On a remote Scottish island, the sheep are shrinking, and the cause appears to be the warming of winter.

The wild Soay sheep that live on the island of Hirta in the North Atlantic have been under careful scientific observation since 1985, partly because the island ecosystem is a simple one consisting of the sheep and the vegetation they eat.

Timothy Coulson, a professor of population biology at Imperial College London, and his colleagues analyzed the sheep data and found that the weight of the average female Soay had decreased about three ounces a year, or about 5 percent over the past quarter-century.

Image Soay sheep on the island of Hirta. The females are getting smaller. Credit... A. Ozgul/Science-AAAS

That was somewhat surprising, as larger sheep have better odds of surviving and evolution tends to favor those that are stronger.