Climate Change and the Mystery of the Shrinking Sheep

	The Soay sheep (Ovis aries) on St. Kilda Archipelago. Click here for more information.

In an example of how climate change can override natural selection, researchers have found that changing winter conditions are causing Scotland's wild Soay sheep to get smaller despite the evolutionary benefits of having a large body, reports the July 3 issue of Science. 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. Arpat Ozgul and colleagues analyzed 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, although larger sheep should generally have a better chance of survival, the sheep have, on average, been decreasing in size for the last 25 years. The results suggest that the decrease in body size 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, lambs now do not need to put on as much as weight in the first months of life in order to survive to their first birthday. Even the slower-growing ones now have a chance of surviving.

Article #20: "The Dynamics of Phenotypic Change and the Shrinking Sheep of St. Kilda," by A. Ozgul; T. Coulson at Imperial College London in Berkshire, UK; S. Tuljapurkar at Stanford University in Stanford, CA; T.G. Benton at University of Leeds in Leeds, UK; J.M. Pemberton at University of Edinburgh in Edinburgh, UK; T.H. Clutton-Brock at University of Cambridge in Cambridge, UK