BEAVER

(Castor), any member of the aquatic rodent family Castoridae (order Rodentia), highly prized for its fur and well known for its dam-building activities. Beavers are thickset animals with small, rounded ears, short legs, and large, webbed hind feet. They may grow to about 1.3 metres (4 feet) long, including the flat, scaly, 0.3-m tail, and may weigh more than 27 kilograms (60 pounds). Both sexes possess musk glands that produce a liquid, castoreum, used in perfumes. The coat, consisting of a dense, fine underfur overlaid with many coarse guard hairs, is glossy tan to dark brown above, paler below. The search for this fur stimulated some of the early 19th-century explorations of western North America; the pelts were a standard medium of exchange. New World beavers (C. canadensis) once ranged from northern Mexico to the Arctic. Extensive trapping has reduced their numbers. Old World beavers (C. fiber) are now found only in the Elbe and Rhne drainages of Europe. Both species are now protected. Beavers show preference for streams and small rivers but also live around the margins of forest-edged lakes. Their dams of sticks, stones, and mud may last for years, impounding pools that sometimes cover many acres. Eventually, silt fills the ponds and produces meadows. Beavers build dome-shaped island lodges of sticks plastered with mud. The interior may be 1.8 m high. The mud freezes in winter, and the lodge becomes impregnable to predators; one or more tunnel entrances open below the ice level. In rivers and lakes, beavers often burrow into banks. Food usually consists of the tender bark and buds of trees. Saplings and even large trees are felled by gnawing, cut into portable lengths, and dragged or floated through beaver-made canals to the pond. Branches, twigs, and small logs are anchored in the bottom mud in deep water for winter food. Beavers live in colonies, one or more family groups to a lodge. A family usually consists of a mated pair and two sets of offspring. Breeding takes place in midwinter, and two to eight (usually four) young, or kits, are born four months later. Of placid disposition, beavers often work cooperatively. When alarmed, they immediately seek safety in the water. They warn others of danger by slapping the surface of the water with their tails.