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Western Pygmy Shrew - Sorex eximius

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Species of Concern
Native Species

Global Rank: G4
State Rank: S3
(see State Rank Reason below)

Agency Status

External Links

State Rank Reason (see State Rank above)
Observations of this species are infrequent resulting in limited data to assess threats. Species may only breed once in its brief life, so is more vulnerable than many small mammal species.
  • Details on Status Ranking and Review
    Western Pygmy Shrew (Sorex eximius) Conservation Status Review
    Review Date = 05/03/2018
    Range Extent

    ScoreF - 20,000-200,000 km squared (about 8,000-80,000 square miles)

    Comment116,880 square Kilometers from Natural Heritage Program range maps

    Long-term Trend

    ScoreU - Unknown. Long-term trend in population, range, area occupied, or number or condition of occurrences unknown

    CommentUnknown, species distribution is poorly documented in Montana

    Short-term Trend

    ScoreU - Unknown. Short-term trend in population, range, area occupied, and number and condition of occurrences unknown.

    CommentNo data


    ScoreU - Unknown. The available information is not sufficient to assign degree of threat as above. (Severity, scope, and immediacy are all unknown, or mostly [two of three] unknown or not assessed [null].)

    CommentSpecies is found in conifer and riparian forested habitats, but its associations and distributions within these habitats are not well understood.

    Intrinsic Vulnerability

    ScoreB - Moderately Vulnerable. Species exhibits moderate age of maturity, frequency of reproduction, and/or fecundity such that populations generally tend to recover from decreases in abundance over a period of several years (on the order of 5-20 years or 2-5 generations); or species has moderate dispersal capability such that extirpated populations generally become reestablished through natural recolonization (unaided by humans).

    CommentSpecies has relativly large litters, but may only breed once or twice in its lifetime

    Environmental Specificity

    ScoreB - Narrow. Specialist. Specific habitat(s) or other abiotic and/or biotic factors (see above) are used or required by the Element, but these key requirements are common and within the generalized range of the species within the area of interest.

    CommentMesic areas within forests

    Raw Conservation Status Score

    Score 3.5 + 0 (geographic distribution) + 0 (environmental specificity) + 0 ( trend) + -0.25 (intrinsic vulnerability) = 3.25

General Description
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.

Diagnostic Characteristics
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).

Species Range
Montana Range Range Descriptions


Observations in Montana Natural Heritage Program Database
Number of Observations: 56

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



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

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).

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: 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
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).

Reproductive Characteristics
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.

