SCOTLAND:The declining number of dark sheep among a wild herd in Scotland comes down to genes, researchers said yesterday.
About three-quarters of the wild Soay sheep on the isolated island of St Kilda are dark, but their dwindling numbers have puzzled scientists. This is because dark animals tend to be bigger, which should give them an evolutionary advantage to survive harsh winters.
"If being big is good and dark sheep are bigger, we would expect the frequency of dark sheep to increase," said Jon Slate, a researcher at the University of Sheffield. To solve the riddle, the scientists analysed versions of genes that determined colour. Like all animals, sheep inherit one version of each gene from each parent and these sheep can inherit either a gene for a dark coat or a gene for a white coat from each parent.
The researchers determined that the gene for a dark coat is dominant - dark sheep carry either two dark genes or a dark gene and a light gene. But they also found that having a light gene boosts fitness, so the best combination in evolutionary terms for the sheep is a mix of genes that produce a dark coat.
This explains the decline of dark sheep because those with a pair of dark genes are the least fit, even though they are big, Mr Slate said.