Species: Castor canadensis

American Beaver

Species

Show on Lists

Encyclopedia of Puget Sound

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.

Articles:

The secret legacy of tidal beavers

Beavers are typically associated with freshwater environments, but scientists have learned that they also survive and thrive in the shoreline marshes of the Salish Sea. New research is shedding light on the vital connection between tidal beavers and salmon.

: A beaver emerges from a wire trap cage surrounded by tall reeds and wetland vegetation.

Can the age-old affinity between beavers and salmon be restored?

September 26, 2023

Salmon restoration groups are learning how to work with beavers to create better salmon habitat. The process hinges on reducing human-beaver conflicts while taking a natural approach to ecosystem recovery. The beavers are happy to help.

A beaver swimming in a pond with vegetation in the background.

Classification

Kingdom

Animalia

Phylum

Craniata

Class

Mammalia

Order

Rodentia

Family

Castoridae

Genus

Castor

NatureServe

Source:
ELEMENT_GLOBAL.2.100922

Classification

Other Global Common Names

Beaver - North American Beaver - castor du Canada

Informal Taxonomy

Animals, Vertebrates - Mammals - Rodents

Formal Taxonomy

Animalia - Craniata - Mammalia - Rodentia - Castoridae - Castor

Ecology and Life History

Migration

true - false - false - >

Non-migrant

true

Locally Migrant

false

Food Comments

The diet of the beaver changes throughout the year. Use of woody vegetation extends from fall to spring, peaking in winter and reaching a minimum in summer. Favored woody plants in many regions include aspen (POPULUS TREMULOIDES), willow (SALIX), cottonwood (POPULUS sp.), alder (ALNUS sp.), birch (BETULA), sweetgum (LIQUIDAMBAR STYRACIFLUA), blackgum (NYSSA), and pine (PINUS). Aspen is especially favored where it is available. Apparently alder often is cut for construction material and not for food, except when other sources are unavailable (see Novak 1987a). In general the beaver is flexible in its choice of woody plants and eats the leaves, bark, and twigs of most species. Beavers commonly debark pine trees and lick the pitch. Summer foods include aquatic succulents such as pond lilies (NUPHAR, NYMPHAEA), bur-reed (SPARGANIUM), duckweeds (LEMNA, etc.), pondweeds (POTAMOGETON), algae, and fleshy rootstocks of many other species, as well as a wide variety of upland or riparian herbaceous plants. In addition, beavers in some areas commonly visit fields to eat and/or harvest clover, alfalfa, corn, soybeans, and other crops, some of which may be used as construction materials. Relatively little is known about the possible use of below-surface aquatic vegetation in winter. See Novak (1987a) for an overview of beaver food habits in different regions. Naiman et al. (1988) cited various studies indicating that in northern regions beavers annually cut at least a metric ton of wood within approximately 100 m of their pond. In Wisconsin, it was estimated that a beaver family may cut about one-half acre (0.2 ha) per year in pole stands of 500 stems per acre (1250 stems per ha) (Knudsen 1962). Extensive tree cutting occurs only in late summer and fall (Adirondacks; Stegeman 1954). In northern areas, beavers store food in late summer and fall for use in winter. Food cache construction begins at the end of August at the Mackenzie Delta (Aleksiuk 1970). Branches of woody plants are cached in deep pools near the den. Normally branches that are cut and carried to the cache are under 13 cm in diameter. The food pile may be capped with alder, peeled logs, or conifers. Caching provides an under-ice food supply in winter. In areas that rarely become heavily ice-covered, beavers do not always or regularly cache food (Swenson et al. 1983).

Reproduction Comments

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.

Ecology Comments

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.

Length

117

Weight

27000

Conservation Status

NatureServe Global Status Rank

Global Status Last Reviewed

1996-11-08

Global Status Last Changed

1996-11-08

Other Status

LC - Least concern

Distribution

Conservation Status Map

Global Range

H - >2,500,000 square km (greater than 1,000,000 square miles) - H - North America except arctic tundra, peninsular Florida, and much of the desert area of the Southwest, including parts of the northern edge of Mexico; introduced into Eurasia.

Global Range Code

Global Range Description

>2,500,000 square km (greater than 1,000,000 square miles)

ELEMENT_GLOBAL.2.100922