Weasels belong to the Mustelidae family, which also includes mink, martens, fishers, wolverines, badgers, river otters, black-footed ferrets, and four species of skunks. Although members of the weasel family vary in size and color , they usually have long, slender bodies, short legs, rounded ears, and anal scent glands.

A weasel’s hind legs are barely more than half as long as its body (base of head to base of tail). The weasel’s forelegs also are notably short. These short legs on a long, slender body may account for the long-tailed weasel’s (Mustela frenata) distinctive running gait. At every bound the long body loops upward, reminding one of an inch-worm.

In the typical bounding gait of the weasel, the hind feet register almost, if not exactly, in the front foot impressions, with the right front foot and hind feet lagging slightly behind. The stride distance normally is about 10 inches (25 cm).

Male weasels are distinctly larger than females. The long-tailed and short-tailed (M. erminea) weasels have a black tip on their tails, while the least weasel (M. nivalis) lacks the black tip

General Biology, Reproduction, and Behavior
Weasels are active in both winter and summer; they do not hibernate. Weasels are commonly thought to be nocturnal but evidence indicates they are more diurnal in summer than in winter.

Home range sizes vary with habitat, population density, season, sex, food availability, and species . The least weasel has the smallest home range. Males use 17 to 37 acres , females 3 to 10 acres . The short-tailed weasel is larger than the least weasel and has a larger home range. Male short-tailed weasels use an average of 84 acres , and females 18 acres , according to snow tracking.

The long-tailed weasel has a home range of 30 to 40 acres , and males have larger home ranges in summer than do females. The weasels appear to prefer hunting certain coverts with noticeable regularity but rarely cruise the same area on two consecutive nights.

Weasel population densities vary with season, food availability, and species. In favorable habitat, maximum densities of the least weasel may reach 65 per square mile (169/km2); the short-tailed weasel, 21 per square mile (54/km2); and the long-tailed weasel, 16 to 18 per square mile (40 to 47/km2). Population densities fluctuate considerably with year-to-year changes in small mammal abundance, and densities differ greatly among habitats. Weasels, like all mustelids, produce a pungent odor. When irritated, they discharge the odor, which can be detected at some distance .

Long-tailed weasels mate in late summer, mostly from July through August. Females are induced ovulators and will remain in heat for several weeks if they are not bred. There is a long delay in the implantation of the blastocyst in the uterus, and the young are born the following spring, after a gestation period averaging 280 days. Average litters consist of 6 young, but litters may include up to 9 young. The young are blind at birth and their eyes open in about 5 weeks. They mature rapidly and at 3 months of age the females are fully grown. Young females may become sexually mature in the summer of their birth year.

Habitat

Some authors report finding weasels only in places with abundant water, although small rodents, suitable as food, were more abundant in surrounding habitat. Weasels are commonly found along roadsides and around farm buildings. The absence of water to drink is thought to be a limiting factor .

A typical den has two surface openings about 2 feet (61 cm) apart over a burrow that is 3 to 10 feet (0.9 to 3 m) long. Other weasel dens have been found in the trunk of an old uprooted oak, in a bag of feathers, in a threshing machine, in the trunk of a hollow tree, in an old mole run, a gopher burrow, and a prairie dog burrow.