The sheep on Scotland’s St Kilda archipelago. Credit: Arpat Ozgul/Imperial College, London

New Delhi, July 4: Climate change appears to have triumphed over the rules of natural selection on a remote Scottish island where native wild sheep have been shrinking in size over the past two decades, scientists have reported.

An Indian-American scientist at Stanford University and his colleagues have shown that milder winters are causing wild sheep on the island of Hirta, northwest of Scotland, to get smaller despite the evolutionary benefits of a larger body.

Studies in the past have hinted at evolutionary responses to a warming world — squirrels are breeding earlier in Canada and some birds have changed migratory patterns in Europe.

But the study on the Scottish sheep is among the most exhaustive explorations of how climate change is altering the way ecological and evolutionary processes intersect. The findings appeared in the journal Science yesterday.

“The sheep provided a paradox to science,” Tim Coulson, a team member and professor of population biology at Imperial College, London, told The Telegraph. The theory of evolution suggests that over time, the sheep should have become bigger because larger animals are more likely to survive and reproduce than smaller ones.

But the average size of the island sheep has decreased 5 per cent over the past 24 years.

The researchers also observed that younger females produce lambs that are smaller than they were at birth — daughters weigh on average about 150 grams less than their mothers. This parent-to-offspring effect counters the effect of natural selection that adds on average about 185 grams.

The scientists suggest that as the winters have become milder as a consequence of global warming, lambs do not need to put on as much weight in the first months of their lives to survive as they did when the winters were colder.

The team analysed body-weights and life history of the sheep and used a mathematical model to precisely measure the contributions of natural selection, parent-to-offspring effect and the environment.

“We show that the first two of these (nearly) cancel each other out. So without environmental change, we would have no evolutionary change and no change in weight or size,” said Shripad Tuljapurkar, a team member at Stanford University.

“Environmental change is responsible for the (size) changes that we observed,” Tuljapurkar said. He and Coulson have developed a method of sorting out different effects that can influence weight, height, or blood pressure.

Changing winter conditions have lengthened the period of grass growth on the island, reducing the length of time a sheep has to depend on stored fat reserves. The findings explain why sheep are smaller than they used to be.

“We’ve observed how a subtle change in climate can produce a substantial effect on wildlife populations,” said Arpat Ozgul, another team member who’s a post-doctoral research associate at Imperial College.