Climate change shifts sheep shape

Soay sheep live on a remote island in the Outer Hebrides

Climate change could have an impact on animal evolution and ecology, scientists believe.

A 20-year study of Scottish sheep found weather patterns were driving changes in body shape and population size.

Harsh winters led to larger sheep, which brought about changes in population size, yet in milder winters this effect was not seen.

The team says the study, published in the journal Science, is the first to connect these different factors.

"Until now, it has proven really quite difficult to show how ecology and evolutionary change are linked, but we have developed a way to tie them together," said Tim Coulson, an author of the paper and a scientist at Imperial College London.

Dr Coulson and colleagues did this by examining a population of Soay sheep on the island of Hirta in the Outer Hebrides.

"The reason we looked at these sheep is they have been studied in enormous detail. Where they live is like a natural laboratory - it is a really simple system - there is just sheep and grass on the island," Dr Coulson explained.

Reproductive success

The scientists looked at data recorded since 1985, analysing sheep population sizes and body measurements.

"To determine how ecology influences evolution and vice versa, an important step is to be able to see how population dynamics are influenced by traits such as body size or eye colour that are, in part, controlled by genes."

The researchers discovered body size was linked to animal numbers: when lots of large sheep came into the population, the numbers tended to fluctuate quite widely, possibly because body size is linked to reproductive success.

During harsh winters, bigger sheep were favoured

But the researchers also discovered the sheep's body size was in turn influenced by their environment.

"We used a measure of how bad the winters were in Scotland, and this has been changing over the duration of the study," said Dr Coulson.

During the harsher winters in the 1980s, the data showed big sheep were genetically favoured, he said.

"But over the years, winters have been getting a little bit better; and as winters have got better, we have found there is not as much natural selection for large animals as we saw in the past, as there is less advantage to being big."

He said the study revealed how environmental factors were driving evolutionary and ecological change, and predicted that as the climate changed, and winters became less frequently harsh, the sheep would get smaller and the population size would be more stable.

"People have argued for a long time that climate change is leaving an ecological legacy, but we have shown it will leave an evolutionary legacy too," he added.