The masked shrew is the most widely distributed shrew in North America, ranging over almost all of the continent’s northern half. It occurs throughout Pennsylvania. Overall length, 3.3 to 4.3 inches; tail, 1.4 to 1.8 inches; weight, 0.12 to 0.2 ounces (less than a dime).
Masked shrews molt twice a year. In winter, they are dark brown to almost black on their upperparts, lighter brown or grayish on their under parts. Summer coloration is lighter and browner. Sorex cinereus closely resembles the slightly smaller pygmy shrew.
Masked shrews inhabit wooded areas, living under rocks, logs, and in the leaf litter, often in swamps or along stream banks or spring runs. Rarely found in dry fields, they occasionally inhabit hedgerows and stone walls in open country. Masked shrews spend most of their lives in underground runways they construct, or in the tunnels of mice or other small mammals.
Masked shrews sometimes climb into low bushes or fallen trees. They are good swimmers but rarely enter the water. Their ability to see and smell are poor, but their sense of touch is well-developed. Masked shrews eat insects, worms, centipedes, slugs, snails, mollusks and spiders, vegetable matter such as moss and seeds, and carrion. They probably do not store food. One observer reported that a captive ate over three times its body weight daily.
The species nests under logs, stumps or rocks, in fist sized nests of leaves, grass and fine rootlets. Breeding runs from March to September. After an 18-day gestation, 2 to 10 (usually about 7) young are born. They are blind and helpless but grow quickly and are on their own after about a month. They mature sexually at 20 to 26 weeks of age. An adult female may raise three litters; the male remains with the family during the early life of the young. Masked shrews are active day and night, but especially at dusk. An individual’s heart beats 1,200 times per minute, evidencing its rapid metabolism. Owls, hawks, herons, shrikes, weasels, foxes, cats and the larger shrews kill masked shrews, few of which reach their maximum lifespan of about 18 months.