|[Image Jackhynes][St Kilda Field Mouse]|
Because no predators exist on St Kilda, this subspecies was able to grow to twice the size of its ancestor, the field mouse. The St. Kilda field mouse generally weighs between 50 grams and 70 grams. It has black eyes, small peaked ears, and is fairly uniform in colour: mainly brown, with a lighter shade of fur on its underside. It also has longer hair and a longer tail. The St Kilda field mice can survive by eating snails, insects, moss and seeds, can feed on the carcasses of dead sheep and birds, but and will also readily eat food brought in by visitors and intended for their own consumption.
The increased size enables the St. Kilda field mouse to preserve its heat and increase fat storage, something that is necessary in the harsh clime of St Kilda.
When the archipelago was abandoned by its human population and their houses were no longer heated, none of the islands' house mice could survive the bleak conditions and all perished within two years.
 Boudewijn Büch: Eilanden -1991