Tim Coulson of Imperial College, London, and his colleagues examined the weights of about 2,000 female sheep that lived on the island in the two decades of their study. They combined this information with detailed histories of individual animals. They found that daughters were, on average, lighter than their mothers had been at the same age. Their legs were shorter, too, suggesting that the breed really was shrinking.

Why is this happening? The researchers, who published their results in the current issue of Science, suspected that it might have something to do with climate change. To explore this they used an index of weather severity called the North Atlantic oscillation. This tracks the difference in air pressure between Iceland and the Azores and, during the winter months, determines the strength of the winds, temperatures and rainfall in the St Kilda archipelago. From this index the researchers concluded that the winters on Hirta are getting shorter and warmer, and that may be because of climate change.

This has two consequences for the sheep. First, they do not need to have such large fat reserves to live off if the winters they face are getting milder. Second, more sheep survive the winter, so lambs face more competition with larger animals for food, so they grow less fast. If the researchers’ explanation is right and climate change is involved in shrinking the sheep of Hirta, it shows how rapidly environmental change can change populations.

© The Economist Newspaper Limited 2009