Global warming is leading to a reduction in size of a breed of wild sheep on Hirta, St Kilda. Scientists say milder winters are helping smaller Soay sheep to survive. This goes against classic evolutionary theory which predicts that wild sheep gradually get bigger, as the stronger, larger animals survive into adulthood and reproduce. The researchers found that the local environment had a stronger effect on the animals than the evolutionary pressure to grow larger. “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, because of climate change, there is grass available for more months of the year. This means smaller individuals are becoming increasingly commoner. 5%. Sheep have also tended to give birth to smaller lambs – a phenomenon known as “the young mum effect“. The sheep have shrunk by an average of around 5% since the study began in 1985.