The mysterious case of St Kilda's vanishing black sheep, animals whose ancesters were roaming the group of remote Scottish islands more than 4,000 years ago, has been solved by geneticists.

The small, horned, Soay sheep arrived before the Vikings with the first human settlers in this area of north-west Scotland. Deriving from the earliest domestic flocks, they are mostly black or brown, but some have a light sandy coat. In the past few decades the numbers of black animals in the flocks have declined. The baffling question was why, since the darker sheep are larger so more resilient to the severe winters that regularly kill off the more frail animals. Soay itself is home to a few hundred sheep; on Hirta, the largest isle, there are up to 1,000.

To crack the colour mystery the study began with a scan of the sheep's genes - and the tests revealed a remarkable demonstration of evolution in action.

The black sheep get their dark coats from a single gene, one inherited alongside another gene for bulkiness. But the lighter coloured sheep, though more slender, seemed to have evolved as the fitter form, inheriting also genes for extra toughness and reproductive ability.

"This was an enigma, but we're excited to have solved it," said Jon Slate, an evolutionary biologist at Sheffield University, whose study appears in the US journal Science. The lighter sheep were benefiting from genes that made them both more fit and more able to have offspring. "That's why we're seeing the population of the darker sheep go down. It's evolutionary theory in action."

The scientists also identified a third genetic variety - dark Soays that also carry a gene for light colouring. So they too inherit good survival genes.

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