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Microtus chrotorrhinus - Rock vole, Yellownose Vole

Physical Description:

Microtus chrotorrhinus is usually a grayish brown color dorsally, frequently with black-tipped hairs.  The distinguishing face is yellowish to orange rust that is most vibrant on the snout and fades toward the ears.  The degree and intensity of this coloration varies due to age and locality.  The rock vole is an average sized vole measuring 140 to 185 mm from snout to tail tip, with the tail comprising 42 to 64 mm of its length.  Males of the species tend to be slightly larger  (Kirkland and Jannett 1982).  It has a dental formula of 1/1, 0/0, 0/0, 3/3 totaling sixteen teeth (Kurta 1995).  The molars are rootless, growing continually and the occlusal surfaces are helpful in distinguishing the rock vole from other voles (Kirkland and Jannett 1982).

Distribution:

M. chrotorrhinus ranges from northeastern Minnesota and southwestern Ontario, east to Labrador and the Maritime Provinces in Canada.  In eastern North America, it is found as far north as Cape Breton Island and south through the Appalachian Mountains to the Virginias and North Carolina (Kirkland and Jannett 1982; Pagels 1990). Rock voles are relatively scarce and tend to live in small, isolated patches.  While they were traditionally thought to inhabit boreal habitats at higher elevations (above 600m), they have since been trapped in openings in moist forest, in Transition Zone habitats, hardwood forests, and in dense grass or meadow habitats (Christian and Daniels 1985; Timm et al. 1977).  This indicates that rock voles are geographically more widespread and have a greater ecological range than previously known (Kirkland and Knipe 1979).  Though acceptable habitats do exist in Wisconsin, especially in the North and West areas of the state, throughout its range the rock vole does not inhabit seemingly appropriate environments (Hamilton and Whitaker 1979) and this appears to be the case in Wisconsin.

Ontogeny and Reproduction:

M. chrotorrhinus breeds early in the spring until mid to late fall with a gestation period lasting 19 to 21 days (Kirkland and Jannett 1982; Jannett 1987).  Females produce three or more litters during the season and those born in spring will produce litters their first summer.  An average litter size is 3.5 young (Timm et al. 1977) and range from 2 to 5 with larger, older females producing the most young (Kirkland and Jannett 1982).  Females then quickly abandon juveniles after weaning (Jannett 1987).

Ecology and Behavior:

As the common name implies, rock voles inhabit areas where rocks or boulders are prominent environmental elements, and they seem especially preferential to rock crevices.  Mosses, ferns and forbs also tend to be included in their habitat (Kirkland and Knipe 1979; Pagels 1990).  Black Spruce (Picea mariana), Paper Birch (Betula papyrifa), White Cedar (Thuja occidentalis), and Balsam Fir (Abies balsamea) are often the dominant trees though they are found most often in areas of open canopy.  Downed trees that expose underlying boulders and crevices are sites that frequently shelter rock voles (Christian and Daniels 1985).  Additional flora dominating the rock vole’s habitat includes Bunchberry (Cornus Canadensis), Blueberry (Vaccinium angustifolium), Aster (Aster macrophyllus), and Clinton’s Lily (Clintonia borealis).  Furthermore, water, either standing or flowing; surface or subsurface is an important habitat component (Timm et al. 1977; Kirkland and Knipe 1979).

Food habits of Microtus chrotorrhinus in the northeastern portion of their range were studied by the analysis of the stomach contents of 47 specimens.  Whitaker and Martin (1977) found Bunchberry to be an elemental part of the rock vole’s diet.  They found it accounted for 47 percent of the stomach volume, and their laboratory study confirmed the species partiality toward this plant.  They also found it to eat a variety of green vegetation, mosses, and lepidopterous larvae.  Forbs and leaves from ericaceous plant species have also been noted as an important dietary component (Donato 1998).  Additionally, captive voles in Minnesota ate Blueberry, Raspberry (Rubus strigosus), and many types of insects (Timm et al.  1977).

Little is known about the behavior of M. chrotorrhinus (Kirkland and Jannett 1982).  Systems of subterranean runways are found in conjunction with the rock voles habitat and many are captured in these or similar subsurface places (Kirkland and Knipe 1979; Timm et al. 1977).  This is a good indication that the rock vole spends the majority of its time below ground (Kirkland and Jannett 1982).  From trapping data, it seems to be most active in the day and night, being less so during the afternoon hours (Timm et al. 1977).  This behavior likely arises from a desire to avoid predation thought they do fall prey to Bobcats (Lynx rufus), Weasels (Mustela sp.), Short-tailed Shrews (Blarina Brevicauda), snakes, and some raptors  (Kirkland and Jannett 1982; Donato 1998).  They also have little social structure (Jannett 1987), though the females will sometimes exclude males from the nest while they are nursing (Kirkland and Jannett 1982).

An important aspect of Microtus chrotorrhinus is that its populations are discontinuous, isolated, (Christian and Daniels 1985; Kirkland and Jannett 1982) and small to the point where trapping capture nearly all individuals of a colony (Hamilton and Whitaker 1979).  This geographic isolation reduces gene flow likely accounting for variation amongst the three subspecies—Southern populations of rock voles have slightly darker pelage and somewhat more robust skulls than their northern counterparts (Kirkland and Jannett 1982).  However, the frequency of inbreeding in M. chrotorrhinus is consistent with other rodents, and the level of genetic heterozygosity the voles display is actually above average for rodent populations (Kilpatrick and Crowell 1985).  This would seem indicative of at least some interpopulation migratory action by the rock voles.

Remarks:

There are three subspecies of Microtus chrotorrhinus:  M.c. chrotorrhinus being the most widespread inhabiting Minnesota, Northern Appalachia, and most Canadian species, M.c. carolinensis occupying Southern Appalachia, and M.c. ravus, the smallest, is restricted to Labrador (Kirkland and Jannett 1982).

Microtus chrotorrhinus stems from the Greek chrotorrhinus meaning, “colored nose” (Donato 1998).

Literature Cited:

Christian, D.P. and J.M. Daniels.  1985.  Distributional Records of Rock Voles, Microtus chrotorrhinus, in Northeastern Minnesota. Canadian Field-Naturalist, 99(3): 356-359.

Donato, S.  1998.  Habitat use and Dietary Habits of Yellownose voles (Microtus chrotorrhinus), meadow voles (Microtus pennsylvanicus), and red-backed voles (Clethrionomys gapperi) in Logged and Mature Black Spruce Stands in the Claybelt of Northeastern Ontario.  Laurentian University.

Hamilton, J.H., Jr. and J.O. Whitaker, Jr.  1979.  Mammals of the Eastern United States. Comstock Publishing Associates.  Cornell Univ. Press, Ithaca and New York.

Jannett, F.J. Jr.  1987.  Habitat Breadth and Population Stability and Structure of the Rock Vole, Microtus chrotorrhinus, in Northeastern Minnesota.  American Zoologist 27(4): 45A.
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