Castor canadensis
(American Beaver)

Order: Rodentia
Order Description:Rodents
Family: Castoridae
Family Description: Beavers

The beaver is our largest rodent. It is typically dark, reddish brown but varies from a yellowish brown to almost black. They have a very distinctive flattened, scaly tail, which is a fat storage area. Their hind feet are webbed and clawed, and their front feet are smaller and not webbed. Their aquatic habits are accommodated by valves that can close off the ears and nose when underwater, and a clear membrane that closes over the eyes. Their lips seal behind their incisors, which allows them to gnaw wood underwater. Their dense underfur is grayish-brown, and they have long, reddish brown guard hairs. Total length is 40 to 52 inches (1000-1350 mm), tail length is 10.4 to 13 inches (280-325 mm), and they weigh 22 to 75 pounds (10-35 kg).

Following local extinction because of beaver trapping during the 1800's, conservation has allowed their populations to increase. Presently they are widely distributed through North America north of Mexico, excluding Florida, southern California, and southern Nevada. They are common throughout much of Idaho where ever there are streams, ponds, lakes and even along rivers.

Beavers are dependent on slow-flowing brooks, streams, and rivers for dam construction, but they do also live in small lakes and fairly large rivers in Idaho. They prefer adjacent stands of successional growth trees such as aspen, willow and cottonwood rather than mature forests. Beavers are highly capable of altering their own environment by damming streams and other flowing water to create ponds

They prefer bark of deciduous Click word for definitiontrees such as aspen, willow, birch and even maple as well as various woody shrubs. Willow and aspen seem to be their favorites. They also eat roots of tuberous aquatic plants, especially pond lilies which they utilize more during summer months. They do not eat fish or other aquatic animals, which is contrary to what many people think.

Beavers dam flowing water to create deeper, water held in ponds. Their dams consist of a mixture of tree trunks, branches and mud and can be as high as 6 to 8 feet and in extreme cases up to ¼ mile long. Most dams are much smaller. Once their dams are constructed, the water deepens and provide safety for them as well as a place to construct their lodges and store food for the winter. They build two types of lodges, those that are constructed in open water of ponds and bank lodges typically along the banks of larger lakes and rivers. Their lodges consist of hollow mounds of sticks and mud with two or more underwater entrances. Inside they build a resting platform above the water level where they rest, feed and have their young. Bank lodges have tunnels beginning underwater and extending up into higher banks along ponds and rivers. The entrance areas are usually covered by large mounds of sticks and mud on the bank. The sticks and mud provide good insulation for their lodges during the winter. Beavers move onto land to cut woody vegetation both for building materials and for food. Beavers can energetically afford to go only so far for their sources of food on land. Beyond a certain point they are too vulnerable to predation as they are too far from the safety of their pond, or they are expending too much energy getting the wood they may cut down to their pond. In effect, they can eat themselves out of "house and home". Thus, the length of time a colony site is occupied depends in part on rate of replenishment of food resources. Over the years, siltation Click word for definitioncan also cause their ponds to get too shallow for their use, in which case they are forced to move on to other suitable habitat. Beavers are usually active from dusk to dawn cutting saplings and trees or shrubs to acquire food. Prior to the winter they store branches and twigs in underwater piles in their ponds. This food supply sustains them during the winter when their activity is reduced. They commonly move overland when searching for a mate or locating unoccupied water source. A colony typically consists of 4 to 8 related individuals (pair of adults plus offspring from current and previous year). Colony densities may reach 3 per 300 acres (121 ha); 0.6 to 0.9 per km2 has been reported in northern Minnesota. On rivers, population densities may range from 2 to 15 beavers per 2.6 km2. In an Idaho study, males and juveniles showed the greatest migration Click word for definitiontendency; a common migration Click word for definitionpattern was from upstream to downstream. Beavers can cause extensive flooding and considerable damage by cutting trees. However, they also provide great habitat for waterfowl, numerous song birds, and many small mammals who favor moist, riparian Click word for definitionhabitat. Here in Idaho, beavers have also been found to be helpful in maintaining good riparian Click word for definitionhabitat for grazing livestock. A colony of beavers is a social unit that helps all colony members survive. They have a well-known danger signal: a loud, resounding slap of their tail and all beavers in the colony instinctively dive. More than one colony can coexist in large ponds, and members do move from one family to another. Predators include wolves, bobcats, coyotes, and others while on land. Their genus name, Castor, is named after their paired anal scent glands with which they deposit "castor Click word for definition" or scent on scent stations around their ponds.

Beavers breed from mid-January to early June depending on whether they're in the south or northern part of their range. gestation Click word for definitionlasts 106 days which is much longer than for most rodents. parturition Click word for definition, or birth, occurs from April to-June in Idaho. litter Click word for definitionsize varies from 1 to 9 young, but the average is 3 to 5 (larger in north than in south). Young are weaned Click word for definitionin about 6 weeks. A female produces 1 litter per year. A pair can mate for life, and the male cares for young by maintaining the dams, gathering food and helping watch for predators. Young mature and disperse in 1.5 to 2 years.



Game species

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Important State References:
Leege, T.A. 1968. Natural movements of beavers in southeastern Idaho. J. Wildl. Manage. 32:973-976.

Information written by Donald Streubel,© 2000
Map image provided by
Stephen Burton,© 2000
Design by Ean Harker©1999, 2000.