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Montana Animal Field Guide

Montana Field Guides

Dwarf Shrew - Sorex nanus

Species of Concern

Global Rank: G4
State Rank: S2S3

Agency Status
USFWS:
USFS:
BLM:
FWP SWAP: SGCN2-3


 

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General Description
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).

Diagnostic Characteristics
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).

Species Range
Montana Range

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Western Hemisphere Range

 


Summary of Observations Submitted for Montana
Number of Observations: 20

(Click on the following maps and charts to see full sized version) Map Help and Descriptions
Relative Density

Recency

 

(Observations spanning multiple months or years are excluded from time charts)



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

Habitat
In general, the Dwarf Shrew is found in a variety of habitats, including rocky areas 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 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 et al. 1985, Foresman 2012).

Ecological Systems Associated with this Species
  • Details on Creation and Suggested Uses and Limitations
    How Associations Were Made
    We associated the use and habitat quality (common or occasional) of each of the 82 ecological systems mapped in Montana for vertebrate animal species that regularly breed, overwinter, or migrate through the state by:
    1. Using personal observations and reviewing literature that summarize the breeding, overwintering, or migratory habitat requirements of each species (Dobkin 1992, Hart et al. 1998, Hutto and Young 1999, Maxell 2000, Foresman 2012, Adams 2003, and Werner et al. 2004);
    2. Evaluating structural characteristics and distribution of each ecological system relative to the species' range and habitat requirements;
    3. Examining the observation records for each species in the state-wide point observation database associated with each ecological system;
    4. Calculating the percentage of observations associated with each ecological system relative to the percent of Montana covered by each ecological system to get a measure of "observations versus availability of habitat".
    Species that breed in Montana were only evaluated for breeding habitat use, species that only overwinter in Montana were only evaluated for overwintering habitat use, and species that only migrate through Montana were only evaluated for migratory habitat use.  In general, species were listed as associated with an ecological system if structural characteristics of used habitat documented in the literature were present in the ecological system or large numbers of point observations were associated with the ecological system.  However, species were not listed as associated with an ecological system if there was no support in the literature for use of structural characteristics in an ecological system, even if point observations were associated with that system.  Common versus occasional association with an ecological system was assigned based on the degree to which the structural characteristics of an ecological system matched the preferred structural habitat characteristics for each species as represented in scientific literature.  The percentage of observations associated with each ecological system relative to the percent of Montana covered by each ecological system was also used to guide assignment of common versus occasional association.  If you have any questions or comments on species associations with ecological systems, please contact the Montana Natural Heritage Program's Senior Zoologist.

    Suggested Uses and Limitations
    Species associations with ecological systems should be used to generate potential lists of species that may occupy broader landscapes for the purposes of landscape-level planning.  These potential lists of species should not be used in place of documented occurrences of species (this information can be requested at: http://mtnhp.org/requests/default.asp) or systematic surveys for species and evaluations of habitat at a local site level by trained biologists.  Users of this information should be aware that the land cover data used to generate species associations is based on imagery from the late 1990s and early 2000s and was only intended to be used at broader landscape scales.  Land cover mapping accuracy is particularly problematic when the systems occur as small patches or where the land cover types have been altered over the past decade.  Thus, particular caution should be used when using the associations in assessments of smaller areas (e.g., evaluations of public land survey sections).  Finally, although a species may be associated with a particular ecological system within its known geographic range, portions of that ecological system may occur outside of the species' known geographic range.

    Literature Cited
    • Adams, R.A.  2003.  Bats of the Rocky Mountain West; natural history, ecology, and conservation.  Boulder, CO: University Press of Colorado.  289 p.
    • Dobkin, D. S.  1992.  Neotropical migrant land birds in the Northern Rockies and Great Plains. USDA Forest Service, Northern Region. Publication No. R1-93-34.  Missoula, MT.
    • Foresman, K.R.  2012.  Mammals of Montana.  Second edition.  Mountain Press Publishing, Missoula, Montana.  429 pp.
    • Hart, M.M., W.A. Williams, P.C. Thornton, K.P. McLaughlin, C.M. Tobalske, B.A. Maxell, D.P. Hendricks, C.R. Peterson, and R.L. Redmond. 1998.  Montana atlas of terrestrial vertebrates.  Montana Cooperative Wildlife Research Unit, University of Montana, Missoula, MT.  1302 p.
    • Hutto, R.L. and J.S. Young.  1999.  Habitat relationships of landbirds in the Northern Region, USDA Forest Service, Rocky Mountain Research Station RMRS-GTR-32.  72 p.
    • Maxell, B.A.  2000.  Management of Montana's amphibians: a review of factors that may present a risk to population viability and accounts on the identification, distribution, taxonomy, habitat use, natural history, and the status and conservation of individual species.  Report to U.S. Forest Service Region 1.  Missoula, MT: Wildlife Biology Program, University of Montana.  161 p.
    • Werner, J.K., B.A. Maxell, P. Hendricks, and D. Flath.  2004.  Amphibians and reptiles of Montana.  Missoula, MT: Mountain Press Publishing Company. 262 p.

Food Habits
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.

Ecology
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 et al. 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).

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.

Reproductive Characteristics
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 year (Hoffmann and Owen 1980).

Management
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.

