The Incredible Shrinking Sheep

The average body size of Scotland's wild Soay sheep has shrunk by 5 percent since 1985. Credit: Arpat Ozgul/Science

Like a wool coat in the rain, sheep are shrinking. But it's not rain that's doing it.

Researchers have found that milder winters are causing Scotland's wild Soay sheep to get smaller despite the evolutionary benefits of having a large body.

The results highlight how wide-ranging the effects of global climate change can be, adding further complexity to the changes we might expect to see in natural populations in future, according to a summary of the work published by the journal Science.

Arpat Ozgul at Imperial College London in Berkshire and colleagues analyzed body-weight measurements and life-history data (which record the timing of key milestones throughout an individual's life) for the female members of a population of Soay sheep. The sheep live on the island of Hirta in the St. Kilda archipelago and have been studied closely since 1985.

The researchers plugged their data into a numerical model that predicts how a trait such as body size will change over time due to natural selection and other factors that influence survival and reproduction in the wild, according to the statement from the journal. They selected body size because it is a heritable trait, and, although larger sheep should generally have a better chance of survival, the sheep have, on average, been decreasing in size for the last 25 years.

The results suggest that the decrease in body size is primarily an ecological response to environmental variation over the last 25 years; evolutionary change has contributed relatively little. More specifically, lambs are not growing as quickly as they once did. As winters have become shorter and milder, lambs now do not need to put on as much as weight in the first months of life in order to survive to their first birthday. Even the slower-growing ones now have a chance of surviving.