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Shrinking sheep Wild Soay sheep on the islands of the St. Kilda archipelago in the North Atlantic have been measured and monitored by scientists for decades. All this data now explains why females have been slowly getting smaller with generations.Arpat Ozgul

Climate change now hits home for tongue twister fans. Shorter, sweeter winters shrink sheep, scientists say (slowly).

Female wild Soay sheep on the remote North Atlantic island of Hirta have shrunk by about 5 percent during the past two decades, says Tim Coulson of Imperial College London’s campus in Berkshire. To see what’s driving that change, a weight loss averaging 81 grams per year, Coulson and his colleagues applied a new analytical approach to a mountain of data. It turns out that evolutionary forces favor the opposite trend, toward bigger sheep. But environmental changes have softened winters, overwhelming those evolutionary effects, the team reports online July 2 in Science.

For climate change, “the effects people tend to focus on are the ecological ones,” Coulson says. Studies have documented creatures shifting their ranges or changing the timing of migrations or blooming. “We’re showing that the effects extend beyond the ecology, down to individual attributes,” Coulson says.

The results show that influences on size are complex, says Kaustuv Roy of the University of California, San Diego. “We urgently need more case studies like this to really make sense of how populations and species will respond to ongoing warming,” he says.

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Mouths to feedAn overall trend for shrinking sheep comes mainly from milder winters, researchers say. More of the small, weak lambs survive until spring so there are more mouths competing for food and thus slower growth rates.Arpat Ozgul

Thanks to detailed monitoring that began on Hirta in 1986, Coulson and his colleagues could figure out why females in this population are shrinking. Soay sheep, with brown coats and curling rams’ horns, resemble early forms of domesticated sheep and have roamed for several thousand years on the St. Kilda archipelago, “a group of godforsaken rocks halfway to Iceland,” Coulson says.

To parse out what governs body size in such a harsh climate, Coulson and his colleagues started with basic equations that population biologists use to describe how traits change over time. The researchers combined and refined these equations to create a type of bookkeeper’s ledger that mathematically describes all the factors that in theory could cause a trait to vary.

After plugging in detailed sheep data, the researchers found conflicting forces at work.

The evolutionary force of natural selection favored bigger body sizes, the researchers concluded. Size is partly inherited, and larger youngsters survived better than smaller, weaker ones because sheep need to draw on fat reserves during the winters.

A quirky effect of milder winters, however, drove body size toward the diminutive. Over the past 25 years, spring has shifted two to three weeks earlier in Northern Europe. In years with shorter winters, more of the small, weak lambs survived. With more sheep competing for food in spring, growth rates slowed among the surviving youngsters. Environmental factors linked to less-violent winters were the most important determinant of body size and overcame the evolutionary effect, the researchers say.

Fossil studies of other species such as deep-sea crustaceans called ostracods also show climate and body size changing together, says paleobiologist Gene Hunt of the Smithsonian Institution’s National Museum of Natural History in Washington, DC. These patterns show on a long time scale what the sheep researchers saw on a short one. “Of course, they are able to dissect the body size into genetic and environmental effects in a much more sophisticated manner than is possible for fossil data.”

That ability to parse effects makes the sheep study especially interesting, says evolutionary geneticist Mark Rausher of Duke University in Durham, N.C.  Biologists have been working on ways to distinguish genetic from environmental effects for decades, often by comparing the specific form, or phenotype, of individuals of the same species at different altitudes.

“What’s nice about this new study is that it may be the first that convincingly shows that a change in phenotype over time is driven by the direct action of the environment on phenotype,” Rausher says.

More studies using this type of model could follow since “the method is very, very general,” Coulson says. He’s even considering applying it to people, whose body size is definitely not shrinking.

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