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% 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 UC 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 colder 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 around them 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.
"It's like an outside laboratory," Ozgul said.
For the new study, Ozgul and his colleagues analyzed a plethora of data, including the number of lambs born each year since 1985, the age of ewes giving birth and the survival rates of sheep at different ages. The team included body-weight measurements and the length of the hind leg to see whether the sheep were just thinner or actually smaller overall.
To measure the effect of climate, the researchers also incorporated the North Atlantic Oscillation index, which affects the strength of westerly winds in Europe and determines whether winters will be wet and mild or cold and dry. All of these terms were plugged into a mathematical formula that allowed them to measure the individual components that contributed to the change in sheep body size.
As expected, the researchers found that evolutionary pressure pushed the sheep to grow bigger. But that was offset by another, unexpected factor: Lambs born to yearling ewes instead of fully grown sheep weighed less at birth than their mothers did and remained smaller throughout their lifetimes.
Still, the "young mum effect" wasn't enough to account for the decrease in sheep size.
When the researchers added in the effect of changing environmental conditions, "you get just about the exact rate of decline that we've seen," said study coauthor Shripad Tuljapurkar, a biology professor at Stanford.
The researchers found that, on average, 1-year-old sheep now weigh 3.3 ounces less than they did in 1985. They attributed the decline to shorter, milder winters that allowed grass to grow later into the year. As a result, sheep can make it through the coldest months with fewer fat reserves, so more lambs born to young mothers survive in spite of their small size.