Soay sheep in the St Kilda archipelago are shrinking says a new study.
Credit: Science/AAAS
LONDON: Milder winter weather has caused a wild breed of Scottish sheep to shrink in size by around 5% over the last 25 years, and now experts think they understand why.
Curiously this has occurred despite the evolutionary benefits of large body size in the harsh conditions in which the animals live.
The research, reported today in the U.S. journal Science, suggests a mechanism by which climate change can very rapidly act to alter the size and shape of a species.
Shape shifters
Biologists have started to realise that animals can change their shape fairly rapidly over several generations, but classic evolutionary theory has not been able to explain this, said Tim Coulson a population biologist and one of the lead authors of the study.
To understand more, a team led by Coulson and Arpad Ozgul, both at Imperial College in London, England, looked to data from a study of rare Soay sheep on the wind battered Scottish archipelago of St Kilda. They examined life history and body weight data of the population between 1985 and 2009.
The results revealed that slower growing, smaller sheep are able to survive the milder winters, meaning that small animals are not being weeded out of the population. The research also suggests a 'young mum effect', whereby these smaller, younger animals are having relatively smaller offspring, because they are unable to produce large lambs.
"What we found is that climate change has moved spring to come a little bit earlier... and fewer sheep have died," which has contributed to a decline in the growth rate, said Coulson.
Ecological, not evolutionary
"In the past only big, healthy sheep and large lambs that had piled on weight in their first summer could survive the harsh winters on [the island of] 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," he said. "Smaller individuals are becoming increasingly prevalent in the population."
Because these factors have not yet resulted in heritable genetic changes to the population, they are not evolutionary changes, but ecological ones.
"Traditionally we think of ecological changes happening on shorter time scales than evolutionary changes, although some recent studies are also showing that evolutionary changes can be very rapid," commented Andrew Sugden, a managing editor with Science in Cambridge, England.
