Beaver - biology.

Beaver - biology. I INTRODUCTION Beaver, semiaquatic rodent noted for the building of dams. There are only two kinds of beavers, the American beaver and the Eurasian beaver. The two species share similar behavior and anatomy, although they have some small physical differences, such as the shape of the nasal bones. 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. II 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. III 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. IV 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. V 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. Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.