Scotland's sheep are shrinking: Blame climate change

Add to the ever-growing list of emerging signs of climate change … shrinking sheep.

Yes, Scotland’s wild sheep are apparently becoming smaller as warmer winters kill off fewer off the smaller, weaker animals, which can then go on to breed offspring.

Those findings, by researchers at Imperial College London, are published in this week’s Science Express.

The strange decrease in the size of wild Soay sheep on the Scottish island of Hirta was first observed 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 per cent 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. 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.

Lead author Tim Coulson from Imperial’s Department of Life Sciences, 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.

“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,” Coulson 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.”

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.

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,” said 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).