True weasels belong to the genus Mustela, with species found in Eurasia, N Africa, and the Americas. Weasels are very active and chiefly terrestrial but are able to climb trees. They prey on small animals by night, often killing more than they eat, and spend the day in dens made in holes in the ground, rock piles, or hollow stumps. Although they are notorious for destruction of poultry, the damage they do is far outweighed by their value as destroyers of rodents. Weasels are usually brown, with white underparts.
Species living in snowy regions acquire white coats in winter and are then known as ermine .
The most widely distributed weasel, Mustela erminea, is known in Europe as the stoat and in North America as the short-tailed weasel. It is about 16 in. It ranges from the Arctic Ocean to central Asia, S Europe and the central United States. The much smaller M. nivalis, known in Europe simply as weasel, is found in Europe, N and central Asia, and N Africa. It turns white only in the extreme northern parts of its range.