The dusky shrew, Sorex monticolus, is a small to medium sized member of the long-tailed shrew genus Sorex, found within the family Soricidae. In general, shrews are tiny animals that resemble mice with the addition of a long, pointed snout. Sorex monticolus is no exception, weighing about 5.5 to 7 g and measuring between 86 and 142 mm (Smith and Belk, 1996).
Some of the smallest mammals on earth are shrews, with the tiniest member of the soricid family being the dwarf shrew, which only weighs about two grams (Findley, 1987). As a result of their small size and relatively large surface area, shrews rapidly lose body heat and must make up for the loss through frequent eating. Not surprisingly, shrews are very active and voracious predators, foraging for insects and other small invertebrates day and night. Sorex monticolus and other species may even consume more than their own body weight in food in a single twenty-four hour period (Findley, 1987). In order to achieve this gastric feat, the dusky shrew has been known to supplement its invertebrate diet with conifer seeds, fungi, and lichens found among the surface litter that it usually inhabits (Smith and Belk, 1996).
Sorex monticolus is also known as the montane shrew, an appropriate name since it is normally restricted to the mountainous or boreal regions of a corridor stretching from northern Alaska to northern Mexico and extending from the Pacific Ocean east to the Rocky Mountains (Smith and Belk, 1996). The name dusky, though more frequently used, is a less meaningful descriptor that refers to the brownish-gray fur color common to many shrews. Recognition of S. monticolus apart from the more than 16 other Sorex species and sub-species found within its range becomes almost impossible when relying on color, form, and size alone.
Identification of shrew species, especially within the genus Sorex, is no easy task. Most of the time, members can only be distinguished by analyzing the upper rows of teeth. Unique to Sorex, the upper jaw will normally contain five unicuspid teeth between the incisors and premolars (dental formula: 1/5/4) (Smith and Belk, 1996). Most members of the genus, including S. monticolus, also have a pigmented ridge on the inside of the unicuspids. Sorex merriami, S. arizonae, and S. towbridgii, the three species that lack this character, are also set apart by their possession of two foramina (mandibular and postmandibular) on the lower jaw. This differs from the usual Sorex condition as seen in S. monticolus where only the more anterior mandibular foramen is present (Jameson and Peters, 1988). The dusky shrew can be distinguished from the remaining Sorex species through a combination of dental and cranial characteristics including a convex skull, upper incisor pigment that extends above the median tine or lobe, and the possession of a third unicuspid that is smaller than the fourth (Jameson and Peeters, 1998). Many of the Sorex species exhibit one or two of these traits, so all three must be present together for a positive identification of S. monticolus.
Literature
Findley, James S. 1987. The Natural History of New Mexican Mammals. The University of New Mexico Press, The New Mexico Natural History Series.164pp.
Jameson, E.W. Jr., and Hans J. Peeters. 1986. California Mammals. University of California Press, Berkeley. California Natural History Guides:52. 403 pp.
Smith, Michael E. and Mark C. Belk. 1996. Sorex monticolus. Mammalian Species. No. 528, pp.1-5.
Links
Information and pictures of shrews on Animal Diversity Web (Univ. of Michigan Museum of Zoology).
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