Shrinking¹ sheep 'defy' natural selection

Sheep living on a remote island off the coast of Scotland have been shrinking for 20 years. Shorter
winters caused by climate change may be responsible.
Soay sheep are a primitive breed of domestic sheep living free on the island of Hirta in the St Kilda
archipelago. Over the last two decades, the average size of the sheep has been falling.
5 Scientists say milder² winters help smaller sheep to survive, resulting in this "paradoxical decrease
in size".
Classic evolutionary theory would predict that wild sheep gradually get bigger, as the stronger,
larger animals survive into adulthood and reproduce.
Scientists first began studying Soay sheep, on the island of Hirta in the St Kilda archipelago, in
10 1985.
Since then, the sheep have decreased in size by 5% - their legs getting steadily shorter and their
body weight decreasing.
The lead researcher in the study, Tim Coulson from Imperial College London, said "The island is
almost like a natural laboratory - there are only the sheep and the vegetation there," he said.
15 They 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 weight³ in their first
summer could survive the harsh winters on Hirta," said Professor Coulson.
Because of climate change, he explained, grass for food is now available for more months of the
20 year on the island.
"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 common in the
population," he said.
The team also found that younger sheep tended to give birth to smaller lambs - a phenomenon they
25 termed "the young mum effect". As for the future of the sheep, the team believes that they are still
shrinking. But it's too early to say if, in 100 years, we will have chihuahuas herding4 pocket-sized
sheep!
Extract from BBC News, by Victoria Gill, Thursday, 2 July 2009
Sum up this article and explain the main ideas using your scientific knowledge.
¹Shrinking: becoming smaller
²Milder: less harsh climate conditions
³To pile on weight: to accumulate fat
4Herding: number, group of animals