SOAY SHEEP
The departing St Kildans left behind them a rare breed of sheep. Known as Soay sheep – they were kept on the island of the same name (Soay means ‘sheep island’ in Norse which suggests that there have been sheep on St Kilda since at least the time of the Vikings). Genetically, Soays have ancestry from two different waves of sheep domestication. The first is typified by the Mediterranean mouflon, with which Soays share a ‘dark, wildtype’ coat colour. It seems these sheep reached all parts of Europe by the Bronze Age but were then supplanted by a second domestication wave of more productive sheep.
While Soays come from the island of Soay, there is another more modern breed on Boreray called the Boreray sheep. ‘ Soay is appreciably harder to land on than Boreray, and it seems to me the Soays must have landed there very early and were never replaced by an improved breed,’ says Professor Josephine Pemberton, a leading member of the Soay Sheep Project team and professor of natural history at the University of Edinburgh. Written and genetic evidence shows that the St Kildans shifted sheep between populations in historical times, as the Soays and Borerays share some genetics which Pemberton thinks came from a now extinct breed called the dunface. In 1932 – two years after the last islanders departed – 107 Soays were transported to Hirta and have been maintained as a feral population ever since. They currently number around 1,800 and have been studied continuously since 1952 in what has developed into one of the world’s leading monitoring programmes of a species left by humans to its own devices.
The animals are tiny. In August, mature females average around 24kg while mature males are around 38kg, making them about one third the size of most modern domestic sheep. They are also highly variable in appearance. Many have a dark coat with a light belly and rump patch, others are the same colour all over.
The studies have revealed that Soay sheep have rather unusual population dynamics. Unlike many other populations of ungulates (for example, red deer on Rum), the population rises to a peak and then crashes at irregular intervals. ‘The population crashes when the sheep go into winter at high density, the population contains a high proportion of vulnerable individuals – young, old and male – and meets bad weather,’ says Pemberton.
However, a crash has not been recorded for some years and Pemberton wonders whether climate change – with warmer winters and longer grass-growing seasons – is making the island more favourable to the sheep.
The studies continue and cover areas such as ecology, evolution, genetics, behaviour, parasitology and immunology. ‘I guess the overall purpose could be said to understand what makes individuals successful and how this builds up into population dynamics and trends over time,’ says Pemberton.