Adirondack Ecological Center
Beaver

(Castor canadensis Kuhl)

From: Saunders, D. A. 1988. Adirondack Mammals. State University of New York College of Environmental Science and Forestry. 216pp.

Order: Rodentia
Family: Castoridae

Range and Habitat

The range is most of North America from the tree line south into northern Mexico, except for much of Florida, and parts of Nevada and California. The beaver is an abundant, widespread species in the Adirondacks where it occupies most of the waterways near deciduous and mixed forests.

The beaver, the state mammal of New York, played an important role in the history of the region because of the value of beaver pelts. The early fur traders shipped 8,000 pelts from the New Netherlands in 1633. By 1640, the beaver was extirpated from what is now New York State except for the colonies surviving in the Adirondacks, and even these were eventually reduced, until by 1903 only one colony was thought to exist in an area northwest of Upper Saranac Lake. In that year, the legislature appropriated $500 to restock beavers in the Adirondacks. Fewer than 50 were released, and following their complete protection, the population expanded rapidly. Extensive stands of aspen and birch, a result of the widespread fires in the Adirondacks during the early part of the century, created ideal food supplies for the beaver. By 1924, amidst a storm of protests centered on property, timber, deer yard, and trout habitat destruction, the trapping season was reopened.

Beaver population estimates at the time varied almost as much as the damage claims; possibly more than 20,000 beavers lived in the Adirondacks. At present, the Department of Environmental Conservation estimates that 50,000-75,000 beavers inhabit the Adirondacks, occupying most of the available habitat. All animals alter their environments, but few do so profoundly as the beaver. Dams, lodges, burrows, canals, and trails mark its presence. Beavers create deep water ponds and lakes of 0.4 80 ha (1-200 acres) by damming flowing water with the trunks, limbs and twigs of trees supplemented with mud and even rocks. Trickles and torrents alike succumb to barricades ranging from a flimsy latticework of branches to massive structures 90 m (300 ft) in length, and 2.4 3.6 m (8-11 ft) in height.

Beavers may dig burrows, sometimes reinforced with mud and branches, with underwater entrances. More often, they build conical shaped lodges along the shorelines of lakes or by first towing branches to a portion of the pond with a slight elevation on the bottom. As the accumulation grows higher, additional material is placed on the perimeter and top. Mud, stones, and peeled branches are added as long as the beaver family is in residence. Some Adirondack lodges are immense structures, 8.5 10.7 m (28-35 ft) in diameter, 1.8 2.1 m (6-7 ft) tall, and several decades in age.

The beavers form a passageway, and then excavate a central cavity or living chamber a few centimeters above the water level both by gnawing and using their backs to push up against branches. They may add bedding of shredded wood, leaves, moss or even grass to the floor of this chamber.

Over a period of years, a single family of beavers may build multiple dams and lodges, but construction does not end with them. Trips overland along the same pathway produce trials 20-50 cm (8-20 in) wide. More elaborate canals of about the same width may radiate from the pond and enable the beavers to transport branches more easily. Always, freshly peeled twigs, new layers of mud on lodges and dams, and gleaming white tree stumps indicate the beaver's continued use of an area.

Beavers have many adaptations for their woody diet. Large jaw muscles power the bevel edged, continuously growing incisors that slice abrasive wood and the broad, flat molars that grind the fibrous bark. The digestive system includes several glands, such as the cardiac gland in the stomach, to accommodate the massive diet of wood cellulose. The large cecum contains commensal organisms (mostly bacteria and fungi) that aid in assimilating about 30% of the dietary cellulose.

Food and Feeding Behavior: The beaver is a herbivore. Woody plants provide all the winter food, about half the spring and autumn requirements, but only 10% of the summer diet when herbaceous plants such as sedges and aquatic plants become available. Beavers eat the inner bark of living trees, preferring aspens but using many other deciduous species such as maples, birches, willows, beech, alder, and black cherry. An adult requires 0.6 2.0 kg (20-72 oz) of bark per day for food. Smaller trees provide relatively more edible, nutritious bark than larger ones, and beavers fell trees less than 15 cm (6 in) in diameter first. Colonies relocate after cutting all suitable tree, large and small, within 92-152 m (300-500 ft) of the water, apparently limited by the energy that must be expended to transport materials farther, and the increasing risk of predation away from the water.

A beaver usually works alone while felling a tree, but colony members may section the fallen trunk and limbs, and drag branches to shallow water feeding platforms before peeling and eating the bark. This activity is not coordinated nor synchronized. To fell a tree, a beaver places the forefeet against the trunk, and braced in place by the tail, uses the incisors to gouge chips from the trunk in an hourglass pattern. A beaver removes about 140 ragged chips of wood from a tree 14 cm (5.5 in) in diameter, toppling it in a few minutes.

Beavers transport and store branches in the water near their lodge or burrow beginning in August and continuing until a permanent layer of ice forms. These food caches may be as much as 12 m (40 ft) in diameter and 3 m (10 ft) in height, and provide the winter food supply. When hungry, a beaver carries a section of a branch, swimming beneath the ice, to its living chamber.

