WASHINGTON: Like the wool sweater that emerges from the dryer a size too small, global warming seems to be shrinking sheep.
On average, wild Soay sheep on Scotland’s island Hirta are 5% smaller today than they were in 1985, according to a team of researchers led by Tim Coulson of Imperial College London. “The decrease in body size was due to a reduction in growth rates caused, in part, by the changing climate,” Coulson said.
The law of evolutionary theory says the brown, thick-coated ungulates should have got progressively bigger. Evolution favors the development of large sheep, which can more easily survive harsh winters, Coulson explained. So the researchers became curious about the overall decline in size of the animals on Hirta.
The team pored over data for the animals’ body size and life history over 24 years. They found that the sheep were not growing as fast as they once did and smaller sheep were likelier to survive into adulthood instead of perishing as lambs. This gives smaller sheep a shot at reproduction, which means that the average sheep size has fallen — by 81 grammes per year on average.
In addition, Coulson noted, there is what he termed the “young mum effect,” with the younger mothers physically unable to produce large offspring.
The find adds to the understanding of how change occurs in many types of animals, he said, including birds, fish and mammals.
It shows how evolution and ecology each play a role in change, Coulson said: “And that, for wild sheep at least, climate change is having a detectable effect on body size — a trait partly determined by genes — and that this complements previous research showing how climate change can influence population size.”
“This study addresses one of the major goals of population biology, namely to untangle the ways in which evolutionary and environmental changes influence a species’ traits,” said Andrew SugdenT, deputy managing editor at Science, which published the report.
Climate change is already having an impact on species. But scientists say it is hard to predict which will be winners and losers from the change, partly because of the complexity of separating out evolutionary pressures from environmental factors.
