It was the curious case of the shrinking sheep. For nearly a quarter of a century the wild Soay sheep on the Scottish island of Hirta have been getting smaller when evolution should have made them bigger.
It was a conundrum that had mystified the scientists who began studying the
flock back in the mid-1980s, but now they believe they have come up with a solid explanation: it all comes down to climate change.
A succession of milder winters and earlier springs have allowed smaller lambs to survive the harsh Hirta winters, with the result that the average body size of the typical Soay ewe has shrunk by about 5 per cent over the past 24 years.
Scientists believe that the findings might begin to explain the subtle interactions between ecological pressures that act on a species' body size over short periods, and the longer-term evolutionary pressures that can lead to the sort of extreme "island dwarfism" seen in fossils of extinct animals trapped on islands, such as pygmy hippos.
"Our findings have solved a paradox that has tormented biologists for years - why predictions did not match observation," said Professor Tim Coulson, of Imperial College London, who led the study, published in the journal Science, with colleagues Josephine Pemberton and Tim Clutton-Brock.
Another contribution was the "young-mum effect". The scientists found that ewes pregnant for the first time were physically incapable of giving birth to large lambs.
"It's a little bit early to predict that we'll be having Chihuahuas running around herding pygmy sheep in say a hundred years from now," Professor Coulson said.
- INDEPENDENT
