Warmer weather is causing a famous breed of Scottish sheep to get smaller, according to research published in a leading scientific journal.

Evolutionary theory holds that species get bigger and stronger over time because larger, more dominant animals are more likely to reproduce. But wild sheep on the Scottish island of Hirta have baffled scientists since 2007 when the animals' average size appeared to be shrinking.

A study, published in the July 2 issue of Science, fingers climate change as a possible cause. Survival conditions on Hirta are becoming less challenging, which means slower-growing, smaller sheep are more likely to survive the winters than they once were.

The researchers analyzed body size and life history data for Soay sheep on Hirta over a 24-year period beginning in 1985.

They found that sheep on the island aren't growing as quickly as they once did, and smaller sheep are becoming more likely to survive into adulthood, bringing the average sheep size down over time.

Soay sheep on the St. Kilda Archipelago in Scotland. A study led by researchers at Imperial College London theorizes that global warming is causing the average sheep size on the island of Hirta to decrease by making it easier for smaller animals to survive long enough to reproduce. (A. Ozgul/American Association for the Advancement of Science)

To be sure, the rate of shrinking is not overly dramatic. Average body size has decreased by about five per cent over the last 24 years, the data show.

But the lead researcher on the project, Prof. Tim Coulson of Imperial College London's department of life sciences, says the decline is still significant.

He suggests that shorter, milder winters, caused by global climate change, mean that lambs don't need to put on as much weight in the first months of life to survive to their first birthday as lambs 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."

Ecological response

The results suggest that the decrease in average body size seen in Hirta's sheep is primarily an ecological response to environmental changes over the last 25 years.

"Our findings have solved a paradox that has tormented biologists for years — why predictions did not match observation," Coulson said. "Unfortunately, it is too early to tell whether a warming world will lead to pocket-sized sheep."

Researchers also theorize that the age of Soay ewes when they give birth is playing a role. Younger Soay ewes are physically unable to produce offspring that are as big as they themselves were at birth.

Though the exact causes remain unclear, climatic changes have allowed more of these young ewes to reproduce, another factor in why the sheep of Hirta are defying biologists' expectations.

"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."