The study explains a mystery that has bedevilled scientists for the past two years.

The wild Soay sheep live on Hirta, in the St. Kilda archipelago in the storm-battered Outer Hebrides, and have been closely studied for nearly a quarter of a century.

The law of evolutionary theory says the brown, thick-coated ungulates should have got progressively bigger.

Tough winters mean that bigger sheep have a better chance of survival and of reproducing than smaller ones, and eventually they would dominate in the flock's numbers.

But in 2007, stunned researchers realised that the average size of the Hirta sheep, instead of rising, had been progressively falling.

The answer, British biologists said on Thursday, lies in climate change.

A team led by Tim Coulson, a professor at Imperial College London, pored over data for the animals' body size and life history over 24 years.

They found that the sheep were not growing as fast as they once did and smaller sheep were likelier to survive into adulthood instead of perishing as lambs.

This gives smaller sheep a shot at reproduction, which means that the average sheep size has fallen -- by 81 grammes per year (2.85 ounce) per year on average.

Coulson believes that shorter, milder winters mean that lambs do not need to put on as much weight in the first months of life in order to survive to their first birthday, as they did when winters were colder.

"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," he said.

"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."

Another factor in the sheep shrinkage is a so-called "young mum effect."

Ewes that give birth earlier tend to produce smaller sheep, thus adding to the smaller average size.

Man-made climate change is already having an impact on species in terms of habitat and migratory patterns.

But scientists say it is hard to predict which will be winners and losers from the change, partly because of the complexity of separating out evolutionary pressures from environmental factors.

The new study, published in the US journal Science, could help, said Coulson.

"Biologists have realised that ecological and evolutionary processes are intricately intertwined, and they now have a way of dissecting out the contribution of each," he said.

"Unfortunately, it is too early to tell whether a warming world will lead to pocket-sized sheep."

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