Characteristics

                        The beaver is usually about 76 cm (about 30 in) long. It stands less than 30 cm (12
                        in) high, and the broad, flat, scaly tail is about 25 cm (about 10 in) long. The body is
                        plump, the back arched, the neck thick, the hind feet webbed, and all the digits
                        clawed. The fur is usually reddish-brown above and lighter or grayish below. The
                        eyes are small and the nostrils closable. The skull is massive, with marked ridges
                        for fixing the muscles that work the jaws. The two front teeth on either jaw are like
                        those of other rodents, wearing away more rapidly behind so as to leave a sharp,
                        enameled chisel edge. With these the beaver can cut down large trees. It usually
                        selects trees 5 to 20 cm (2 to 8 in) in diameter, but it can fell trees with diameters
                        as large as 76 cm (30 in). Beavers have a pair of anal scent glands, called castors,
                        that secrete a musklike substance called castoreum, probably for marking
                        territories. The animals tend to be monogamous and have a life span of 20 years or
                        more. The female has one litter a year, usually of two to four young. 

                        The Lodge

                        Beavers are social animals. In areas where food is abundant and the locality
                        secluded, the number of families in a beaver community is rather large. The
                        so-called beaver lodge is a unique structure. Three distinct kinds exist, their
                        differences depending on whether they are built on islands, on the banks of ponds,
                        or on the shores of lakes. The island lodge consists of a central chamber, with its
                        floor a little above the level of the water, and with two entrances. One of these, the
                        "wood entrance," is a straight incline rising from the water, opening into the floor of
                        the hut. The other approach, the "beaver entrance," is more abrupt in its descent to
                        the water. The lodge itself is an oven-shaped house of sticks, grass, and moss,
                        woven together and plastered with mud, increasing gradually in size with year after
                        year of repair and elaboration. The room inside may measure 2.4 m (8 ft) wide and
                        up to 1 m (3 ft) high. The floor is carpeted with bark, grass, and wood chips,
                        sometimes with special storerooms adjoining. The pond lodge is built either a short
                        way back from the edge of the bank, or partly hanging over it, with the front wall built
                        up from the bottom of the pond. The lake lodge is built on the shelving shores of
                        lakes. 

                        The Dam

                        The dams used by beavers to widen the area and increase the depth of water
                        around their homes are constructed either of sticks and poles or more firmly and
                        solidly of mud, brushwood, and stones. As time goes by the beaver repairs and
                        adds to the dam. Floating material lodges there, and vegetation growing on the top
                        adds its roots to the strength of the dam. Frequently the beaver builds a smaller
                        dam downstream in order to back up some water against the original dam and thus
                        decrease the pressure of water on it from the other side. The dams are about 1.5 m
                        (about 5 ft) high, usually more than 3 m (10 ft) wide at the base, and narrow at the
                        top. A beaver dam more than 300 m (1000 ft) long was found in Rocky Mountain
                        National Park, Colorado. Beaver ponds eventually fill with sediment, and the animals
                        move to a new location. The abandoned area becomes good meadowland. Beaver
                        dams also help control runoff. 

                        Although the beaver is a powerful swimmer, it has difficulty dragging over the ground
                        the logs and branches it needs for building and for food. Colonies of beavers
                        therefore often dig canals from the pond to a grove of trees. Such canals are up to 1
                        m (3 ft) wide and deep and often a few hundred meters long. The timber is then
                        readily floated down the canal toward the pond. 

                        Beavers and Humans

                        Beavers have long been exploited for their fur, and for many years during the 18th
                        and 19th centuries hundreds of thousands of beaver skins were exported to Europe
                        from North America annually. The animals were also sometimes destroyed because
                        of the damage they did to forests and the flooding occasionally caused by dams.
                        Ceaseless slaughter led to near extinction of beavers in both Europe and North
                        America. The beaver is still almost extinct in Europe, but is becoming reestablished
                        in Canada and in protected areas of the U.S.