Beaver, semiaquatic mammal noted for the building of dams. One species of
beaver occurs in North America, the other in Eurasia. 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 (about 35 lb), but
specimens as heavy as 40 kg (90 lb) have been found, and some extinct beavers
were almost bearlike in size.
CHARACTERISTICS
The beaver is usually about 76 cm (about 30 in) long and stands less than 30 cm
(12 in) high. The broad, flat, scaly tail is about 25 cm (about 10 in) long and
serves as a warning signal when slapped against the water, as a support when the
beaver is standing on its hind legs, and as a rudder while swimming. 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 may live
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 attract fish, ducks, and
other aquatic animals. Although the dams cause local flooding, they also help
control runoff and reduce flooding downstream. The ponds eventually fill with
sediment, and the animals move to a new location. The abandoned area becomes
good meadowland.
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. Some
observations and experiments suggest that dam construction is the beaver’s
response to the sound of running water.
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 United States. Beavers are
sometimes viewed as pests, particularly in suburban areas of the eastern United
States. Several states have limited trapping seasons.
Scientific classification: Beavers make up the family Castoridae, in the
rodent order. The North American species is classified as Castor canadensis. The
Eurasian species is classified as Castor fiber.
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