Activity and Movement

Beavers spend most of the daylight hours sleeping or resting in their living chambers, emerging towards dusk. An exception to their nocturnal habits comes with autumn and its food hoarding demands, when beavers are more likely to be out during the late afternoon. Feeding predominates the first half of each night, and other activities such as dam and lodge building the second half. Beavers tend to remain near the lodge with the approach of dawn. Strong winds, pouring rain, and severe cold deter activity. On land, beavers move with a slow waddle, but when alarmed, gallop back to the water. In water, they swim at speeds up to 3.2 km per hour (2 mph) with alternate strokes of the powerful hind feet propelling movement, traveling quietly along the surface or underwater. The tail functions as a s diving plane and rudder for underwater swimming; special transparent eye membranes serve as underwater goggles, protecting the eyes but permitting vision. The valvular ears and nostrils close, and the flexible lips fold behind the incisors during submergence which may last for periods up to 15 minutes, but is typically 2-3 minutes.

Reproduction

Beavers mate in the water from mid-January to mid-March. Females bear 1-9, usually 3-5 young, 106-128 days later, from mid-March to early June. At birth, the fully furred kits weigh 0.2-0.6 kg (8-22 oz) and are approximately 30 cm (12 in) long with eyes partially opened. At 2-3 weeks of age, they begin eating the foliage of fresh branches the female carries to them. By 6 weeks they are weaned, and by the following spring they may weigh 7.7 kg (17 lb). Sexual maturity occurs at 1.5-2 years. The average life span is 10-12 years; a few may survive 20 years. One Adirondack beaver is known to have survived to 18 years at which time it weighed 32 kg (70 lb).

Predators

Coyotes, bobcats, foxes, formerly the lynx, and occasionally large birds of prey are predators of adults and young.

Social Behavior

Social System

The “colony” or family consists of 1-12 individuals, with 5-6 an average, which includes the monogamous pair, their kits of the year, and usually their young of the previous year (yearlings) which may disperse to new areas with the birth of a new litter. Density may influence the probability of dispersal of both yearlings and two-year olds. The dispersing two-year olds may move 11-48 km (7-30 mi), occasionally farther, to find suitable habitat. A colony may reside as an isolated family or contiguous with others, sharing a common dam but never sharing lodges, burrows, and feeding areas. The effective territory is approximately 805 m (0.5 mi) in radius. This elaborate social system reflects the distribution and relative abundance of the beaver's food, and the beaver's aquatic life style.

Communication

Beavers are well known for the thunderous warning slaps or splashes of their tails that alert other family members to the presence of intruders, an example of a mechanical sound used in signaling. Vocalizations include the young's contact calls, (high-pitched, rising-falling whines), and the adults low-pitched ventriloquial whine and “hiss” given during aggressive encounters. A beaver colony marks an area by building mounds of mud near the lodge and dam, on or near trails. On these mounds, which usually vary from 2-7 in number and are 5.0-7.5 cm (2-3 in) in height and 30 cm (12 in), in diameter, colony members place secretions from the castor sacs and anal glands. Scent mounds are most prevalent during early summer when the two-year olds or yearlings, or both are dispersing, and are likely deterrents or “fences" to exclude them from occupied habitat. The castorum of the castors is still used as a fixative for perfumes. Until the 1700's it was widely prescribed as a cure for a variety of maladies, from headaches to hysteria, and in some instances it may have alleviated symptoms because of the salicylic acid it contains-the basic ingredient of aspirin. Beavers engage in mutual grooming, which may plat a minor role in tactile communication. Visual signals are relatively unimportant, but include various postures.

Additional References

Hodgdon, H. E. and J. S. Larson. 1980. A bibliography of the recent literature on beavers. Massachusetts Agricultural Experiment Station Research Bulletin, 665:1-128.

Jenkins, S. H. and P. E. Busher. 1979. Castor canadensis. Mammalian Species, 120:1-8.

Johnson, C. E. 1927. The beaver in the Adirondacks: its economics and natural history. Roosevelt Wildlife Bulletin, 4:499-641.

Lancia, R. A., W. E. Dodge, and J. S. Larson. 1982. Winter activity patterns of two radio-marked beaver colonies. Journal of Mammalogy, 63:598-606.

Müller-Schwarze, D. and S. Heckman. 1980. The social role of scent marks in beaver. Castor canadensis. Journal of Chemical Ecology, 6:81-95.

Müller-Schwarze, D. and R. M. Silverstein, ed. 1982. Chemical signals in vertebrates 3. “Experimental modulation of behavior of free-ranging mammals by semiochemicals.” Plenum Press, New York. 368pp.

Patric, E. F. 1952. A beaver management program for the Huntington Forest. M.S. Thesis, SUNY College of Forestry. 117pp.

Radford, H. V. 1907. History of the Adirondack beaver. New York State Forest, Fish and Game Commission Annual Report for 1904, 1905, 1906. 389-418.

Sterling, E. A. 1913. The return of the beaver to the Adirondacks. American Forestry, 19:292-299.

Tevis, L., Jr. 1950. Summer behavior of a family of beavers in New York State. Journal of Mammalogy, 31:40-65.

Will, G. and M. Brown. 1981. Adirondack furbearers. The New York State Conservationist, 35:32-37.