Land Mammals

Besides the wren, two other species demonstrate how the St Kilda environment has moulded the appearance and habits of animals into unique subspecies, the housemouse and the fieldmouse, both of which had to rely on human assistance to reach St Kilda.

The St Kilda Housemouse

St Kilda housemouseThe housemouse undoubtedly came with the early human settlers, at most only a few thousand years ago. Its association with people in such a remote and challenging environment enabled it to evolve distinctive characteristics. It was first noted by Seton in 1878 and collected in 1894 when it, like the wren, was first accorded full specific status as Mus muralis. In 1906, however, it was realised to be merely a robust and pale form of Mus musculus. Its fate was sealed, however, when the islanders evacuated St Kilda in 1930. So dependent had it become on humans and their dwellings that within a few years it had become extinct. Scientists were able to study the last few in the Village in 1931 but by 1938 the unique St Kilda housemouse Mus musculus muralis had vanished altogether.

The St Kilda Fieldmouse

St Kilda fieldmouseThe St Kildan fieldmice are much larger and heavier than their Scottish mainland counterparts, measuring around 17cm in length and weighing over 70g when fully grown. They occur all over Hirta but are rare in short grass and heather. The mice are, however, especially common in the old cultivations around the Village where, having less of a burrowing habit than mainland mice, they prefer the shelter of stony recesses, old walls, cleitean and other buildings. In 1931 no fieldmice were ever caught near the houses but in 1938, without competition, from the housemice they were everywhere, a good example of the phenomenon known as niche expansion. Robert Atkinson in his book entitled Island Going (1938) described trapping fieldmice in the byres and houses, the cleitean and the old walls – all former housemice habitat – as well as in the long grass, even to the summit of Conachair. At some time past, somehow, they had also colonised Dun, but the St Kildans were careful to ensure they did not infest Boreray. Parallel evolution seems to have moulded the St Kilda fieldmouse to the unusual and rigorous conditions presented by the St Kilda archipelago.

British races of fieldmice

In 1895 it was recognised that ‘a sharply differentiated local form’ of the fieldmouse Apodemus sylvestris was to be found in the Outer Hebrides, larger and darker than the mainland form but with smaller ears. It was named as a new species Apodemus hebridensis. Four years later another new species was described from St Kilda – Apodemus hirtensis, larger still but with more yellowish-brown on the underside. But the following year (1900) a review of all the specimens in the British Museum concluded that these two forms were only sub-species. Undeterred a new species from Fair Isle was described in 1906- A. fridariensis. By 1940 no less than 15 subspecies were being postulated, from islands all round the British coast. It was not until 1961 that some rationality was introduced to the problem, and the fieldmice from Rum, the largest of the island forms, was the only one to retain its subspecific status – Apodemus sylvaticus hamiltoni. No longer was it speculated that the mice had reached these offshore islands by a complex system of land bridges or Ice Age refugia, but there was general agreement that humans were involved. One anatomical study highlighted how more similar the island mice were to those from Norway than to British mainland populations. The Vikings were proposed as the most likely agent of dispersal, especially since the St Kildan and Icelandic mice were the most Norwegian of all. Mice could easily stow away in the belongings, foodstuffs and animal fodder carried by sea-faring colonists, but whether the Vikings or some more recent immigrants were responsible, remains a matter of debate.


Boreray Sheep

Toward the end of the 19th century the ubiquitous blackface sheep were kept on Hirta and Boreray. The islanders cleared Hirta of sheep when they departed in 1930 but those on Boreray, some 6km to the northeast and difficult of access at the best of times, were abandoned to their fate.

Boreray ramWith dark collars and white or tan markings they look like a cross between Soay sheep (the true St Kildan sheep) and blackface sheep, but in fact they are survivors of a cross between early blackface and the old Scottish shortwool sheep that still survive on North Ronaldsay in Orkney and in the Shetland Islands. Atkinson in 1938 described the Boreray sheep as ‘…often striped and mottled in a curious manner, and long in the leg, giving one very much the idea of reverting to Nature.’

They have large curving horns, especially on the rams, and have a largely creamy-brown coat; some are grey-brown, a few blackish and an occasional one is tan in colour. The face varies from black and white to greyish, with a few completely black, tan or white. Both Soay and Boreray lambs withstand cold and wet better than commercial sheep but freely make use of shelter in the cleitean. They are also skilled climbers so the varied topography of the island cliffs offers shelter too.

Numbers of sheep vary, probably due to weather. Counting sheep on Boreray is difficult, however, and land counts are better than those undertaken from a boat offshore. Totals fluctuate from 350 or so to nearly 700, although ram mortality is high, presumably because they expend so much energy in the rut, just before winter sets in.

Only about 60ha of Boreray’s rocky 77ha can act as pasture for the sheep so the density of animals is high, about 12 per ha, five times the density of hill sheep in the Hebrides; lowland sheep only reach densities of 15 per ha. Although soaked in salt spray the vegetation of Boreray is well manured by guano from the nesting seabirds. The diversity of plants making up this well-grazed sward has a thick and extensive root system which, together with compaction of the soil by the sheep flock, helps reduce erosion. Thus such a high density of sheep on such steep, wind-swept slopes does not appear to be a problem. Lambing percentages on Boreray can be high, equivalent to shepherded hill farms in the Scottish Borders.

