The masked shrew, Sorex cinereus, is widely distributed, occurring in Alaska, most of Canada, and much of the northern United States. In Utah, the species occurs primarily in mountainous areas, where it is most often found in moist habitats.
The masked shrew is a generalist feeder, eating insects, worms, other invertebrates, small vertebrates, and even seeds. Because of its high metabolic rate, the masked shrew must consume a great deal of food, often eating more than its body weight in food each day. The species mates from late spring to early fall. Gestation lasts approximately eighteen days, and females give birth to two or three litters of approximately seven young each year. The masked shrew is active both day and night throughout the year.