Montana Field Guide

The Western Pygmy Shrew is a relatively small member of the shrew family. The upperparts in summer are reddish-brown or grayish-brown, becoming paler on the flanks (especially in winter), with whitish, grayish, or rusty-gray underparts. In winter the pelage is brighter, drab above, and paler below. Ranges in external measurements (in millimeters) are: total length 67 to 98, tail length 25 to 34, hind foot 8.5 to 11.5, and a mass of 2.2 to 6.6 grams. Condylobasal length of the skull is 13.4 to 15.8 millimeters, and maxillary breadth is 4.9 to 5.6 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. The medial edge of the first incisor has a lengthy tine; U3 and U5 are small (U3 disc-like), easily overlooked upon superficial examination, and make it appear as though there are only three unicuspids. On the lower jaw, the length of the dentary is usually less than 6.1 millimeters, I1 is set at an angle more than 10 degrees from the horizontal ramus of the dentary, the length of C1-M3 is less than 4.2 millimeters, and the height of the coronoid process is usually 3.1 to 3.4 millimeters (Long 1974, Diersing 1980, Junge and Hoffmann 1981, Carraway 1995).

Previously the Western Pygmy Shrew was called the Pygmy Shrew (S. hoyi). However, recent analysis of the genetic structure of this species has provided support for splitting it into several species with the Western Pygmy Shrew occurring in Montana.

Pygmy Shrew can be distinguished from all other North American Sorex by its small disc-like U3 and the long medial tine on I1 (Junge and Hoffmann 1981). Upon superficial examination, there appear to be only 3 upper unicuspid teeth, instead of 4 to 5. The combination of lower jaw traits (length of the dentary usually less than 6.1 millimeters, I1 set at an angle more than 10 degrees from the horizontal ramus of the dentary, length of C1-M3 less than 4.2 millimeters, height of the coronoid process usually 3.1 to 3.4 millimeters) distinguishes Sorex hoyi from sympatric S. haydeni, S. preblei, and S. merriami using only dentaries (Carraway 1995).

No information is available on movements in Montana. Based upon information from other locations, the species is generally considered non-migratory; apparently only local movements are made.

Throughout its range, the Pygmy Shrew is found in a variety of habitats. It appears to prefer grassy openings in boreal forest, with moist habitats preferred over dry areas. Mesic portions of dry, open coniferous forests (ponderosa pine, western larch) appear to be preferred by Pygmy Shrews in western Montana. Individuals have also been captured in mesic Douglas-fir-lodgepole pine forests, and sagebrush-steppe in northern Beaverhead County (Foresman 1999, 2012). Understory plants include Amelanchier alnifolia, Berberis repens, Arnica cordifolia, Symphoricarpos occidentalis, Ribes spp., Equisetum spp., and Carex spp. A skull recovered in Hill County (Jean and Hendricks 2001, Hendricks 2001) was found in an area largely of open prairie vegetation (Bouteloua gracilis, Koeleria macrantha, Poa secunda, Stipa comata, Carex filifolia, Pascopyrum smithii) but may have originated in one of the forested uplands nearby. However, other specimens have been captured in similar mesic prairie habitats in the pothole region of Sheridan County (Montana Natural Heritage Program Point Observation Database).

In Kentucky and Tennessee, it was much less active above ground than the Southeastern Shrew (S. longirostris) (Feldhamer et al. 1993). Nest sites are not well known. Habitat associations in Manitoba, Wisconsin and Minnesota include mesic forests of hemlock, white pine, aspen, black spruce-tamarack, maple, jack pine, and sometimes in marshy areas (Long 1972, 1974, Wrigley et al. 1979). In eastern South Dakota an individual was captured in cattail-rush habitat on the edge of a slough (Jones et al. 1983). It has been captured in mesic grand fir-subalpine fir-Engelmann spruce forest in the panhandle of Idaho, and ponderosa pine-lodgepole pine-Douglas-fir forest in northeastern Washington (Stinson and Reichel 1985, Foresman 1986). Farther south, in southern Wyoming and northern Colorado, it is associated with bogs and moist spruce-fir forest meadows (Brown 1966, 1967, Armstrong 1972, Clark and Stromberg 1987).

