Dusky shrews are one of the most common members of the genus Sorex in North America. They can be found from northern Alaska to New Mexico and from the Pacific coast to central Manitoba (Smith & Belk, 1996). They also inhabit the Rocky Mountains, Blue Mountains, and the Sierra Nevada. In addition dusky shrews can be found on Vancouver Island and Queen Charlotte Island (Willson & Ruff, 1999).
Dusky shrews occupy a wide range of habitats including tundra, alpine meadows, forests, and prairies (Forsyth, 1985). The main component of suitable microhabitat is dense ground cover, which may aid in predator avoidance. Shrews are often found in forest floor litter and almost never burrow (Smith & Belk, 1996). Habitats with high quantities of coarse woody debris lead to higher reproductive rates in(Lee, 1995). They are closely associated with riparian zones and studies show that most shrews can be found within 100 meters of streams or rivers (Smith & Belk, 1996). Shrews prefer habitats with acidic soils and nearby coniferous forest (Forsyth, 1985).
are small, long tailed shrews. Total length varies between 103 and 142mm, and tail length between 40-62mm (Smith & Belk, 1996). In summer shrews are brownish dorsally with silvery white or gray ventral pelage (Willson & Ruff, 1999). In September or October the pelage becomes darker and thicker. In general molting occurs twice per year and starts from the rump and nose and spreads out, finishing between the ears. The timing of molt differs between sexes, around March for females and May for males (Smith & Belk, 1999). The tail is indistinctly bicolored. Musk glands on the flanks are visible in breeding males and 30% of breeding females. There is no significant sexual dimorphism (Willson & Ruff, 1999).
Dusky shrews have one incisor with two cusps, five unicuspids and four molars in the upper jaw. There is one incisor, two unicuspids and three molars in the lower jaw (Smith & Belk, 1996). Dental formula: 3/1, 1/1, 3/1, 3/3=32 teeth (Forsyth, 1985).
- Range mass
- 5.9 to 7.2 g
- 0.21 to 0.25 oz
- Average mass
- 5.5 g
- 0.19 oz
The breeding season may last from February to August. Around February males show signs of reproductive activity by increasing their body size by 50%. Females become receptive in March (Smith & Belk, 1996). The exact timing of reproduction varies with geographic location so, in general, young are born in spring and summer. Little is known about the precise length of gestation period but it is estimated to be about 13 to 28 days (Forsyth, 1985). Young shrews are born in a small nest made from grass and placed under a rock or fallen tree. They are blind and naked but they mature very quickly, weaning takes only 3 weeks (Findley, 1987). Females experience postpartum estrus and may produce 3 or 4 litters with an average of 5 to 6 young per litter (Smith & Belk, 1996). The expected longevity of these small animals is 12 to 18 months and usually females do not start breeding until after their first winter (Willson & Ruff, 1999).
- Key Reproductive Features
- gonochoric/gonochoristic/dioecious (sexes separate)
Dusky shrews are solitary animals. They do not hibernate and are active 24 hours with three peak activity cycles (two during night and one in early morning) that last 3 to 4 hours. During the day activity periods are more brief (Smith & Belk, 1996). Home ranges of dusky shrews vary between sexes. After they mature males tend to leave their original home range, whereas females stay in their original home range throughout their entire life (Smith & Belk, 1996). When breeding, males tend to have bigger home ranges than females. A male's home range may encompass home ranges of five females (Willson & Ruff, 1999). Home ranges of non- breeding individuals average 1.22 square meters. Home ranges of breeding individuals were bigger averaging 4.02 square meters (Smith & Belk, 1996).
Young shrews were observed to be somewhat territorial but, in general, non-breeders maintain their territories only to survive winter, whereas breeding adults are not territorial (Willson & Ruff, 1999).
- Key Behaviors
Communication and Perception
Dusky shrews are insectivorous. Their small size results in rapid heat loss due to the small surface to volume ratio. In order to maintain a constant body temperature shrews have to maintain a high metabolic rate and, therefore, consume large quantities of prey (Findley, 1987). They must spend most of their time hunting and feeding. The diet ofconsists of insects and their larvae, earthworms, spiders, snails, and, rarely, small salamanders. The largest possible prey size was estimated to be >30mm (Smith & Belk, 1996). In addition, dusky shrews were observed eating conifer seeds, lichens, and fungi (Rhoades, 1986). Although dusky shrews were described as aggressive hunters, little has been mentioned as to how they capture their prey.
Economic Importance for Humans: Positive
Nothing was found on the positive benefit of this species to humans.
Economic Importance for Humans: Negative
Nothing was found on the negative effect of this species on humans.
Since it is one of the most common and widespread species of Sorex in North America it is not protected and no (known) steps have been taken to protect these species.
The nameis from the Latin mons (mountain) and colere (to inhabit) (Smith & Belk, 1996). Sorex means "the shrew mouse" (Forsyth, 1985).
Prior to 1977and Sorex vagrans were considered conspecific (Smith & Belk, 1996). These shrews are extremely difficult to distinguish. In general differs from S. vagrans in cranial dimensions, structure, color of the teeth, and the number of friction pads on the toes. Anything published before 1977 cannot be assigned to either S.monticolus or S. vagrans with certainty (Smith & Belk, 1996).
Katarzyna Banasiak (author), University of Toronto.
living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.
- bilateral symmetry
having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.
uses smells or other chemicals to communicate
animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.
forest biomes are dominated by trees, otherwise forest biomes can vary widely in amount of precipitation and seasonality.
having the capacity to move from one place to another.
This terrestrial biome includes summits of high mountains, either without vegetation or covered by low, tundra-like vegetation.
- native range
the area in which the animal is naturally found, the region in which it is endemic.
reproduction that includes combining the genetic contribution of two individuals, a male and a female
uses touch to communicate
A terrestrial biome with low, shrubby or mat-like vegetation found at extremely high latitudes or elevations, near the limit of plant growth. Soils usually subject to permafrost. Plant diversity is typically low and the growing season is short.
Churchfield, S. 1990. The natural history of shrews. London, USA: Christopher Helm Ltd..
Findley, J. 1987. The natural history of New Mexico mammals. Albuquerque: University of New Mexico Press.
Forsyth, A. 1985. Mammals of the Canadian wild. Camden: Camden House Publishing Ltd..
Hoffmeister, D. 1986. Mammals of Arizona. Tucson: University of Arizona Press.
Lee, S. 1995. Comparison of population characteristics of three species of shrews and the shrew - mole in habitats with different amounts of coarse woody debris. Acta Theriologica, 40 (4): 415-424.
Rhoades, F. 1986. Small mammal mycophagy near woody debris accumulations in the Stehekin River Valley, Washingtonl. Northwest Science, 60 (3): 150-153.
Smith, M., M. Belk. 17 May, 1996. Sorex monticolus. Mammalian species, No. 528: 1-5.
Willson, D., S. Ruff. 1999. The Smithsonian book of North American mammals. Washington and London: Smithsonian Institution Press.
Woodward, S. 1994. Identification of *Sorex monticolus* and *Sorex vagrans* from British Columbia. Northwest Science, 68 (4): 277-283.