Climate change and the mystery of the shrinking sheep

Changing winter conditions are causing Scotland's wild Soay sheep to get smaller despite the evolutionary benefits of having a large body, researchers report in a study that shows how climate change can trump natural selection. 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. The study will be published online by Science, at the Science Express website, on 2 July, 2009. Science is published by AAAS, the nonprofit, international science society.

"It's only in the last few years that we've realized that evolution can influence species' physical traits as quickly as ecological changes can. This study addresses one of the major goals of population biology, namely to untangle the ways in which evolutionary and environmental changes influence a species' traits," said Andrew Sugden, Deputy and International Managing Editor at Science.

"Sheep are getting smaller. Well, at least the wild Soay sheep living on a remote Scottish island are. But according to classic evolutionary theory, they should have been getting bigger, because larger sheep tend to be more likely to survive and reproduce than smaller ones, and offspring tend to resemble their parents," said study author Tim Coulson of Imperial College London.

"Our findings have solved a paradox that has tormented biologists for years – why predictions did not match observation. Biologists have realized that ecological and evolutionary processes are intricately intertwined, and they now have a way of dissecting out the contribution of each. Unfortunately it is too early to tell whether a warming world will lead to pocket-sized sheep," Coulson said.

Coulson and his colleagues investigated this paradox by analyzing 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. They selected body size because it is a heritable trait, and because the sheep have, on average, been decreasing in size for the last 25 years.

The results suggest that the decrease 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, due to global climate change, lambs now do not need to put on as much as weight in the first months of life to survive to their first birthday. So, even the slower-growing ones now have a chance of surviving, according to Coulson.

Also contributing to this trend is what Coulson and his colleagues call the "young mum effect." The researchers found that younger mothers are physically unable to produce offspring that are as big as they were at birth. The reasons behind this are still unclear, but the young mum effect counters the effect of natural selection, which favors larger lambs, the authors report.

Source: American Association for the Advancement of Science