I.

Introduction

Beaver, semi-aquatic mammal noted for the building of dams. One species of beaver occurs in Eurasia, the other in North America. 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; an average adult beaver weighs about 16 kg (35 lb), but specimens as heavy as 40 kg (90 lb) have been found, and some extinct beavers were almost as large as bears.

II.

Characteristics

The beaver is usually about 76 cm (30 in) long and stands less than 30 cm (12 in) high. The broad, flat, scaly tail is about 25 cm (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 for propulsion 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 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 orange-yellow front teeth in each 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 5 to 20 cm (2 to 8 in) in diameter, but it can fell trees with diameters as large as 76 cm (30 in). They feed mainly on plants and tree leaves, although in winter their diet comprises branches that were taken in the summer and stored underwater. 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 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, although not all beavers build them. 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 (39 in) high. The floor is carpeted with bark, grass, and wood chips, sometimes with special storerooms adjoining. The pond lodge is built either a short distance 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 logs or more firmly and solidly of mud, brushwood, and stones. As time goes by the beaver repairs and adds to the dam. Floating material that becomes caught in the dam, and the roots of vegetation that grows on the top both strengthen the whole structure. 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 (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 (1,000 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 to control run-off 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 is awkward on land and has difficulty dragging 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 metres long. The timber is then readily floated down the canal towards the pond. Some observations and experiments suggest that dam construction is the beaver's response to the sound of running water.

V.

Beavers and Human Beings

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—mainly for making hats. The animals were also sometimes destroyed because of the damage they did to forests and because of the flooding occasionally caused by dams. Ceaseless slaughter led to near extinction of beavers in both Europe and North America. The beaver is being reintroduced and is becoming re-established 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.

Scientific classification: Beavers make up the family Castoridae, in the rodent order. The Eurasian species is classified as Castor fiber and the North American species as Castor canadensis.