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Masked Shrew
(Sorex cinereus Kerr)

From: Saunders, D. A. 1988. Adirondack Mammals. State University of New York, College of Environmental Science and Forestry. 216pp.

Order: Insectivora

Family: Soricidae

Description: The masked shrew displays characters typical of all shrews. The slender, cylindrical body has a short, velvety, directionless fur. The long, tapered head ends in a flexible, tubular snout, with the narrow nostrils located on the outer edges of the tip. The small external ears are barely visible or are completely hidden by the fur. The eyes are tiny, the tail scantily haired, and the limbs short. All are features adapting shrews to a semi-fossorial life between the interface of soil particles and plant debris.

The masked shrew has an arched brow and a rounded forehead. The upper parts are an ashy to cinnamon color in summer, darker brown in winter. The lower portions of the body are a lighter shade of the dorsal color. The relatively long tail is brown above and a paler buff below, the tip darker. The total length of an average-sized adult is 101 mm (4.0 in) which includes a tail about 41 mm (1.6 in) long. Adults weigh 3-6.5 g (0.11-0.23 oz.). The masked shrew is intermediate in size between the smaller, more slender pygmy shrew and the larger, stockier smoky shrew.

Range and Habitat: The masked shrew has the largest range of any North American shrew, and occurs throughout Alaska, Canada, the northern third of the U.S., as well as portions of the Great Basin, Rockies, and Appalachians. This shrew is found in all terrestrial habitats, natural or disturbed, from the lowest to highest elevations in the Adirondack Park, but is most abundant in cool, moist places within mixed forests, bogs, and swamp edges. The nest, made of plant fibers and 7.6-12.7 cm (3-5 in) in diameter, may be in decaying logs, stumps, the burrows of other mammals, or in the 19 mm (0.75 in) diameter tunnels the masked shrew digs in loose soil.

Food and Feeding Behavior: Insects comprise 65% of the diet, which also includes centipedes, spiders, earthworms, and carrion. Occasionally, masked shrews kill and consume small animals such as salamanders and nestling birds. Small, soft seeds may form part of the winter diet. A masked shrew requires large quantities of food each day, and one kept in captivity ate 3.3 times its own body weight in a 24 hour period. Merriam (p. 174) described three captive shrews kept in one cage, “....Almost immediately they commenced fighting, and in a few minutes one was slaughtered and eaten by the other two. Before night one of these killed and ate its only surviving companion, and its abdomen was much distended by the meal. Hence in less than 8 hours one of these tiny little beasts had attacked, overcome, and ravenously consumed two of its own species, each as large and heavy itself!” Starvation, brought on by insect populations diminished by autumn cold spells, may explain the dead masked shrews seen along woodland trails at this time of the year.

Activity and Movement: Masked shrews are active year round, even tunneling through snow to explore the surface. Although not strictly nocturnal, 85% of their activity takes place at night, peaking at 1:00-2:00 am. Clouds and rain increase nocturnal movement. While foraging, this species shows the jerky movements typical of shrews, and like other shrews, it keeps the tail extended out and curved upward while running. Individuals react to sudden disturbances by darting into the burrow or rooting under soil or leaf litter. Masked shrews can leap 15 cm (6 in), and although they tend to avoid water, can swim. They climb vines and shrubs which provide access to birds’ nests and arboreal insects.  

Reproduction: The breeding season extends from April to November. Females bear one to three litters of 2-10 young. The gestation period is 19-22 days. At birth the hairless young weigh 0.25 g (0.009 oz.) and are about 16 mm (0.6 in.) in length which includes a 3 mm (0.1 in.) tail. Fine hairs develop at 10 days. The eyes open at 17-18 days, and weaning begins at day 20, at which time each weigh about 3.5 g (0.12 oz). By day 30 the young disperse from the nest. Disturbance of the natal nest may elicit caravan behavior from older young, in which siblings move from the nest single file, each with its nose buried in the fur of the rump of a sibling. Although Eurasian shrews exhibit variations of this behavior, the masked shrew may be the only North American species to engage in it.

Young born in the spring often breed later in the same year. The average life span of a masked shrew is less than 14 months.

Predators: Larger shrews, owls, hawks, snakes, frogs, and fish are a few of their predators. Large mammals, e.g., foxes, weasel, bobcats, may kill and eat them, but often leave them on the ground, presumably because of the species’ strong odor.

Social Behavior:

  • Social system - The social behavior of all North American shrews is little known. A few causal observations of a male near a pregnant or lactating female suggest the masked shrew may be monogamous. Studies completes in other parts of the species’ range indicate a home range of 0.2- 0.6 ha (0.5-1.5 acre) which overlap conspecifics. Densities vary from 2.5-100 per ha (1-40 per acre) with 3-25 per ha (1-10 per acre) being more common, and may change dramatically from one year to the next within any particular area. 
  • Communication - Aggression between two opponents may lead to fighting or to “sparring” in which both opponents rest on their hind feet and push each other with their forefeet until one or both tumble over. Staccato squeaks accompany aggression. Other vocalizations include “faint twitters” during foraging, and teeth “gritting” while resting. Masked shrews produce ultrasonic sounds which may help them echolocate. Males have lateral skin glands that produce substances which may influence social behavior.

Additional References 

Blossum, P.M. 1932. A pair of long-tailed shrews (Sorex cinereus cinereus) in captivity. Journal of Mammalogy, 13:136-143.

Buckner, C.H. 1966. Populations and ecological relationships of shrews in tamarack bogs of southern Manitoba. Journal of Mammalogy, 47:181-194.

Doucet, G.J. and J.R. Bider. 1974. The effects of weather on the activity of the masked shrew. Journal of Mammalogy, 55:348-363.

Horvath, O. 1965. Arboreal predation on bird’s nest by masked shrew. Journal of Mammalogy, 46:495.

Kirkland, G.L., Jr. and D.F. Schmidt. 1982. Abundance, habitat, reproduction, and morphology of forest dwelling small mammals of Nova Scotia and southeastern New Brunswick. Canadian Field Naturalist, 96:156-162.


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