Image: Sheep on Scottish island
Arpat Ozgul  /  Journal Science
The shrinking sheep on Scotland's Hirta Island have proven that climate can trump natural selection, researchers say. The average body size has shrunk by 5 percent since 1985. staff and news service reports
updated 7/2/2009 2:10:14 PM ET 2009-07-02T18:10:14

Two years after scientists concluded that a breed of wild sheep on a remote Scottish island was shrinking over time, a study released Thursday revealed why: milder winters tied to global warming.

Due to milder winters, lambs on the island of Hirta do not need to put on as much as weight in the first months of life to survive to their first year, according to the study in the peer-reviewed journal Science. As a result, even the slower-growing ones now have a chance of surviving.

"In the past, only the big, healthy sheep and large lambs that had piled on weight in their first summer could survive the harsh winters on Hirta," lead author Tim Coulson, a researcher at Imperial College London, said in a statement.

"But now, due to climate change, grass for food is available for more months of the year, and survival conditions are not so challenging — even the slower growing sheep have a chance of making it, and this means smaller individuals are becoming increasingly prevalent in the population."

Evolutionary theory upended
The study upends the belief that natural selection is a dominant feature of evolution, noting that climate can trump that card.

"According to classic evolutionary theory," Coulson added, the sheep "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."

The sheep on Hirta have been examined closely since 1985 and experts concluded in 2007 that average body size was shrinking. By this year, it had decreased by 5 percent since 1985.

Coulson's team analyzed body-weight measurements and key life milestones for a selected group of female sheep. They then plugged the data into a computer model that predicts how body size will change over time due to natural selection and other factors.

The results suggest that the decrease in average size is primarily an ecological response to warming, the authors said, and that natural selection has contributed relatively little.

'Young mum effect'
Coulson's team also found what they call a "young mum effect": younger females are physically unable to produce offspring that are as big as they were at birth. Why is still unclear, the authors report, but the effect counters the effect of natural selection.

"The young mum effect explains why Soay sheep have not been getting bigger, as we expected them to," Coulson said. "But it is not enough to explain why they're shrinking. We believe that this is down to climate change. These two factors are combining to override what we would expect through natural selection."

"Our findings have solved a paradox that has tormented biologists for years — why predictions did not match observation," he added. "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."

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