The Soay Sheep.

In 1952 Morton Boyd – then a student at Glasgow University – wrote that ‘St Kilda presents many superb problems in plant and animal ecology, and none would be more rewarding than a study of the wild sheep.’ As soon as he joined the Nature Conservancy, Boyd initiated just such a study, which has continued virtually uninterrupted to this day, and is now supervised by the Institute of Cell, Animal and Population Biology at the University of Edinburgh under the direction of Dr Josephine Pemberton.


Tan-coloured Soay ewe with lambSoay sheep have been described as ‘a remarkable survival of the type of domestic sheep that people kept in the Bronze Age’. Indeed bones excavated from earlier Neolithic sites in Britain seem identical to those of the Soay sheep of St Kilda. Scottish shortwool and then, from 1872 or so, blackfaces were kept on both Hirta and Boreray, but the inaccessible island of Soay retained the original primitive Soay sheep where they were ‘hunted’ rather than shepherded. They scatter rather than let themselves be rounded up by dogs or men, so are effectively wild sheep.

Adult males reach 36kg or more in summer, and adult females about 25kg. About 5% of the sheep have miscellaneous white markings on the face or body. The population is dimorphic with about three times as many having a light brown coat than light fawn. Rather more than half of the ewes are polled (i.e. hornless), the remainder carrying horns, some of which (10-12% males and 65% females) are scurred (i.e. having small and crumpled horns). The light coat frequency has remained at about 20-25% while over the same period the frequency of scurred individuals has fluctuated around 20%.

Flock of Soay sheep - HirtaWhen the inhabitants finally abandoned St Kilda in 1930 they removed all their sheep from Hirta, but two years later the islands’ proprietor, the Marquis of Bute, had 107 Soays of mixed age and sex transferred from Soay to Hirta. They comprised 20 tups, 44 ewes, 22 ram lambs and 21 ewe lambs but it could not have been an easy task! Despite this limited gene pool, however, the Soay sheep of Hirta have been found to have a remarkably high degree of genetic variation.

The presence of this second flock helped ensure the future of the breed. By 1939 the flock on Hirta was estimated at about 500, and the first organised census in 1952 revealed 1,114. Since Soay was so difficult of access, the Hirta flock, especially those in Village Glen, have become a study population, and have been censussed annually since 1955. The Hirta population averages 1,200 animals, (with crashes every four years or so), representing a density of some 0.9 ewes per hectare – a relatively low figure in comparison with the average mainland stocking density being about 2.5 sheep per hectare.

Soay ramThe rut takes place in November, triggered by the shortening days of the northern autumn. After a few weeks of intense activity, without much time to feed, adult rams have to enter the winter in poor condition and mortality can be high. There is thus a preponderance of ewes in the population, varying annually from three ewes to every ram, to as many as eight.

Lambs are born after a gestation of 151 days; this is several days longer than modern breeds, which also mature faster. Most births are clumped within 10 days on either side of 20 April. A small number of twins are born (and at least one instance of triplets is recorded) but their survival is poor. Lambs weigh about 2kg at birth and are weaned by July. The ewes can then spend the rest of the summer regaining condition before the rut in November.

Those animals that survive population crashes can enjoy a ripe old age; the oldest tagged ewe being 15 when she died; others have lived to be 14, 13 and 12 years of age. However, for a ram to live to 10 is rare, however, and most die before they are six.

The counts reveal periods of rapid increase to high density, followed by periodic crashes when up to 60% of the population can die in a single winter. These crashes seem to have become more regular in recent years but weather effects can obscure this; one of the highest population counts ever was in 1996, but the population declined little the following winter implying that high density does not necessarily trigger a crash. Crashes have also occurred from relatively low populations when the sheep might have been expected to be in good condition. Gales in March appear to be an important factor but the overwinter mortality seems to be exacerbated by parasites, particularly gut worms.

Parasite burdens increase as the density of sheep increases, imposing a strong natural selection for parasite resistance. Indeed Soay sheep demonstrate one of the highest resistances to parasites anywhere in the world. They share the same parasites as blackfaces but seem to be more effective in dealing with them, which has important implications for commercial sheep husbandry. High parasite loads in domestic sheep can cause heavy stock losses and helminth worm treatments are costly so this aspect of the Soay study has attracted much interest.


Peter Jewell concluded that ‘these long-term studies can probe more deeply into the processes of evolution itself and add profoundly to our better understanding of the world that we live in. More than any other feature of the natural history of the archipelago of St Kilda, the Soay sheep are a treasure of surpassing value’. From time to time, following a crash when so many carcasses of sheep are scattered over the island, a debate is reopened as to whether they ought to be left unmanaged. It must be remembered that the sheep population of Soay has not had any management to speak of for some considerable time, hundreds perhaps thousands of years, and the sheep population there seems to behave in a similar manner to the Hirta Soay sheep; as in fact do the sheep on Boreray. Morton Boyd concluded that to lose the sheep, would remove assets of great cultural, historical and scientific interest. ‘Removal of the sheep would also reduce the diversity of plant life that their grazing sustains on the islands. . . The sheep are now adjusted over centuries to meet the rigours of their world.’