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 were also introduced, for the fur trade, into Tierra del Fuego, where they have become an invasive pest and caused significant ecological damage.

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.

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