Masked shrews are the second smallest shrew species in North America, pygmy shrews are slightly smaller. Male and female masked shrews are about the same size and color. The fur of their back is almost uniformly brown and their underparts are greyish-white. Their tail is brown above and lighter below, with a black tip. Fur color tends to be darker in the winter. Tail length averages 39.9mm, almost half of their total body length. Average body length of adults is 99 mm. Young are born hairless and with fused eyelids, they weigh from 0.2 to 0.3 grams and are 15 to 17 mm long including a 3 mm long tail.
Masked shrews are the most widely distributed shrew in North American. They are found throughout the northern United States, most of Canada, and Alaska. They do not occur on Vancouver Island, the Queen Charlotte Islands, in tundra habitats, on arctic islands, or in extreme northern Quebec.
Masked shrews occupy a wide variety of habitats. They are found in open and closed forests, meadows, along river banks, lake shores, and willow thickets. Preferred habitats are always close to water and the largest numbers of masked shrews can be found in moist environments. Masked shrews also do well in disturbed habitats such as those disturbed by fire or logging. The average home range size is 0.6 hectares.
Masked shrews probably do not live much past 1 to 2 years old, most probably die before reaching adulthood.
Masked shrews are most active after dark, when 85 per cent of activity occurs. They are especially active when there has been a rainfall or on very dark nights. Their primary activity is hunting. Masked shrews hunt mainly on the ground but may also climb into low vegetation and shrubs. They run quickly, can jump to 10-15 cm high, and dig in loose soils.
Little is known of communication in masked shrews. They have an excellent sense of smell and can see fairly well. They use their sensitive whiskers to find their way around and detect prey. Masked shrews also probably squeak and hiss as a way of communicating.
Because masked shrews inhabit a wide range there is a lot of variability in their diet. Ants represent 50% of the food source for masked shrews in Michigan. In other areas they eat mostly insect larvae. In general, masked shrews consume a variety of invertebrates including insect larvae, ants, beetles, crickets, grasshoppers, spiders, harvestmen, centipedes, slugs, and snails. They are also important predators of forest insect pests such as jack pine budworms and larch sawflies. Seeds and fungi are also eaten occasionally.
Masked shrews avoid being preyed upon by staying under cover and being active mostly at night, they are rarely seen.
Masked shrews can be very abundant in the communities in which they live. They can have a dramatic impact on insect communities because they have to consume such large quantities of insects. They are also important prey items for many small predators.
The negative impacts of masked shrews are unknown. They may affect populations of some beneficial organisms or inhibit reproduction of some plants by consuming seeds.
The extent to which masked shrew populations affect humans is unknown. However, they have a significant impact on populations of insect pests and are important members of communities.
Masked shrews are common and widespread.
Wendy Lee (author), University of Toronto.
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