Castor canadensis (Kuhl 1820)

The largest North American rodent and the only one with a broad, flat, scaly tail, the Beaver is now common and widespread, even in areas it did not inhabit during pre-colonial times. The modifications it makes to the environment by felling trees and building dams result in changes to plant, animal, and microbial communities that are sometimes desirable and sometimes not. The Beaver itself is not easily seen, being nocturnal and secretive, but it can be spotted in ponds, lakes, or large streams at twilight by a quiet observer. Its pelage is brown, with gray underfur, and is prized by trappers. The webbing on its hind feet help it to swim; claws on the digits of its forefeet give it dexterity in handling food; comblike claws on its hind feet help it in careful grooming; and it can close its mouth behind its front teeth, so that it can carry woody material without taking in water. Beavers cache and consume the inner bark of both deciduous and evergreen shrubs and trees, as well as terrestrial and aquatic plants. Their young, called kits, leave the colony at the age of six months.

Adaptation: The shape of the Beaver skull, and the rough texture of its bone, is evidence of a powerful set of chewing muscles. The groove-like depression at the back of the snout and the round one near the jaw joint are areas where large muscles attach. The ridge of bone that marks the curvature below the back angle of the mandible is another muscle attachment site.

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Length: 117 cm

Weight: 27000 grams

Mating is monogamous. Outbreeding is the rule, with rare parent-progeny mating; matings between parent and offspring apparently occur only when a family unit is disrupted by the death of one adult (Taylor 1970, Svendsen 1980). In Ohio, Svendsen (1989) found that 56% of all pairs were formed in September, October, and November. Pair-bonds tended to last longer in areas with more stable conditions (lakes) than they did in comparatively unstable sites (streams).

Breeding (i.e., copulation) occurs January-March in the middle part of the range, mainly February-March in cold northern areas, mid-February in Newfoundland (Bergerud and Miller 1977), and over a longer season (late October-March) in the south (near the winter soltice in Mississippi; Wigley et al. 1983).

Gestation is thought to be 105-107 days, though also reported as about 128 days (Miller 1983) or around 100 days (Bergerud and Miller 1977). Parturition occurs in April, May, or June in Missouri (Schwartz and Schwartz 1981), April or May in Texas, generally late March and early April in Mississippi (Wigley et al. 1983), late May or early June in Newfoundland (Bergerud and Miller 1977), and June in Saskatchewan (Gunson 1970). In Ohio, Svendsen (1980) first heard the whining of kits in lodges in June.

Litter size ranges from one to nine, with three or four being typical in many areas. Factors influencing litter size include food supply, growing season, female size and age, and harvest rate. A female beaver produces one litter per year. Weaning occurs at an age of about six weeks and a weight of 4 pounds (1.8 kg).

In most cases, the young disperse from their family group in late winter or early spring, at an age of almost two years, before the new kits are born, or dispersal may occur later in summer. Some authors state that dispersal occurs at the end of the first year. Dispersal occurs over land and via waterways (Leege 1968). Apparently the young may remain in the family group longer than two years in high quality habitats and/or in habitats that are saturated with beavers (Boyce 1974, Gunson 1970, Bergerud and Miller 1977, Novakowski 1965). Dispersers often move to another area and begin a new pond. Sometimes they may return to their natal site (Svendsen 1980, Ryden 1988).

Survival of the young can be quite high in untrapped populations. Svendsen (1980) found that survival of the kits through their second summer was 95% (based on cohorts that lived long enough to emerge from their natal lodge). Some other studies also found that losses in the first year were very low, but other research indicates higher mortality rates (see Novak 1987a).

Compared to other rodents, beavers attain sexual maturity at a relatively late age. Females normally first give birth on or near their third birthday, and may remain productive for up to at least ten years (Stegeman 1954), though only a few live that long. Sometimes females breed when one (rarely) or two years old, though this is rare at the northern and southern range limits (Hill 1982). Males generally first breed at an age of about 21 months, though a variable proportion of yearlings may breed and sometimes older males may be functionally sterile.

Dimorfismo Sexual: Ninguno

Longitud:
Rango: 1,000-1,200 mm

Peso:
Rango: 16-30 Kg.