Animal Field Guide

Beaver

Castor canadensis
(Castoridae)

Global Rank: G5
State Rank: S5

Agency Status
USFWS:
USFS: none
BLM: none

The largest rodent in North America north of Panama. On land is a large, clumsy, hump-backed animal. In the water, becomes sleek and torpedo-shaped. Propels itself with powerful webbed hind feet. Beavers use their large dorsally flattened, scale-covered tail to maneuver in water. Slapping the tail on water surface is used as a signal of alarm. Split nail on the second hind toe is used for grooming. Incisors are large and continually growing. Fur is rich brown with black to reddish guard hairs. Under-fur is soft and extremely dense with excellent insulating qualities. Both sexes have a pair of anal glands and castor sacs located ventrally. Beavers emit anal gland secretions year round. Total length: 34 to 40 inches. Weight: 30 to 60 pounds. Builds stick and mud dams across streams. Mostly nocturnal. May build large conical houses at the edge of a lake or burrow into the bank for a den along rivers. Beaver life is based on a family unit consisting of a pair of adults, yearlings, and kits. (FWP). The information in next section is from Miller (1983), Hill (1982), and Novak (1987a), unless otherwise indicated. The beaver is the largest North American rodent; most adults weigh 16-23 kg (35-50 pounds), rarely 32-38 kg (70-85 pounds) or more. Kits weigh about 0.5 kg or a little less (1 pound) at birth (Hill 1982), when they are about 38 cm (15 inches) long (Schwartz and Schwartz 1981). The relationship between beaver age and mass is roughly as follows: < 0.5 years old, < 5 kg (11 pounds); 0.5-1.5 years old, 6-11 kg (13-24 pounds); 1.5-2.5 years old, 10- 13 kg (22-29 pounds); > 2.5 years old, > 14 kg (31 pounds). The preceding weight-age data are often accurate, but it must be realized that weight is influenced by various factors other than age. The total length of a beaver varies with age as follows: yearlings are 26-34 inches; adults of 2-3 years, 35-40 inches; older adults, 47+ inches (maximum about 4.5 feet). in the warmer months and generally ends at age 4-5 years. Pelage color varies geographically from yellowish-brown to black (this range of color may be found in a single watershed). There is one annual molt. The pelt is prime from late fall to early spring. The sexes are difficult to distinguish externally. Hodgen (1978) provided information on sexing beavers by their behavior in a live trap. See Novak (1987a) for additional references on sexing methods.

River otter - tail covered with fur. muskrat - smaller, tail slender (fwp). an inexperienced observer could mistake a muskrat (ondatra zibethicus), round-tailed muskrat (neofiber alleni) or a nutria (myocastor coypus) for a beaver, but these other rodents do not have a broad flattened tail.

Problems posed by the beaver are several. Hill (1982) and Miller (1983) listed the following as especially important: flooding of roads, fields, pastures, home sites, and septic systems; damage to timber by flooding and cutting (most serious in the southeastern U.S. where the land is relatively flat); damage to dikes, ditches, and dams (mainly in the arid western U.S. where gravity-flow irrigation systems are used); blockage of culverts and drain pipes to such an extent that they must be dynamited or otherwise removed and replaced; damage to high-value trees; and degradation of trout waters (northeastern and northcentral U.S.). Though implicated in contaminating public drinking water supplies with the pathogen Giardia lamblia, recent research has indicated that this is unlikely (Woo and Paterson 1986, cited by de Almeida 1987). The beaver is an important fur bearer. The annual harvest of beavers from trapping is about 270,000 in the U.S., and 400,000 in Canada. In 1990, the state of New York Department of Environmental Conservation received 1721 complaints about beavers (up from 706 in 1975 when beaver populations were much lower) and in recent years annually expended $180,000 or more (almost 75% of beaver management costs) in resolving those complaints. Along the Platte River Greenway in Denver, Colorado, beavers reportedly annually have caused $150,000-200,000 in damage to trees and landscaping. Likewise, other states are faced with similar situations. Novak (1987) concluded that the economic losses caused by beavers far surpass the value of harvested pelts (though of course beavers and their habitats have substantial economic benefits other than pelts).

Occupies a wide variety of habitats in North America. Water and associated woody vegetation are the most essential components of beaver habitat. Ideal beaver living sites include ponds, small lakes, meandering streams, and rivers.

Beavers are strict herbivores. Beavers eat a variety of woody and herbaceous species. Willows, mountain alder, and aspen are important foods. Will also consume herbaceous vegetation during summer. Much of the woody vegetation beavers cut is not used for food. Caches woody vegetation near shore for winter food.

Breeds from January through March. Two to four young are born after a 105- to 107-day gestation period. One litter per year is produced. Two-year-olds are eventually driven from or leave the parental home.