The island of Hirta, on the western coast of Scotland, is home to a special breed of sheep. Soay sheep, named after a neighbouring island, are the most primitive breed of domestic sheep and have lived on the isles of St Kilda for at least a millennium. They’re generally smaller than the average domesticated sheep, and that difference is getting larger and larger. Over the last 20 years, the Soay sheep have started to shrink.
They are becoming gradually lighter at all ages such that today’s lambs and adults weigh around 3kg less than those from 1986. Their hind legs have also shortened to a similar degree, suggesting that they have indeed shrunk, rather than fallen increasingly ill.
The reasons behind this downward trend have now been revealed by a group of British scientists led by Arpat Ozgul from Imperial College. Using decades’ worth of data, the team showed that natural selection normally favours larger sheep, as the odds of survival increase with body size. But this evolutionary pressure has been overwhelmed by the effects of climate change. Warmer winters have led to easier conditions, and less need to pile on the pounds in the first years of life. The lambs can afford to grow more slowly and they become smaller adults, who are only physically capable of raising small young themselves.
Soay sheep live in a closed population that doesn’t have to deal with human interference, predators, migrants (either in or out), or significant competitors. That makes them an ideal population to study if you’re an evolutionary biologist interested in how animal populations change over time. One such group, including Ozgul and his colleague Tim Coulson, have been studying the Soay sheep since 1985 and have brilliantly called themselves SLAPPED (short for Studies in Longitudinal Analysis of Population Persistence and Evolutionary Demography).
The group wanted to work out the extent to which the sheep’s shrinking size is due to the influence of natural selection and to what extent it is just an ecological response to changing environments. To that end, they developed a mathematical job designed to analyse their 24 years of data and tease apart these contrasting effects.