sewellel beaver: see mountain beaver.

mountain beaver, stout, short-limbed North American rodent, Aplodontia rufa, not closely related to the true beaver. Also called sewellel beaver after the Chinook word for a robe made from its pelts, it is among the most primitive of the rodents and the only living member of its family. The mountain beaver is about 12 in. (30.5 cm) long, grayish or brownish red in color, and nearly tailless. With small eyes and ears and a blunted muzzle, it resembles a tailless muskrat. Its enlarged claws make it an excellent burrower, and it is also a good swimmer and tree climber. Generally nocturnal, the mountain beaver is found along the Pacific coast from British Columbia to California. Inhabiting damp, wooded country near streams, the rodent eats bark, leaves, and twigs. It builds complex colonial burrows with chambers for food storage, sleeping, and nesting. The location of mountain beavers is in part explained by the fact that they cannot adequately regulate the temperature of their bodies and must therefore live in stable, cool, moist environments. Mountain beavers are classified in the phylum Chordata, subphylum Vertebrata, class Mammalia, order Rodentia, family Aplodontidae.

beaver fever: see giardiasis.

beaver, either of two large aquatic rodents, Castor fiber and Castor canadensis, known for their engineering feats. They were once widespread in N and central Eurasia except E Siberia, and in North America from the arctic tree line to the S United States. The mountain beaver of W North America is not a true beaver, but a nonaquatic rodent of a different family.

The beaver is the largest living rodent except the capybara, and is distinguished by its extremely broad, horizontally flattened tail. Beavers are 3 to 4 ft (91-120 cm) long, including the tail (12 in./30.5 cm long, 6 in./15.2 cm wide), and about 15 in. (38 cm) high at the shoulder; they usually weigh about 60 lb (27 kg). Their long, dense fur is reddish brown to nearly black; the naked, scaly tail is black. Both sexes have scent glands located in a pouch in the anal region. The musky secretion, castoreum, which may function as a sexual attractant, was once believed to have medicinal properties, and the glands, or castors, were of commercial value. Beavers have been extensively trapped for their pelts, once considered the most valuable of furs, and were exterminated over a large part of their range. Because of their great importance in maintaining the natural environment, they have been reintroduced in many areas of North America and Russia and are now increasing in numbers.

Beavers build lodges up to 3 ft (91 cm) high and 5 ft (1.5 m) wide of sticks and mud; the entrances are below water level, with ramps leading to the living quarters, located on a platform above water level. They may also build burrows in banks with underwater entrances. They create deep ponds, or maintain the water level in old ones, by building dams across streams. These are made of sticks and logs, and the upper surfaces are reinforced with stones and mud. Materials are gathered by collecting wood and felling small trees by gnawing; often the beavers dig canals for floating these to the right spot. Most, if not all, of these activities are done mechanically, as a result of instinct; captive animals persist in building useless dams, and even in the wild beavers will attempt to reinforce solid, manmade dams with sticks.

Although they form monogamous families and live in colonies, there is little social contact among beavers and they work independently. A colony consists of a cluster of lodges, each occupied by a family of the parents and their last two litters. The beavers sleep by day and spend the night foraging for food and building or repairing their structures. They feed on a variety of aquatic and shore plants, surviving in winter largely on bark. Sticks for winter food are stored in the lodges and underwater. Excellent swimmers, they can stay underwater for up to fifteen minutes. When alarmed, a beaver slaps the water with its tail, making a loud noise that sends other beavers hurrying to the safety of deep water. Females give birth to two to eight young in the spring; these mature in two years. Beavers are responsible for creating many of the woodland ponds that support lush vegetation and eventually become meadows.

Classification

Beavers are classified in the phylum Chordata, subphylum Vertebrata, class Mammalia, order Rodentia, family Castoridae.

Beaver Island, 14 mi (23 km) long, from 3 to 6 mi (4.8-9.6 km) wide, off N Mich., in Lake Michigan. It is the largest island of the Beaver Archipelago and has forests, lakes, beaches, and a harbor at St. James village. The island's permanent inhabitants are mostly fishermen. James J. Strang had a Mormon settlement there from 1847 to 1856.

Beaver Falls, city (1990 pop. 10,687), Beaver co., W Pa., on falls of the Beaver River near its junction with the Ohio; settled c.1793, inc. 1868. A steel center in an area of coal mines, natural gas deposits, and clay pits, it is known for its cold-drawn steel. The plates for U.S. currency are manufactured there. The city was founded on a Native American trail that later became a pioneer road. Geneva College is there.

Beaver Dam, city (1990 pop. 14,196), Dodge co., SE Wis., on Beaver Dam Lake, in a productive farm and dairy area; inc. 1856. Industries included food processing, metal and metal products fabrication, printing, and machinery manufacturing.

Either species of the aquatic rodent family Castoridae (genus Castor), both of which are well known for building dams. Beavers are heavyset and have short legs and large, webbed hind feet. They grow as large as 4 ft (1.3 m) long, including the 1-ft (30-cm) tail, and as heavy as 66 lb (30 kg). Beavers build their dams of sticks, stones, and mud in small rivers, streams, and lakes, often producing sizable ponds. With their powerful jaws and large teeth, they can fell medium-size trees, whose branches they use in their dams and whose tender bark and buds they eat. One or more family groups share a dome-shaped stick-and-mud lodge built in the water, with tunnel entrances below water level. American beavers (C. canadensis) range from northern Mexico to the Arctic. Their prized pelts stimulated the exploration of western North America, and by 1900 beavers were trapped to near extinction. Eurasian beavers (C. fiber) are now found in only a few locations, including the Elbe and Rhône drainages of Europe. The mountain beaver of the Pacific Northwest is unrelated.

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