Pygmy Shrews appear to prey primarily on invertebrates, although there are few studies of the food habits. Examination of stomach contents revealed larval Lepidoptera, larval and adult Coleoptera, larval and adult Diptera, earthworms, snails, and slugs (Jones et al. 1983). The diet in New Brunswick included mainly insect larvae, beetles, and spiders (Whitaker and French 1984). Captives have fed on meat (mice), fish, grasshoppers, and other insects. The diet in Montana has not been reported or studied.

Pygmy Shrew is active throughout the year, with records from all months (Brown 1966, Long 1972, Stinson and Reichel 1985, Foresman 1986, Feldhamer et al. 1993). Records in Montana are mostly from June to August, with early October the latest (Koford 1938, Setzer 1952, Hoffmann et al. 1969).

The Pygmy Shrew is not expected to occur in high densities (Van Zyll de Jong 1983). Other small mammal associates of Pygmy Shrew across its range include the shrews Sorex arcticus, S. cinereus, S. fumeus, S. longirostris, S. monticolus, S. vagrans (Brown 1967, Long 1972, Foresman 1986, Feldhamer et al. 1993); it is often reported associated with Southern Red-backed Voles (Clethrionomys gapperi). In Montana, Pygmy Shrew has been found associated with the shrews S. cinereus, S. monticolus, and S. vagrans, as well as the vole Clethrionomys gapperi (Foresman 1999).

Vital statistics, measures of population trends, and estimates of population density are usually not available. In Michigan, densities of 0.1 to 1.9 individuals per hectare were estimated (Baker 1983, Smallwood and Smith 2001). Pygmy Shrew appears to be relatively common locally at some sites (Hoffmann et al. 1969, Foresman 1999), but is considered uncommon across large landscapes. Reported predators include gartersnakes, domestic cats, and hawks (Buteo spp.). The only non-human predator of Pygmy Shrew reported in Montana is an unidentified species of raptor (Hendricks 2001).

No study of the reproductive biology of Pygmy Shrew in Montana is available. Specimen data indicate litters are produced in spring and early summer, and parous females (pregnant or lactating) have been captured in early August. Recorded litter sizes are 3 to 6 (Hoffmann et al. 1969, Long 1974, Foresman 1999, 2001).

Pregnant and lactating females have been captured in July in Wisconsin; August captures included lactating females as well as young-of-the-year. Records from Wyoming and Colorado in July and August include pregnant or lactating females as well as juveniles (Long 1972, 1974); thus in the West and upper Midwest, parturition appears to occur in spring and summer. Conversely, in Kentucky and Tennessee the primary period young are born is November to early March, with fewest births during April to July (Feldhamer et al. 1993); individuals entered the trappable population about 8 weeks later. Gestation lasts probably 2 to 3 weeks. Litter size is 3 to 8. There is little evidence that more than one litter per year is produced; an Idaho female with mature mammary glands was also in early pregnancy, indicating previous suckling of a litter or a lost litter with subsequent rebreeding (Foresman 1986). Sexual maturity in the West is probably reached late in the first year. Longevity is reportedly 14 to 16 months; only about 9.7% of individuals in a Kentucky-Tennessee study were more than 1 year old, with the oldest individuals about 17.5 months (Feldhamer et al. 1993) and probably surviving through two winters.

No management measures have been enacted for pygmy shrew in Montana. It appears to be relatively common locally at some sites (Hoffmann et al. 1969, Foresman 1999), but is considered uncommon across large landscapes. Its occurrence in Swan Valley timberlands harvested 1 to 3 years prior may indicate tolerance to a moderate level of disturbance, although this, and response to other kinds of disturbance, needs more study.

Additional surveys for pygmy 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.