Climate change and the mystery of the shrinking sheep

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.

The model showed that natural selection favours heavier individuals, who are more likely to make it past the first two years of life. But these effects were paltry and largely counteracted by a far more important influence - the difference in body weights between parents and their young. Every August, year on year, the ewes were rearing daughters that were around 150g lighter than they were at the same age.

The sheep are also growing more solely than they used to, putting on about 93g less in their first year than they used to 20 years ago. This explains both why the adult sheep are smaller, and why the birth weights of lambs are falling - ewes simply cannot produce larger young when they themselves are reproducing earlier and failing to reach full adult body size. So today's Soay sheep run the race to adulthood with a poorer start and a slower pace - no wonder they're shrinking.

But why are the sheep growing more slowly than they used to? Ozgul says that the answer is climate change. The growth of the Soay sheep turned out to be very sensitive to shifting climate. Since 1980, winters on Soay Island have become warmer, milder and shorter, and grass grows for more of the year. As a result, lambs spend less time depending on their stores of fat for survival, and they have more food to graze on when winter ends.

The result is a more forgiving climate, where even lambs that grow slowly make it past their first year. Compared to earlier generations, these newborns have an easier time of it and they don't need to put on as much weight in their first few months. Whereas in the past, only the largest and fastest-growing lambs would make it to breeding age,  now even the slackers can get away with it. Climate, then, is the culprit behind the mystery of Scotland's shrinking sheep.

Reference: Science 10.1126/science.1173668

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