Montana Field Guide

Dwarf Shrew - Sorex nanus

The dwarf shrew is a small, grayish-brown shrew. Summer pelage is brown above, gray and somewhat buffy below; the tail is indistinctly bicolored to the tip, dark above and buff below; the winter pelage is paler and grayer, especially dorsally. Ranges in external measurements (in millimeters) are: total length 82 to 105, tail length 27 to 45, mass 1.8 to 3.2 grams. Condylobasal length of the skull is less than 15.2 millimeters. 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 and U5 are smaller than U4 (Hoffmann and Owen 1980, Junge and Hoffmann 1981).

The dwarf shrew differs from other shrews in Montana through a combination of the following: small body size, medial tine on I1, U3 smaller than U4, and condylobasal length less than 15.2 millimeters (Junge and Hoffmann 1981). This species differs from S. tenellus in averaging slightly smaller and having slightly darker pelage (Hoffmann and Owen 1980). On each half of the lower jaw (dentary) the height of the coronoid process is usually less than 3.1 millimeters, the angle of insertion of I1 is more than 8 degrees from the horizontal ramus of the dentary, and the length of the dentary is usually less than 6.5 millimeters (Carraway 1995).

Montana Range

Little information is available on movements, but the species is probably non-migratory with only local movements taking place.

In general, the dwarf shrew is found in a variety of habitats, including rocky areas (fellfield, rock stripes and polygons) and meadows in alpine tundra and subalpine coniferous forest (spruce-fir), rocky slopes and meadows in lower-elevation forest (e.g., ponderosa pine, aspen, Douglas-fir) with a mixed shrub component, sedge marsh, subalpine meadow, arid sagebrush slopes, arid shortgrass prairie, dry stubble fields, and pinyon-juniper woodland (Hoffmann and Owen 1980, Berna 1990, Kirkland et al. 1997, Rickart and Heaney 2001, Hafner and Stahlecker 2002).

Habitats where dwarf shrews have been documented in Montana are similar in variety to those occupied elsewhere in the global range. Many have been taken in rocky locations in alpine terrain (fellfield, rock polygons) and subalpine talus (2 to 10 centimeters diameter) bordered by spruce-fir, lodgepole pine, or Douglas-fir and aspen; lesser numbers have been captured in montane grassland, sagebrush-grassland with 22% bare ground, and prairie riparian habitat dominated by green ash, rose, and timothy (Hoffmann and Taber 1960, Pattie and Verbeek 1967, Hoffmann et al. 1969, Thompson 1977, MacCracken 1985, Foresman 2001).

No published studies and few reports mention the diet of the dwarf shrew. In captivity it feeds on vertebrate carcasses, as well as spiders and insects, but refuses slugs. Carabid beetles (Coleoptera) and ants (Hymenoptera: Formicidae) are readily attacked, and extra prey are cached (Hoffmann and Owen 1980, Berna 1990). Stomach and intestinal contents have not been examined in wild-caught animals. The diet in Montana has not been reported or studied.

Very little is known about the life history of the dwarf shrew and its true range has yet to be defined.

Dwarf shrew is probably active all year (Kirkland et al. 1997), although the majority of records are from May through September (Brown 1967, MacCracken 1985, Berna 1990, Rickart and Heaney 2001, Hafner and Stahlecker 2002). Collection records for Montana are only from the summer (Hoffmann and Taber 1960, Pattie and Verbeek 1967, Thompson 1977, MacCracken et al. 1985,
D. Flath personal communication).

At many places across their range, dwarf shrews have been found in sympatry with other shrews, including Sorex cinereus, S. merriami, S. monticolus, S. preblei, and Notiosorex crawfordi (Brown 1967, Armstrong et al. 1973, Hoffmann and Owen 1980, Kirkland et al. 1997, Rickart and Heaney 2001, Hafner and Stahlecker 2002). Dwarf shrew in Montana has been found in close association with each of the preceding shrew species except Notiosorex crawfordi, which does not occur in the state (Hoffmann and Taber 1960, Hoffmann et al. 1969, Thompson 1977, MacCracken et al. 1985).
Vital statistics, measures of population trends, and estimates of population density are not available. At some locations where other shrews are present, dwarf shrew is one of the less common, nowhere appearing abundant, and requiring several hundred trap-nights of effort per individual captured (Kirkland et al. 1997). However, in appropriate sites in Colorado it may be abundant relative to other shrew species (Armstrong et al. 1973). The only known non-human predator is the Barn Owl (Martin 1971). Predators of dwarf shrew in Montana are unknown.

No studies and no data are available for Montana.

There are few data on the reproductive biology of the dwarf shrew. Based upon limited research, at high elevation alpine sites, breeding probably begins in late June to early July, after snowmelt. First litters are produced in late July to early August, post-partum estrus appears to be common, and second litters appear in late August to early September, about a month before snowpack begins to accumulate. Embryo counts for second litters were 6, 6, 6, and 8 (average 6.5). There was no evidence that females bred in the year they were born. At lower elevations, breeding may begin earlier and litter size and frequency may be greater (Hoffmann and Owen 1980). A female captured in mid-July on the Kaibab Plateau, Arizona had 3 developing follicles more than 2 millimeters in diameter (Berna 1990).

Males in breeding condition have been captured throughout July and August on the Beartooth Plateau (Wyoming-Montana border). In low elevation grassland, males in breeding condition were captured in mid-June, but it is unknown how early in spring males come into breeding at any elevation. There is some evidence that males in Colorado may attain reproductive maturity late in the summer of their birth (Hoffmann and Owen 1980).

No management measures have been enacted for dwarf shrew in Montana. However, alteration or removal of grassland and sagebrush through fire, herbicides, or mechanical methods, may impact local lower-elevation populations. Measures taken to protect a diversity of size and cover classes of grassland and sagebrush will likely contribute to the conservation of dwarf shrew. Reclamation/restoration of native prairie appears to provide some measure of effective mitigation for strip-mining activity in prairie regions (Kirkland et al. 1997), but this needs additional study. Surveys for dwarf 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 rare shrew.