References
  • Literature Cited AboveLegend:   View Online Publication
    • Armstrong, D. M., B. H. Banta, and E. J. Pokropus. 1973. Altitudinal distribution of small mammals along a cross-sectional transect through the Arkansas River watershed, Colorado. Sothwestern Naturalist 17:315-326.
    • Berna, H. J. 1990. Observations on the dwarf shrew (Sorex nanus) in northern Arizona. Great Basin Naturalist 50: 161-165.
    • Brown, L.N. 1967. Ecological distribution of six species of shrews and comparison of sampling methods in the central Rocky Mountains. Journal of Mammalogy 48(4): 617-623.
    • Carraway, L.N. 1995. A key to recent Soricidae of the western United States and Canada based primarily on dentaries. Occasional Papers of the Natural History Museum, University of Kansas (175):1-49.
    • Foresman, K.R. 2012. Mammals of Montana. Second edition. Mountain Press Publishing, Missoula, Montana. 429 pp.
    • Hafner, D. J. and D. W. Stahlecker. 2002. Distribution of Merriam's shrew (Sorex merriami) and the dwarf shrew (Sorex nanus), and new records for New Mexico. Southwestern Naturalist 47:134-137.
    • Hoffmann, R. S. and R. D. Taber. 1960. Notes on Sorex in the northern Rocky Mountain alpine zone. Journal of Mammalogy 41(2): 230-234.
    • Hoffmann, R.S. and J.G. Owen. 1980. Sorex tenellus and Sorex nanus. Mammalian Species 131:1-4.
    • Junge, J.A. and R.S. Hoffmann. 1981. An annotated key to the long-tailed shrews (genus Sorex) of the United States and Canada, with notes on middle American Sorex. Occasional Papers of the Museum of Natural History, the University of Kansas 94: 1-48.
    • Kirkland, G. L., Jr., R. R. Parmenter, and R. E. Skoog. 1997. A five-species assemblage of shrews from the sagebrush-steppe of Wyoming. Journal of Mammalogy 78:83-89.
    • MacCracken, J. G., D. W. Uresk, and R. M. Hansen. 1985. Habitat used by shrews in southeastern Montana. Northwest Science 59(1):24-27.
    • Martin, R. A. 1971. New records of the dwarf shrew from South Dakota. Journal of Mammalogy 52:835-836.
    • Pattie, D.L. and N.A. M. Verbeek. 1967. Alpine mammals of the Beartooth Plateau. Northwest Science 41(3): 110-117.
    • Rickart, E. A. and L. R. Heaney. 2001. Shrews of the La Sal Mountains, southeastern Utah. Western North American Naturalist 61:103-108.
    • Thompson, L.S. 1977. Dwarf shrew in north-central Montana. Journal of Mammalogy 58:248-250.
  • Additional ReferencesLegend:   View Online Publication
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    • Allen, K.L., T. Weaver, and D. Flath. 1994. Small mammals in Northern Rocky Mountain ecosystems. Unpubl. report to Bureau of Land Management and United States Forest Service, August 31, 1994. Montana State Univ., Bozeman. 54 pp.
    • Cinq-Mars, R.J., R.S. Hoffmann, and J.K. Jones. 1979. New records of the Dwarf Shrew (Sorex nanus) in South Dakota. Prairie Naturalist. 2(1): 7-9.
    • Flath, D. L. 1979. Annual report: Nongame surveys and inventory February 1, 1978 - January 31, 1979. MTFWP. 36 pp. including appendices.
    • Flath, D.L. 1979. Nongame species of special interest or concern: Mammals, birds, reptiles, amphibians, fishes. Wildlife Division, Montana Department of Fish and Game. Helena, MT.
    • Foresman, K.R. 2001. The wild mammals of Montana. American Society of Mammologists, Special Publication Number 12. Lawrence, KS. 278 pp.
    • George, S. B. 1990. Unusual records of shrews in New Mexico. Southwestern Naturalist 35:464-465.
    • George, S.B. 1988. Systematics, historical biogeography, and evolution of the genus Sorex Journal of Mammalogy 69:443-461.
    • Geppert, T. J. 1984. Small mammals of the Shield Trap, East Pryor Mountain, Montana. M.S. thesis. University of Iowa, Iowa City. 45 pp.
    • Hoffmann, R.S. and D.L. Pattie. 1968. A guide to Montana mammals: identification, habitat, distribution, and abundance. University of Montana, Missoula. 133 pp.
    • Hoffmann, R.S., P.L. Wright, and F.E. Newby. 1969. The distribution of some mammals in Montana. I. Mammals other than bats. Journal of Mammalogy 50(3): 579-604.
    • Jones, J.K., D.M. Armstrong, R.S. Hoffmann and C. Jones. 1983. Mammals of the northern Great Plains. University of Nebraska Press, Lincoln. 379 pp.
    • Mickey, A.B. 1948. A Record of the Shrew Sorex nanus for Wy. Journal of Mammalogy 29(3):295.
    • Reid, F. 2006. Peterson Field Guide to Mammals of North America, 4th Edition. Houghton Mifflin Company: Boston and New York, 608 pp.
    • Spencer, A. W. and D. Pettus. 1966. Habitat preferences of five sympatric species of long-tailed shrews. Ecology 47: 677-683.
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Dwarf Shrew — Sorex nanus.  Montana Field Guide.  .  Retrieved on , from