In the past, evolution has favoured the larger members of the flocks of Soay sheep living on the island of Hirta, as bulking up during the summer months has helped them thrive during long, harsh winters.

But over the last 24 years scientists have charted a gradual shrinking of members of the breed, which is now being blamed on climate change.

Classical evolutionary theory suggests that over time the average size of wild sheep increases, because larger animals tend to be more likely to survive and reproduce than smaller ones, and offspring tend to resemble their parents.

But because survival is becoming less challenging, evolution is being over-ruled by environmental conditions.

Researchers say that the average size is also affected by the recently-discovered ‘young mum’ effect, with younger ewes giving birth to smaller lambs that would not in the past have been viable.

The findings by a research team of scientists from Imperial college, London and the Universities of Leeds, Cambridge, Edinburgh and Stanford have been published in Science magazine.

Lead author Professor Tim Coulson, of Imperial College’s Department of Life Sciences, said: “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, 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.”

Sam Bond

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