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Environmental changes cause smaller sheep

Environmental change is causing some wild animals to become smaller instead of evolving to be larger, research shows.

Soay sheep

Edinburgh researchers studying Soay sheep on the island of Hirta, St Kilda, have shown that the animals’ average body size has decreased by five per cent over the past 24 years.

The discovery follows previous work by University scientists which showed that the sheep's decreasing size was not caused by evolution.

Milder winters

The latest findings show that a large part of the change is attributable to smaller sheep surviving the winter than in previous years.

This may be because recent winters have been milder and more grass is available.

Surprising findings

The reduction in body size opposes the trend that would be expected from evolutionary research.

Large sheep have the most offspring, and body size is inherited from one generation to the next, so sheep should tend to become bigger over time.

Researchers say that in the case of the Soay sheep, the dwarfing effect of the animals' environment has been greater than the opposing effect of evolution.

International study

The research was led by scientists at Imperial College London and carried out in collaboration with scientists from the Universities of Cambridge, Leeds and Stanford.

The work was funded by the Natural Environment Research Council and published in Science.

Photo credit: Arpat Ozgul.