St. Kilda Mouse

Apodemus sylvaticus hirtensis

The St. Kilda mouse evolvedfrom the common wood mouse. It lines on a zuindszuept island, sheltering among the ruined buildings left behind after people moved away. The St. Kilda mouse is typical of many subspecies of small mammals that form tiny populations in remote places and arehighly vulnerable, being found nowhere else in the world. The St. Kilda mouse does not face any particular threats. However, it is found only on the tiny island of Hirta in the North Atlantic and perhaps one other of the St. Kilda archipelago. These windswept, precipitous islands lie 40 miles (65 km) from the most westerly of Scotland’s Outer Hebrides. Hirta was only occupied by the British Army, which had a missile range there. It is now a nature reserve. If there were a major natural disaster on Hirta, the mice could disappear, and they may die out anyway through inbreeding for many generations. Visitors may carry rats, dogs, or cats, to the island, a serious threat to island mammals. Fortunately, this never happened during the time that the island was occupied. However, the St. Kilda mouse population remains small and highly vulnerable. It is typical of many island races of small mammals (and other animals) that have become stranded in remote places and undergone their own process of private evolution until they no longer resemble the mainland forms from which they are descended.

The St. Kilda mouse is derived from the common wood mouse, which is found widely on the mainland of Britain and Europe. We know that the St. Kilda mouse must have arrived quite recently and evolved rapidly (within a few centuries at most) on its remote island home because at the end of the ice Age (10,000 years ago) the islands were still covered by solid ice. The deep sea surrounding St. Kilda means that the mice could only have got there with human help. They must have been transported accidentally among thatching materials or supplies of food carried from the mainland. Studies of skull similarities suggest that the original wood mice reached St. Kilda over 1,000 years ago and that they arrived from Scandinavia, not the nearest parts of mainland Scotland. They were probably carried to St. Kilda by the Vikings during their famous voyages. The islands have no mammalian predators and no other small mammals apart from some house mice, which died out soon after people left the island in 1930.

Adapt and Survive

Left to themselves and cut off from the typical wood mice of the mainland with whom they would have bred, the island mice gradually changed to suit their new home. Over many generations they became better adapted to the windswept, rainy conditions on their treeless island. Their fur is now dense and woolly, quite unlike the sleek, thin coat of mainland mice. They also grew much larger than their mainland relatives-more than twice the size. They became quite tame too, having nothing to fear from predators. St. Kilda mice now look quite different from common wood mice, having changed by natural processes.

Isolated Populations

The IUCN lists more than 125 other rat and mouse species of the family Muridae (that to which the St. Kilda mouse belongs) classified as Endangered or Critically Endangered. Often they, like the St. Kilda mouse, live in small numbers in remote places. Many have evolved to be different precisely because o` he remoteness of their populations. On islands, mountaintops, or in other isolated patches of habitat. Isolation tends to cause change, and it happens fastest in the smaller species of mammals, particularly mice, because they breed very quickly. Each generation takes a small step toward the creation of a new form. Isolated habitats tend to generate new forms of small mammals that are found nowhere else. Often they are not properly studied because their remote homes are rarely visited. Sometimes such peculiar creatures are not seen for decades and are known only from a few skins and skulls in museums, perhaps collected over a half-century ago. The extinction of such species would take away fascinating examples of evolution in action.

St. Kilda mouse
Apodemus sylvaticus hirtensis

• Family: Muridae
• World population: About 1,000
• Distribution: Outer Hebridean island of Hirta, St. Kilda group, off the west coast of Scotland
• Habitat: Windswept, grassy slopes; in ruined buildings
• Size: Length head/body: 4.5-5 in (11-13 cm); tail: 3-4 in (8-10 cm). Weight: 1.4-1.8 oz (40-50 g)
• Form: Brownish above, creamy-gray below; large eyes and long tail. Coat thicker and woollier than in mainland mice
• Diet: Plants, insects, and seeds
• Breeding: About 6-8 young per litter; 2 or 3 litters per year. Average life span likely to be only a few months, but in captivity can live at least 2 years
• Related endangered species: Poncelet’s giant rat ( Solomys ponceleti ) ; Anthony’s wood rat ( Neotoma anihonyi ); western small-toothed rat of Indonesia ( Macruromys elegans ); Balkans mole rat ( Spalax graecus ) ; Florida mouse ( Podomys floridanus )

Related Posts

1. Brown Kiwi
2. Animal life ( from Southeast Asia )
3. Animal Life From California
4. Thylacine
5. Importance To Humans: Mammal