, Roy and others have proposed that the same mechanism could prompt body sizes to shrink when animals stay put but the climate gets warmer.

The Soay sheep of Hirta presented an opportunity to test that theory.

The brown sheep, named for their home island of Soay, were transported to the nearby isle of Hirta in 1932, two years after the last of its human inhabitants abandoned it. In the 1980s, scientists returned to study the flock’s descendants. They visit at least once a year to conduct a census and take measurements.