Scottish sheep are shrinking, according to new research published in the journal Science.

Researchers from Imperial College London have been studying Soay sheep on the Scottish island of Hirta. They've concluded that on average the sheep have shrunk in size by 5% over the last 24 years. The reduction is in both body weight and length of leg.

The reason is believed to be global warming having made Hirta a greener place with more food available. That seems strange at first - you'd think that animals would be able to thrive and grow larger in a less hostile environment. However the researchers suggest that the opposite is true. Previously the Soay sheep needed to gorge on what food was available during the summer in order to survive the harsh winters. Now that more food is available for more of the year this is less necessary. In addition smaller new born ewes can now survive their first winter, further reducing the average size of the flock.

Not everyone agrees that climate change is the direct reason for the reduction in sheep size. Other mechanisms such as the nutritional value of the available grass my be involved.