No rhyme or reason for black sheep's lighter coats

Global warming explains smaller animals on St Kilda, but not change in colour

The mystery of why the normally dark, woolly coats of the wild sheep of St Kilda are getting lighter in colour cannot be explained by climate change, according to scientists who have made a long-term study of the flock.

The Scottish island of St Kilda, also known as Hirta, has been home to a flock of several thousand unmanaged Soay sheep since 1932. Scientists have noticed that they are getting both smaller in size and lighter in colour.

Previous studies suggested that the physical changes were the result of warmer temperatures. But now climate change has been all but ruled out as the cause of the colour change – although it is almost certainly causing the sheep to shrink in size.

A study last year by Shane Maloney of the University of Western Australia suggested that dark coats are more likely to absorb solar radiation, giving the darker sheep an advantage over their lighter-coloured counterparts in cold conditions. The scientists argued that warmer conditions allowed more lighter-coloured sheep to survive, gradually increasing from about 25 per cent to 30 per cent since the mid-1980s.

But a team of scientists – who in 2008 were the first to report the decline of darker Soay sheep in St Kilda – have now disputed this interpretation, arguing that the genetics behind the differences in coat colour do not support the idea that there is any direct advantage associated with having dark coats.

Among them, John Slate of Sheffield University said that for Dr Maloney's hypothesis to be true it would require there to be a difference between light and dark sheep in terms of their survival rates and breeding success.

However, research on the St Kilda flock shows that the dark coats of the sheep actually come in two different genetic varieties, or genotypes. These two genotypes are physically identical in that they both have dark coats, but one genotype has a higher breeding success, which shows that coat colour per se is not responsible.

"We're not saying that climate change is irrelevant to Soay sheep. It's just that it doesn't explain the change in coat colour that we have observed," Dr Slate said.

A more probable explanation is that the genes determining coat colour lie in a region of the sheep's chromosomes containing another, as-yet unidentified gene that confers some advantage to the sheep. In this scenario, the change to the flock's coat colour is occurring simply because it is "hitch-hiking" with the other mystery gene, Dr Slate said.

However, although the reasons for the changing colour of the Soay sheep remain a mystery, scientists are convinced that the sheep's shrinking size is the result of a warmer climate and shorter winters observed since the 1980s.