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Scientists solve mystery of shrinking sheep

Milder winters are causing Scotland’s wild breed of Soay sheep to get smaller, according to new research at the University of Leeds.

The study, published in this week’s Science Express, provides evidence for climate change as the cause of the mysterious decrease in the size of wild sheep o­n the remote Scottish island of Hirta, first reported by scientists in 2007. The researchers believe that milder winters are making conditions o­n Hirta less challenging, enabling slower-growing, smaller sheep to survive the winters more easily than in the past.

Evolutionary theory suggests that the average size of wild sheep increases over time, because larger animals are more likely to survive and reproduce. However, the population of Soay sheep o­n Hirta have decreased in size by approximately 5 per cent over the last 24 years.

The team’s findings showed that the Soays o­n Hirta 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 overall.

Previously, o­nly the big, healthy sheep and large lambs could survive the harsh winters o­n Hirta.  But shorter, milder winters mean that survival conditions are not so tough. Grass is available for more months of the year and lambs do not need to put o­n as much as weight in their first summer to survive. Even the slower growing sheep have a chance of making it through the winter - and this means smaller individuals are becoming increasingly prevalent in the population.

In addition, the team also discovered that the age of a female sheep affects the size of her offspring. They realised that young Soay ewes are physically unable to produce lambs 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 why the sheep have not been getting bigger as expected – but not why they are shrinking. This, the researchers believe, is down to climate change.

Says Professor Tim Benton of the University of Leeds’ Faculty of Biological Sciences: “Many, many studies – in lab and field – have shown that evolution occurs, but disentangling the roles of evolution in a species’ size and the effects of environmental change o­n growth rate and food has been very tricky.  This research shows that it is possible to disentangle these in the short term. It also warns evolutionary biologists to be cautious: just because something is changing over many generations doesn’t mean it is evolving.”

He adds: “A five per cent decrease in body size in such a relatively short period is astonishing. Climate change is not just about temperatures getting a bit warmer.  It’s affecting fundamental processes which are leading to significant changes in the wildlife around us.”

The research was funded by the Natural Environment Research Council (NERC) involving scientists from the Universities of Leeds, Cambridge, Edinburgh and Stanford, and was led by Imperial College London.