The least weasel is the world’s smallest carnivore. It is found in Europe, northern Asia and North America. On our continent it inhabits the Appalachian Mountains from Pennsylvania south to North Carolina, the northern Midwest, Canada and Alaska (it’s absent in New England and the Pacific Northwest). In Pennsylvania, Mustela nivalis is most common in the Allegheny Plateau area of our northwest and in the southcentral part of the state.
Least weasels are 6 to 8½ inches long, including a 1½ inch tail. They weigh 1 to 2 ounces. Males are slightly larger and heavier than females. Coloration is brown above, white below. The chin and feet are white, and the brown tail has no black tip. Sexes are colored alike. In Pennsylvania, some least weasels turn white in winter; in Canada, most or all individuals change into white pelage, including the tip of the tail.
Least weasels are just as aggressive and predatory as the larger weasels and kill in the same manner. If disturbed near its nest, an adult least weasel will chirp at its enemy. The chirp is a threat cry; least weasels also hiss (when afraid or threatened) and trill (in friendly encounters with other least weasels). When agitated, they spray musk from their anal scent glands.
The species preys on mice, voles, small birds, insects, earthworms and small amphibians. Sometimes they kill more than they can eat and cache uneaten prey in their dens. Least weasels are nocturnal, solitary and are seldom seen; they spend most of their time hunting and consume food equaling 40 to 50 percent of their body weight each day.
Least weasels breed and reproduce year-round, with the possible exception of winter. Delayed implantation does not occur, and two or more litters may be produced each year. A female's estrus lasts four days. If bred, she bears 1 to 6 young (usually 4 to 5) following a 35-day gestation period.
Least weasels inhabit meadows, fields, brushy land, or woods. They may take over nests and burrows of mice, moles and voles, lining them with fine grass or fur; in winter, the fur lining may be an inch thick and matted like felt. Least weasels rarely travel more than 100 yards from their home burrows, and the average individual range is estimated at two acres.
This tiny weasel occupies a lower position in the food chain than ermines and long-tailed weasels. It is preyed on by the larger weasels, snakes, owls and cats. Longevity in the wild is not known.