| St Kilda field mouse | |
|---|---|
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Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Mammalia |
| Order: | Rodentia |
| Family: | Muridae |
| Genus: | Apodemus |
| Species: | |
| Subspecies: | A. s. hirtensis |
| Trinomial name | |
| Apodemus sylvaticus hirtensis (Barrett-Hamilton, 1899) | |
TheSt Kilda field mouse (Apodemus sylvaticus hirtensis) is asubspecies of thewood mouse that isendemic to theScottish archipelago ofSt Kilda, the island 40 miles (64 km) west ofBenbecula in theOuter Hebrides, and 100 miles (160 km) from mainland Scotland.[1] Unique to the islands, the mouse is believed to have arrived on the boats ofViking settlers more than a millennium ago. It is not to be confused with theSt Kilda house mouse (Mus musculus muralis), a subspecies of thehouse mouse which is now extinct.[2][3]
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 theSt 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.[3] The islands currently have temporary human habitations. While field mice are widespread onHirta, their concentration is more pronounced in the old village areas where holes provide access into buildings.[4] Though rarely observed by casual visitors, the mouse is common and is present in every part of the habitat, from the harbour to the high point.[2]
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,[2] with a mass of between 50 grams (1.8 oz) and 70 grams (2.5 oz), and has longer hair and a longer tail. The evolution of a larger size has been credited to a lack of predators in itsisland habitat,[5] which allows the mice to grow larger to preserve heat and increase fat storage.[2] The mouse can reach a maximum length of about 17 centimetres (6.7 in).[6] It is found across the main island, Hirta, especially in the remains of human settlements, as well as on the island ofDùn. It is not found onBoreray.[6] Studies of the fur of the mice have recorded theflea speciesCtenophthalmus nobilis, andNosopsyllus fasciatus on the mouse as well as the miteTyphloceras poppei. Studies of the intestines have observed thenematodeTictularia cristata and thecestodeHymenolepsis diminuta.[7]
The mouse is an opportunisticomnivore. Its diet includes insects, snails, seeds, and moss, as well as human litter and animal carcasses.[5] With only one other naturalised mammal, theSoay sheep, which eats grasses and herbs, the St Kilda field mouse faces little competition for food on the islands.[8]
Unique to the islands, the ancestors of the St Kilda's field mouse are believed to have arrived on the ships ofViking settlers.[2] Thetaxon was first described in 1899 byGerald Edwin Hamilton Barrett-Hamilton[9] as the separate speciesApodemus hirtensis. A year later in a specific review of mouse species the taxon was reclassified as asubspecies of the wood mouseApodemus sylvaticus.[10][6]
Studies of the mouse populations on the islands were carried out in 1931, 1939 and 1955. These studies documented the rapid extinction of theendemic house mouse subspecies (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 ofniche expansion.[11]
