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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
The
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
The
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.
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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.
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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.
With
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 Borerays 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.
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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.
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Soay
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%.
When
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.
The
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.
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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.
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