The beaver (Castor) is the only genus of the family Castoridae. The family contains a single genus, Castor, with two species: Castor canadensis, found in the New World, and Castor fibre, found in the Old World. Both species are semi-aquatic rodents noted for the building of dams. The two species differ chiefly in the shape of the nasal bones and are so much alike that some authorities consider them to be varieties of the same species. They are large rodents; the average adult beaver weighs about 16 kg, but specimens as heavy as 40 kg have been found, and some extinct beavers were almost bear- like in size. The beaver is usually about 76 centimetres long and stands less than 30 centimetres high, with a broad, flat, scaly tail about 25 centimetres 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 greyish 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, enamelled chisel edge. With these the beaver can cut down large trees. It usually selects trees five to 20 centimetres in diameter, but it can fell trees with diameters as large as 76 cm.
Beavers have a pair of anal scent glands, called castors, that secrete a musk-like 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.
Beavers are social animals and in areas where food is abundant and the locality secluded, the number of families in a beaver community can be 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. The room inside is carpeted with bark, grass, and wood chips, sometimes with special store rooms 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 lakelodge is built on the shelving shores of lakes. Dams are used by beavers to widen the area and increase the depth of water around their homes and 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 meters high, usually more than three meters wide at the base, and narrow at the top. A beaver dam more than 300 meters 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 one meter wide and deep and often a few hundred meters long. The timber is then readily floated down the canal toward the pond.
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 almost extinct in Europe, but is becoming re-established in Canada and in protected areas of the USA. Research Beaver
Abbreviations
Nature
Research Beaver