Weasel , common name for any of several small, fur-bearing, carnivorous mammals that are most abundant in North America and Europe but also occur in South America, northern Africa, and Asia. The various species of weasel are similar in appearance and habits. They are lithe, slender animals with elongated necks, muscular, snakelike bodies, and short legs. They vary from 13 to 40 cm (5 to 16 in) in length, and the male is larger than the female. Their heads are small and triangular with narrowed snouts. Nearly all weasels are brown above and white beneath, and those found in northern regions turn white in the winter (see Ermine). In warmer regions weasels retain their brown coat throughout the year. The weasel, which preys on mice, rats, birds, and rabbits, is largely nocturnal. Extremely agile, it attacks animals larger than itself. Although sometimes a serious threat to poultry, it also feeds on rodents around farms. When young, the weasel can be tamed. The female makes a nest of straw, leaves, and moss in a ground crevice or a hollow tree. A litter has two to twelve young. Among the best-known species are the ermine, known also as the stoat, and the longtail weasel, of the United States and Mexico. Close relatives are the mink, polecats, and ferrets. The black-footed ferret, the largest and most endangered weasel native to the American prairies, was nearly extinct until a captive breeding program was started. The success of reintroduction is not yet clear .