Scientists say they've solved riddle of the shrinking Soay sheep

Scientists have been studying the evolution of Soay sheep in the Outer Hebrides for more than quarter of a century and believe milder winters have changed their size, trumping natural selection.

The findings are said to be the first evidence that climate change could have an impact on animal evolution and ecology.

The mysterious decrease in the size of Soay sheep on the island of Hirta in the remote St Kilda archipelago was first reported by scientists in 2007.

A follow-up analysis suggests conditions are becoming less challenging as the planet heats up – meaning slower-growing, smaller sheep are more likely to survive the winters than they once were.

This, together with the phenomenon called "young-mum effect", where young ewes produce smaller offspring, explains why average body size among island sheep has fallen by about 5 per cent over the past 25 years.

The sheep 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.

The principal investigator, Professor Tim Coulson, of Imperial College London, said: "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."

He added: "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.

"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."

Prof Coulson suggests milder winters, caused by global climate change, mean that lambs do not need to put on as much as weight in the first months of life to survive to their first birthday as they did when winters were colder.

In addition, the research team discovered that the age at which a female sheep gives birth affects the size of her offspring. They realised that young Soay ewes are physically unable to produce offspring that are as big as they themselves were at birth.

This "young-mum effect" had not been incorporated into previous analyses of natural selection, which explains in part why the sheep of Hirta are defying biologists' expectations.

The study was released yesterday by Science, published by AAAS, the non-profit, international science society.

Andrew Sugden, deputy and international managing editor at Science, said: "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."

The researchers looked at data recorded since 1985, analysing sheep population sizes and body measurements, along with life history events, which times key milestones throughout an individual sheep's life.

A BREED APART

THE Soay sheep are closely related to the wild sheep originally domesticated by man. They run wild on the island, which was abandoned in the 1930s after 2,000 to 3,000 years of occupation.

No-one knows quite when the sheep arrived on St Kilda, but evidence suggests they came with the first human settlers about 4,000 years ago. When the Norse arrived at the St Kilda archipelago in the 9th-10th centuries AD they named the island Sauda-ey – "Island of sheep".

Until 1932, pure-bred Soays were only found on the island of Soay, then a flock of 107 were rounded up and moved on to the main island of Hirta.

Today, unmanaged populations of Soays live on both Soay and Hirta, where their population fluctuations have interested biologists since the 1950s.