Saturday July 4, 2009, 2:19 pm
The new study provides evidence for climate change as the cause of the mysterious decrease in the size of wild sheep on the Scottish island of Hirta, first reported by scientists in 2007. The researchers believe that, due to climate change, 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. This, together with newly-discovered so-called 'young mum effect' whereby young ewes produce smaller offspring, explains why the average size of sheep on the island is decreasing.

Classical evolutionary theory suggests that over time the average size of wild sheep increases, because larger animals tend to be more likely to survive and reproduce than smaller ones, and offspring tend to resemble their parents. However, among the Soay sheep of Hirta, a remote Scottish island in the St Kilda archipelago, average body size has decreased by approximately 5% over the last 24 years.

The research team analysed body size and life history data, which records the timing of key milestones throughout an individual sheep's life, for Soays on Hirta over this 24 year period. They found that sheep on the island are not growing as quickly as they once did, and that smaller sheep are more likely to survive into adulthood. This is bringing down the average size of sheep in the population over all.

Professor Coulson suggests that this is because shorter, 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.

He explains: "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."

Their 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; evolutionary change has contributed relatively little.

In addition, the research team also 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 young mum effect explains why Soay sheep have not been getting bigger, as we expected them to," concludes Professor Coulson, "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."

The research was carried out in collaboration with scientists from the Universities of Leeds, Cambridge, Edinburgh and Stanford. It was funded in the UK by the Natural Environment Research Council (NERC).

Saturday July 4, 2009, 2:38 pm
hmmm... interesting Cowboss... wish global warning would result in making me smaller too... Thanks Cowboss!

Saturday July 4, 2009, 4:35 pm
The rate at which these changes are happening is somewhat alarming. It seems rather benign that the sheep are getting smaller, but this may only be an indicator of a much larger problem.

Sunday July 5, 2009, 1:27 am
This is very interesting, I wonder what Darwin would have made of all this. I had always understod that animals tended to be smaller when the extremes were colder to conserve body heat better (mammoths were smaller than mastondons and this was the theory I had learnt. Not that mammoths were very small critters!) You learn something new every day. Thanks Cowboss, noted with interest.

Sunday July 5, 2009, 2:03 am
thanks cowboss noted .

Sunday July 5, 2009, 8:58 am
I do not see any connection between global warming and the fact that a species of animal is getting smaller on a Scottish Island. In the 14th Century, (long before global warming), Scots took horses and collies to the Shetland Islands in less than 2 centuries these two animals had shrunk to the Shetland Sheepdog or "Sheltie" and the Shetland Pony. Animals adjust their size to the range in which they have to live by evolution. In my Amazonian fish tank, I keep several species that grow to be very large fish in the Amazon, their heritage of being domestic, ornamental fish for the last two centuries has caused them to shrink in size. Hence the descendants of a wild large fish of the Amazon is a small perfectly adjusted fish to a 46 gallon tank in all biological functions including spawning and food needs. People say "global warming" to every species change they see now ignoring present day examples (the keeping of tropical fish) and historic examples like the change in the collie and the horse on the Shetland Islands. Crying "global warming" today is like crying "witch" in former days towards any one who did not share an individual's belief system. Global warming is terrifying and very real but it should be measured in the size of the ice on earth not the size of an animal species adopting to the change in the size of its habitat.