Just in case the world’s public were growing inured to tales of rising sea level, drought and crop failure, scientists have come up with a new reason climate change is bad. It makes sheep shrink.
Despite the fact that larger sheep are more likely to survive when young, the Soay sheep (Ovis aries) on the Scottish island of Hirta have been shrinking in size over the last 20 years. Tim Coulson, of Imperial College London, UK, and his colleagues have been working out which of the myriad of possible factors is most responsible for this change.
In Science they report their analysis of the body-weights and life-history of female sheep from Soay. They found that the animals are not growing as quickly as they once were and that more of the smaller sheep were surviving their early years.
“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,” says Coulson (press release).
“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.”
His team also found that the ewes didn’t seem able to produce offspring as big as they were at birth. Together these factors override the positive selection for bigger-boned sheep.
As the BBC explains:
They used a formula called the “Price equation”, which was designed by evolutionary theorist George Price to predict how a physical trait, such as body size, will change from one generation to the next.
With all of this data, the team was able to “rearrange the equation” and use it to work out how much of a contribution each driver made to the sheep’s body size.
Kaustuv Roy, of UC San Diego, told the LA Times that most thinking on climate and species was “fairly simplistic” when actually the dynamics are “fairly complex”.
“We’re going to have to sort out some of these details if we’re ever going to make predictions about how individual species are going to respond to warming,” says Roy.
How much might the Soay sheep continue to shrink?
“Unfortunately, it is too early to tell whether a warming world will lead to pocket-sized sheep,” says Coulson. He will rule out one cute-overload possibility: “We’re certainly not predicting that we’ll be seeing pygmy sheep being herded by Chihuahuas.” (AFP, Times.)
Image: A. Ozgul. Image © Science/AAAS