Castor canadensis

The information in this 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.

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.

Beavers inhabit permanent sources of water of almost any type in their range, which extends from arctic North America to the Gulf of Mexico and arid Southwest, and from sea level to over 3400 m in the mountains. They prefer low gradient streams (which they modify), ponds, and small mud-bottomed lakes with dammable outlets (Slough and Sadleir 1977, Beier and Barrett 1987, Novak 1987a, McComb et al. 1990). Beavers readily occupy artificial ponds, reservoirs, and canals if food is available. They generally avoid lakes with strong wave action and fast- moving streams. Waters with greatly fluctuating flow or water levels generally are poor habitat. In larger rivers (9th order or larger streams), beavers use floodplains and backwaters. In the north, they require water that is deep enough such that it does not freeze to the bottom and allows the accumulation of a substantial food pile beneath the ice. Beavers are associated with deciduous tree and shrub communities. In many situations beavers modify existing stream courses or other bodies of water and make them more suitable for their own use (see section below on dam building). They may occupy deep sections of streams and existing lakes or ponds without constructing a dam.

See Allen (1983) for a beaver habitat suitability index based on food and water characteristics. See Novak (1987a) for references to other models that have been developed in attempts to predict beaver family density.

Dam Construction Most dam building occurs April-June and especially August-October (Hill 1982), though repairs may be made at any time. In Quebec, the density of dams on small streams (4th order or less) averaged 10.6 per kilometer (range 8.6-16.0) (Naiman et al. 1986). In the northeastern U.S., a beaver family maintains an average of 2.5 dams. Generally trees, together with mud and rocks, supply the material for a dam.

Streams dammed by beavers typically are 2nd to 4th order streams; rarely do they build on 1st-order streams except at uncommonly high population densities. Dams in the main channel of streams of 5th order or larger often are destroyed by spring flooding (Naiman et al. 1986). In some areas (e.g. Missouri), dam building does not occur as commonly as in the north or west, perhaps due to the occurrence of deep swift rivers subject to severe flooding (Schwartz and Schwartz 1981).

Lodges

A characteristic beaver sign is the lodge, a hollow mound of tree branches (or, in the far north, mainly nonwoody material) and mud within which the beaver family sleeps and raises young. A lodge may be built against a bank, on an island, or in shallow water, but it must be near permanently deep water such that the entrance of the lodge is under water and so that the food cache can be reached even when the body of water is ice-covered. In bogs in northern Minnesota, beavers sometimes built lodges on the bog mat and many lodges were floating (Rebertus 1986). allowing for some ventilation. A family may maintain and use more than one lodge in summer in some areas (e.g., Colorado, Massachusetts). In some localities beavers use a different lodge each winter; it is more likely that the same lodge will be used if food in the area is abundant (Buech 1985). Beaver dens are not always in lodges. In some areas, beavers den in burrows in the banks of streams or lakes. This is common in areas subject to floods and fluctuating water levels. Beavers normally stay within 0.8 km of the den throughout the year (Schwartz and Schwartz 1981).

A keystone species that has profound effects on aquatic and riparian ecosystems (Naiman et al. 1986).

In boreal systems, may influence 20-40% of the length of 2nd- to 5th-order streams (Ford and Naiman 1988). Open patches created by beaver in New York and Wisconsin varied from less than 0.5 ha to 30 ha (Remillard et al. 1987, Dickinson 1971, Knudsen 1962), but averaged less than 4 ha.

Among the many changes that occur with beaver activity are the following (see Johnston and Naiman 1987 and Naiman et al. 1988):

1. Storage of precipitation and reduced discharge variability (Naiman et al. 1986, Hill 1982).

2. Increased depth and surface area of water (Hill 1982, Naiman et al. 1986).

3. Increase in open canopy (Naiman et al. 1986)).

4. Reduction of riparian deciduous trees (Beier and Barrett 1987, NYDEC 1991).

5. Enhancement or degradation of fish habitat (Neff 1957, Gard 1961, Hill 1982, Churchill 1980, cited in Munther 1983).

6. Habitat enhancement for species dependent on wetlands or dead trees (Reese and Hair 1976, Hill 1982, Ermer 1988, Dieter and McCabe 1989, Arner and Hepp 1989, Dubec et al. 1988, 1990, NYDEC 1991, Novak 1987a).

7. Increased plankton productivity and an increase in aquatic insects (Naiman et al. 1986).

8. Increased trapping of sediment and decreased turbidity downstream (Naiman et al. 1986).

9. Enhancement of beaver food plants such as willow and alder (Slough and Sadleir 1977).

10. Increase in carbon and nutrients in the channel (Hodkinson 1975, Naiman et al. 1986, Naiman and Mellilo 1984, Francis et al. 1985).

11. Increased resistance of ecosystem to perturbation (Naiman et al. 1986).

Beaver ponds are a shifting mosaic of habitats, dependent on pond age and size, successional state, substrate, hydrology, and nutrients. In boreal regions, there is a complex pattern of ecosystem development that involves the formation of marshes, seasonally flooded meadows, and forested wetlands, which appear to persist in a somewhat stable condition for centuries (Naiman et al. 1988). Food shortage probably is the major factor affecting colony longevity (Hodgdon 1978).

Population Ecology: A typical 'colony' is a family group (Payne 1982) of 3-6 individuals, with one breeding female (Novak 1977). Typical densities range from 0.4 to 0.8 families per square kilometer (Naiman et al. 1986) or from 0.09 to 1.2 families per stream km (Voight et al. 1976); saturation densities 0.4-1.9 families per km. In Newfoundland, reach greatest density in early succession (Northcott 1964). In Massachusetts, density increased with increasing hardwoods and with decreasing gradient, watershed size, and stream width (Howard and Larson 1985).

Mortality Factors: Humans are the only significant predators in most areas. Wolves may prey on beavers when ungulate populations are low (Voight et al. 1976, Shelton and Peterson 1983). In some regions tularemia (the bacterium FRANCISELLA TULARENSIS) has caused large die-offs (see Novak 1987a, Addison et al. 1987). However, most unexploited populations have a low mortality rate (less than 5- 7%), and can grow quickly in areas with abundant resources.

Scent Marking and Territoriality: A family maintains a territory from which other unrelated beavers generally are thought to be excluded (see Hodgden 1978 for evidence to the contrary). An important component of this is the construction of scent mounds, which typically are placed along trails and canals, and on lodges, dams, and pond banks.

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.