Montana Field Guide

Arctic Shrew - Sorex arcticus

The Arctic shrew is a medium-large sized shrew. Adults possess a distinctive tri-colored pelage; the dorsum is very dark brown to black, the sides are lighter brown, and the underparts are grayish-brown. The tail is distinctly bicolored, darker above and lighter below. Ranges in external measurements (in millimeters) are: total length 100 to 125, tail length 36 to 45, hind foot 12 to 15, and a mass of 5.3 to 13.5 grams. Condylobasal length of the skull is 19.0 to 20.0 millimeters, palatal length is 8.1 to 8.7 millimeters, maxillary breadth is 4.8 to 5.5 millimeters, and the length of the maxillary tooth row is 6.7 to 7.9 millimeters (Junge and Hoffmann 1981, van Zyll de Jong 1983, Kirkland and Schmidt 1996). The skull has 32 teeth (dental formula: I 3/1, C 1/1, P 3/1, M 3/3); the 5 upper teeth with single cusps that are posterior to the first incisor are termed the unicuspids (U), and include 2 incisors, 1 canine, and 2 premolars. There is a medial tine on I1, and U3 is as large or larger than U4. On the lower jaw (mandible), the pigment is in two or three segments on I1 (the posterior extent of pigment on the ventromedial edge is restricted to about the first third), the height of the coronoid process is more than 4.3 millimeters, and the length of the dentary is greater than 7.8 millimeters (Carraway 1993).

Throughout its range, the Arctic shrew is the only member of the subgenus Sorex, and is distinguished by its well-developed postmandibular canals and lack of pigmented ridges on the unicuspids (Junge and Hoffmann 1981, Kirkland and Schmidt 1996). In northeastern Montana, no other Sorex shrew is as large in skull or body measurements as the Arctic shrew; the tricolored pelage is also distinctive.

Montana Range

No information is available on movements.

Few descriptions of habitat use by Arctic shrew in Montana are available . All individuals have been captured in wet meadow habitat adjacent to marshes or in the sandy flats of creek flood plains. Dominant plants included Agropyron repens, Sonchus spp., Juncus balticus, Carex spp., Agropyron smithii, Distichlis spicata; Hordeum jubatum was also present (Perry 2000, Foresman 2001).

Food habits studies have not been done for Arctic shrews in Montana. Generally, Arctic shrews feed largely on insects; the diet includes moth larvae, grasshoppers, larval and adult beetles, fly larvae, aquatic insects, and other terrestrial invertebrates (Jones et al. 1983, Kirkland and Schmidt 1996). Destructive larch sawfly larvae seasonally constitute a large part of its diet (Buckner 1964). The diet in Montana has not been reported or studied.

No studies have been completed for Arctic shrew in Montana. Studies from other areas of the species' range reveal Arctic shrews are active throughout the year, and are most often nocturnal (Baird et al. 1983, Kirkland and Schmidt 1996). All Montana captures have occurred in late July (Perry 2000, Foresman 2001). Other small mammal associates of Arctic shrew across its range include the shrews Sorex cinereus, S. hoyi, and S. palustris (Wrigley et al. 1979, Kirkland and Schmidt 1996); S. cinereus has been captured in sympatry with the Arctic shrew in Montana (Perry 2000, Foresman 2001).

Vital statistics, measures of population trends, and estimates of population density are rarely available. Home range size in Manitoba was 5913 square meters (Buckner 1966). Population density in Manitoba tamarack bogs was 4.1 to 5.1 per hectare in July and 7.3 to 7.8 per hectare in September; in Wisconsin marsh habitat, density was 8.6 per hectare (Kirkland and Schmidt 1996). There is high population turnover, with loss of approximately 80% of each generation prior to sexual maturity (Buckner 1966). Predators are largely unreported, but include the Great Horned Owl and weasels (Jones et al. 1983, Schowalter et al. 2002). Non-human predators in Montana are not known.

No reproductive studies or data are available for Arctic shrew in Montana. Studies from other areas of the species' range indicate this species' reproductive season extends from February to August in Wisconsin, and April to September in Minnesota. Gestation lasts 2 to 3 weeks, with a lactation period an additional 3.0 to 3.5 weeks. Litter size is 4 to 9, with an average of 2 litters (1 to 3) produced per year; a few females breed in consecutive years. Young-of-the-year sometimes breed late in the first summer in Minnesota. Mortality of nestling Arctic shrews in the first month of life is estimated to be 50% or more, and 80% of cohort mortality occurs before sexual maturity is reached. Longevity may reach 18 months; adults surviving through the winter are not present in samples after July (Baird et al. 1983, Kirkland and Schmidt 1996).

No management needs are identified nor have any measures been enacted for the conservation of Arctic shrew in Montana; the only occurrence so far known is on a national wildlife refuge. Nevertheless, wetland drainage or alteration has the potential to negatively impact local populations. Additional surveys for Arctic shrew can provide the basis for development of conservation protocols by determining its full distribution in Montana, the array of habitats in which it occurs, its relative abundance in different habitats, and, if properly designed, an idea of how different habitat disturbances affect this shrew at the margin of its global range.