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The sibling vole is an accidentally introduced species in Svalbard and is the only small mammal species on the archipelago. They were first discovered in 1960 and misidentified as the common vole Microtus arvalis.
The sibling vole has grey-brown fur covering its body, including its short ears and short tail (which is about one third of the body length). The animals are 10–16 cm long and weigh approximately 30 g. Males are somewhat heavier than females. The sibling vole is an accidentally introduced species in Svalbard and is the only small mammal species on the archipelago.
This species is found in meadows and agricultural areas in continental Eurasia. The northern limit of its distribution on the continent is central Finland, and they extend south to the Balkan Peninsula.
The sibling voles in Svalbard probably originate from the vicinity of Leningrad in Russia and were most likely introduced in hay shipments (animal fodder for livestock) carried by supply ships to the former Russian mining settlement Grumantbyen sometime between 1920 and 1960. They spread to the nearby productive grassy vegetation associated with seabird colonies where they were discovered on cliff-faces near Grumantbyen in 1960 and misidentified as the common vole Microtus arvalis. In 1990, they were correctly identified as sibling voles via genetic analyses.
The distribution of the sibling vole in Svalbard is restricted to a narrow coastal area spanning a distance of some 20 km in the Grumant area in Isfjorden. When vole densities are high, they sometimes spread out along the coast toward Longyearbyen to the east and Barentsburg to the west, but no permanent colonies have been found outside the Grumant area. Their presence in an area can be detected by grazed vegetation, run-ways and faeces.
The core area of the sibling vole in Svalbard is located in the guano-fertilized slopes of Fuglefjella, between Grumantbyen and Bjørndalen, in Isfjorden, Spitsbergen from sea level up to an altitude of 400 m. These steep slopes are covered by luxurious vegetation, mainly comprised of arctic grasses (Alopecurus spp.), which are the main food for the voles at this site.
The voles prefer areas with dense vegetation mixed with boulder fields, which gives good drainage as well as cover. Their distribution seems to be limited mainly by the availability of forage plants.
The vole population size varies a lot; densities can range from 0 to more than 1000 individuals per hectare, but with no evidence cyclic patterns. These dynamics are mainly controlled by winter weather conditions. Rain-on-snow events that encapsulate the vegetation in ice can cause massive mortality because ground-icing makes the vegetation inaccessible and it offers no insulation against low temperatures.
The whole resident, terrestrial vertebrate community in Svalbard is synchronised by climate patterns. In the most climatically unfavourable winters the voles are able to survive only in scattered patches at high altitudes where snow conditions are fairly stable and favourable. But, given favourable conditions the voles are able to re-colonize quickly because of their very high reproductive rate and dispersal abilities.
Because of its very restricted distribution, the sibling vole in Svalbard has no natural competitors and due to the lack of avian specialist predators in Svalbard its only predator is the Arctic fox.
Similar to other arctic rodents, the sibling vole is active throughout the winter, and can even reproduce under the snow when conditions are favourable.
Females can reach sexual maturity at 17 days of age. The males become sexually mature somewhat later, i.e. at an age of more than 35 days. Pregnancy lasts for 20–21 days and females can mate again a few hours after giving birth. A normal litter contains five or six pups and a female can produce as many as four litters over a breeding season.
The weight of newborn pups is on average 2.1 g. The sibling vole is usually full grown when it is approximately 100 days old, but many individuals do not become this old in Svalbard. The average life span is two or three months at this location. Mortality rates vary a lot between seasons, years and across the range of the species.
Sibling voles have been observed regularly near Longyearbyen since the 1960s. But it was not until 1989 that the species distribution in Svalbard was systematically surveyed and the core area for the species in Svalbard localised. So far there are no indications that the distribution range of the voles has changed.
The scope for expansion of the species in Svalbard is limited since sibling voles seem to require a particular habitat type that is rare in Svalbard outside their present distribution. However, it is possible that climatic warming could result in expansion.
In 1999, it became known that the sibling vole in Svalbard is a host for the larval stage of the tapeworm Echinococcus multilocularis, which can be transmitted to humans and cause a fatal disease Alveolar Echinococcosis, which has an 80–90% mortality rate if left untreated. Transfer can occur via handling of fox faeces (so this activity should be done with gloves, if it must be done at all).
This parasite probably spread to Svalbard with migrating arctic foxes (the host of the adult stage of the parasite) from the Russian Arctic where the parasite is common. The combination of a high proportion of Echinococcus multilocularis positive faeces and a high density of arctic fox faeces in the Grumant area suggest that this area is a high risk area for human infection. Even if infected foxes have the potential to spread this parasite’s eggs far from this area, the risk for human infection drops to low levels at very short distances from the Grumant area. Systematic surveys of the sibling vole – parasite population ended in 2007.
Possible future monitoring of the risk of Echinococcus multilocularis infection should focus on monitoring the stability and eventual spread of vole occupancy in habitats associated with seabird colonies and human settlements at and beyond the limits of the sibling voles’ present distribution range.