St Kilda field mouse

St Kilda field mouse
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Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Order:	Rodentia
Superfamily:	Muroidea
Family:	Muridae
Subfamily:	Murinae
Genus:	Apodemus
Species:	A. sylvaticus
Subspecies:	A. s. hirtensis
Trinomial name
Apodemus sylvaticus hirtensis (Barrett-Hamilton, 1899)

The St Kilda field mouse (Apodemus sylvaticus hirtensis) is a subspecies of wood mouse that is endemic to the Scottish archipelago of St Kilda, which is 40 miles west of Benbecula in the Outer Hebrides, and 100 miles (160 km) from mainland Scotland. Unique to the islands, the mouse is believed to have arrived on the boats of Viking settlers more than a millennium ago. It is not to be confused with the St Kilda house mouse, a species which is now extinct.

The last remaining human inhabitants of St Kilda abandoned the islands on 29 August 1930. Thereafter the mice that survived, even those occupying houses abandoned by the St Kildans, were field mice that had moved into the houses from the hills. The islands' house mice could not survive the harsh conditions for more than two years after the archipelago was abandoned by its human population. The islands currently have temporary human habitations. While field mice are widespread on Hirta, their concentration is more pronounced in the old village areas where holes provide access into buildings. Though rarely observed by casual visitors, the mouse is actually very common, and is present in every part of the habitat, from the harbour to the highpoint. The field mice can survive by eating snails, insects, moss and seeds, can feed on the carcasses of dead sheep and birds, and will also readily eat food brought in by visitors and intended for their own consumption.

Description

The mouse has black eyes, small peaked ears, and is fairly uniform in colour: mainly brown, with a lighter shade of fur on its underside. It is generally twice as heavy as field mice found on the mainland, with a mass of between 50 grams (1.8 oz) and 70 grams (2.5 oz), and has longer hair and a longer tail as well. The evolution of a larger size has been credited to a lack of predators in its island habitat, which allows the mice to grow larger to preserve heat and increase fat storage. The mouse can reach a maximum length of about 17 centimetres (6.7 in). It is found across the main island, Hirta, especially in the remains of the human settlement, as well as on the island of Dùn, but not on Boreray. Studies of the fur of the mice have recorded the flea species Ctenophthalmus nobilis, and Nosopsyllus fasciatus on the mouse as well as the mite Typhloceras poppei. Studies of the intestines have observed the nematode Tictularia cristata and the cestode Hymenolepsis diminuta.

Diet

The mouse is an opportunistic omnivore. Its diet includes insects, snails, seeds, and moss, as well as human litter and animal carcasses. With only one other native mammal, the Soay sheep, which eats grasses and herbs, the St Kilda field mouse faces little competition for food on the islands.

History

Unique to the islands, the ancestors of the St Kilda's field mouse are believed to have arrived on the ships of Viking settlers. The taxon was first described in 1899 by Gerald Edwin Hamilton Barrett-Hamilton as the separate species Apodemus hirtensis. A year later in a specific review of mouse species the taxon was reclassified as a subspecies of the common fieldmouse Apodemus sylvaticus.

Studies of the mouse populations on the islands were carried out in 1931, 1939, and 1955. These studies documented the rapid extinction of the endemic house mouse species (which was dependent for its survival on grain and other commodities used by the islands' human inhabitants), and its subsequent replacement by the field mouse, through a process of niche expansion.