  • Literature Cited AboveLegend:   View Online Publication
    • Armstrong, D. M. 1972. Distribution of mammals in Colorado. University of Kansas Museum of Natural History Monograph 3:26-31.
    • Baker, R. H. 1983. Michigan mammals. Michigan State University Press. 642 pp.
    • Brown, L. N. 1966. First record of the pygmy shrew in Wyoming and description of a new subspecies (Mammalia: Insectivora). Proceedings of the Biological Society of Washington 79:49-52.
    • 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.
    • Clark, S.G. and M.R. Stromberg. 1987. Mammals in Wyoming. University of Kansas Museum of Natural History, Public Education Series Number 10. xii + 314 pp.
    • Dierseing, V.E. 1980. Systematics and evolution of the pygmy shrews (subgenus Microsorex) of North America. Journal of Mammalogy 61(1): 76-101.
    • Feldhamer, G. A., R. S. Klann, A. S. Gerard, and A. C. Driskell. 1993. Habitat partitioning, body size, and timing of parturition in pygmy shrews and associated soricids. Journal of Mammalogy 74:403-411.
    • Foresman, K. R. 1986. Sorex hoyi in Idaho: a new state record. Murrelet 67:81-82.
    • Foresman, K. R. 1999. Distribution of the pygmy shrew, Sorex hoyi, in Montana and Idaho. Canadian Field-Naturalist 113:681-683.
    • Foresman, K.R. 2012. Mammals of Montana. Second edition. Mountain Press Publishing, Missoula, Montana. 429 pp.
    • Hendricks, P. 2001. A significant new record of the pygmy shrew, Sorex hoyi, on the Montana-Alberta border. Canadian Field-Naturalist 115:513-514.
    • Jean, C. and P. Hendricks. 2001. Lund easement baseline biological inventory. Report to the Natural Resource Conservation Service. Montana Natural Heritage Program, Helena, Montana. 15 p. plus appendices.
    • 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.
    • 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.
    • Koford, C. B. 1938. Microsorex hoyi washingtoni in Montana. Journal of Mammalogy 19: 372.
    • Long, C. A. 1972. Notes on habitat preferences and reproduction in pygmy shrews (Microsorex hoyi). Canadian Field Naturalist 86:155-160.
    • Long, C. A. 1974. Microsorex hoyi and Microsorex thompsoni. Mammalian Species (33): 1-4.
    • Setzer, H. W. 1952. Pigmy shrew, Microsorex, in Montana. Journal of Mammalogy 33:398.
    • Smallwood, K. S. and T. R. Smith. 2001. Study design and interpretation of shrew (Sorex) density estimates. Annales Zoologici Fennici 38:149-161.
    • Stinson, D. W. and J. D. Reichel. 1985. Rediscovery of the pygmy shrew in Washington. Murrelet 66:59-60.
    • Van Zyll de Jong, C. G. 1976. Are there two species of pygmy shrews (Microsorex)? Canadian Field-Naturalist 90:485-487.
    • Whitaker, J. O., Jr. and T. W. French. 1984. Foods of six species of sympatric shrews from New Brunswick. Canadian Journal of Zoology 62:622-626.
    • Wrigley, R.F., J.E. DuBois, and H.W. Copland. 1979. Habitat, abundance and distribution of six species of shrews in Manitoba. Journal of Mammalogy 60:505-520.
  • Additional ReferencesLegend:   View Online Publication
    Do you know of a citation we're missing?
    • 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.
    • Boggs, Keith., , Field inventory of wildlife resources . . . WICHE Proj. No. D117. Spec. Oc./Habitat Type.
    • 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 Mammalogists, Special Publication Number 12. Lawrence, KS. 278 pp.
    • George, S.B. 1988. Systematics, historical biogeography, and evolution of the genus Sorex Journal of Mammalogy 69:443-461.
    • Hanauska-Brown, L., B.A. Maxell, A. Petersen, and S. Story. 2014. Diversity Monitoring in Montana 2008-2010 Final Report. Montana Fish, Wildlife & Parks. Helena, MT. 78 pp.
    • Hendricks, P., S. Lenard, D.M. Stagliano, and B.A. Maxell. 2013. Baseline nongame wildlife surveys on the Fort Peck Indian Reservation. Report to the Assiniboine and Sioux Tribes of the Fort Peck Indian Reservation. Montana Natural Heritage Program, Helena, MT. 83 p.
    • Hoffmann, R.S. and D.L. Pattie. 1968. A guide to Montana mammals: identification, habitat, distribution, and abundance. Missoula, MT: University of Montana. 133 p.
    • 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.
    • Joslin, Gayle, and Heidi B. Youmans. 1999. Effects of recreation on Rocky Mountain wildlife: a review for Montana. [Montana]: Montana Chapter of the Wildlife Society.
    • Long, C. A. 1974. Sorex hoyi. American Society of Mammalogists, Lawrence, KS. Mammalian Species No. 33:1-4.
    • Mengak, Micheal T., et. al. 1987. Abundance and distribution of shrews in western South Carolina. Brimleyana (13):63-66.
    • 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.
    • Woodman, N. 2018. American recent Eulipotyphla Nesophontids, Solenodons, Moles, and Shrews in the New World. Smithsonian Contributions to Zoology 650. 108 p.
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