Beavers
Beavers
The true beavers are robust, aquatic herbivores in the family Castoridae, order Rodentia. Many taxonomists believe that two, closely related species of true beavers exist—the American beaver (Castor canadensis ) and the Eurasian beaver (C. fiber ). Other taxonomists, however, classify these as closely related variants of the same species, under the name Castor fiber.
A few other rodents are also called beavers, such as the mountain beaver (Aplodontia rufa ) of western North America and the swamp beaver or nutria (Myocastor coypu ) of South America. However, these two species of rodents are not in the family Castoridae and are not true beavers.
The true beavers are large animals, weighing as much as about 88 lb (40 kg), and they are the largest rodents to occur in Eurasia and North America. Only the capybaras of South America (family Hydrochoeridae), which can weigh as much as 110 lb (50 kg), are larger
rodents. However, a now-extinct species of giant beaver in the genus Castoroides, which occurred in North America as recently as about 10,000 years ago at the end of the most recent ice age, is estimated to have weighed several hundred pounds. This enormous rodent was similar in size to a black bear (Ursus americanus ).
One of the most distinctive features of beavers is their scaly, naked, paddle-like tail. The flattened beaver tail is used as a rudder while the animal swims, using its webbed hind feet to propel itself through the water. The unusual tail is also used as a support while the beaver is standing and as a brace while the animal is dragging logs to the water. If danger is perceived, the tail is energetically splashed onto the water surface to warn other beavers of the threat. However, contrary to what some people believe, the tail is not used as a trowel to daub mud onto the dams that beavers often build.
The American beaver
The American beaver (Castor canadensis ) is widespread in North America, ranging from the limits of the boreal forest in the north, through almost all of the United States, except for the Florida peninsula and parts of the southwestern states. The American beaver has also been introduced beyond its natural range, for example, into some regions in Europe. As a result, some hybridization has occurred with the European beaver, suggesting a close evolutionary relationship between the two species.
The American beaver is a large animal, with the biggest individuals reaching a weight of about 88 lb (40 kg), but more typically being 33–77 lb (15–35 kg). The beaver has a robust body, a broad and blunt head, and a short neck and limbs. Beavers have very large, continuously growing incisor teeth and large cheek teeth used for chewing their food of plant materials. The incisor teeth meet outside of the closed lips of the mouth, enabling the animal to feed easily underwater. The nostrils and ears have skin flaps that serve as valves to keep water out when the animal is submerged. The forepaws have long fingers, useful for dexterous handling of branches and twigs while feeding and building lodges and dams. The hind feet have two serrated claws that are used in preening and oiling the fur, a task in which beavers are commonly engaged. The other three claws on the hind feet are blunt and flat. The pelage of this animal is thick and lustrous, with a dense, brown underfur and longer, coarser guard hairs.
Beavers are social animals, with the basic unit being the family, which forms a colony with a hierarchical structure among the individuals. The oldest female is the central individual in the group. She establishes the colony, and, if she is killed and no daughter exists to take over the matriarchal role, the site is abandoned. The average colony size is about six animals. All of the animals in the colony work cooperatively, especially in building and maintaining the group’s dams and lodge.
Beavers are famous for their industriousness and engineering skills. If an open-water wetland such as a pond is not available locally, beavers will construct one by building a dam of logs, sticks, stones, and mud-plaster across a stream, causing the water to back up. Beavers maintain their dams assiduously, and they seem to be constantly working on improving these structures. This is necessary, of course, because the beaver pond provides essential local habitat for the species, yet it is in some respects artificial, having been created by the animals themselves. Of course, many other species of wildlife benefit greatly from the habitat-creating enterprise of beavers. The dams are generally constructed to create a pool that is 6.5–10 ft (2–3 m) deep in some places, so that in the winter unfrozen water will occur beneath the ice, which can be as much as 3 ft (1 m) thick. Some dams can be hundreds of feet long, and several feet high.
Beavers also build lodges of sticks and mud. Beaver lodges are commonly located in shallow, open water, and their tops project as high as several feet above the water surface. Lodges may also be located near the edge of the beaver pond, rather than in open water. The lodge has a hollow, gnawed-out core, in which the family lives. The roof of the lodge is relatively thin and porous, allowing fresh air to circulate. The interior of the lodge is reached by several underwater passageways. Beavers also burrow into mud banks, and animals living on large rivers or lakes will do this instead of building a lodge.
Beavers are crepuscular, meaning they are most active between sunset and sunrise. They have a slow lumbering gait on land, but are skilled swimmers. Beavers mostly eat the inner bark and cambium of trees and shrubs, as well as the buds, leaves, and flowers of woody plants. They also supplement this diet with aquatic plants and herbaceous terrestrial vegetation during the summer and autumn. Beavers sometimes fell quite large trees (up to 16 in [40 cm] in diameter) to get access to the relatively nutritious branches and twigs of the canopy. Trembling aspen (Populus tremuloides ) is the food species of choice, with willows, birches, poplars, and alders also being favored.
Beavers fell trees by gnawing them at the base, often while standing erect, propped by their tail, and gripping the tree with their forepaws. The stumps of beaver-felled trees have a distinctive, conical top, with clear evidence of the large cuts made by the chisel-like incisors of the animal, which remove substantial chips of wood. The beavers do not gnaw right through the trunk—they leave a central core intact, and rely on rocking motions from a later wind to actually cause the final felling of the tree. Beavers seem unable to plan the direction of the eventual tree-fall, and they are sometimes killed when this actually happens. Beavers occasionally construct canals in wet terrain in order to make their logging areas easier to reach, and they often develop wide, well-trodden paths to facilitate the dragging of branches to their pond.
Beavers do not hibernate—they remain active in their lodges and beneath the ice of their pond. During winter, these animals mostly feed on underwater piles of branches and twigs accumulated for this purpose during the previous summer and autumn. However, beavers will sometimes emerge above the ice and snow to feed if they run short of their stored winter food.
Many animals prey on beavers. When predators are relatively abundant, beavers are wary and do not like to forage or fell trees very far from the safety of their pond.
Beavers and the fur trade
The pelts of American beavers are valuable in the fur trade and are largely used in making coats and hats. During the first several centuries of the European colonization of North America, beaver pelts were one of the most important natural resources to be exported from the northern regions of that continent. The most important markets were in Europe, where the pelts were used to make gentlemen’s hats, also known as “beavers.” In fact, most of the initial exploration and settlement of the interior of North America was undertaken by fur traders, and these intrepid men were most enthusiastically searching for beaver pelts. For many years in vast regions of North America, beaver pelts were the measure of wealth, and were even a common unit of currency. In view of the great importance of the beaver in the early colonial history of Canada, this animal has become a national symbol of that country. However, beavers were similarly important in the northeastern and central United States.
The extraordinary overharvesting of beavers for their pelts caused great reductions in the abundance of these animals, and they were widely extirpated from much of their original range in North America. Moreover, the American beaver will not breed in captivity, so fur-farming is not possible. Fortunately, the implementation of conservation measures after about the 1940s has allowed a substantial rebound in the populations of American beavers. These animals are now re-occupying much of their former range, as long as the habitat has remained suitable for their purposes. Beavers are sometimes hunted for meat, although their use in this way is usually secondary to the taking of their pelts.
Beavers can be viewed as a nuisance, their constructions flooding roads, culverts, railroads, lawns, and agricultural land. Beavers also may cut down valuable ornamental trees in some places where they are living in proximity to humans. As a result, many states and provinces will live-trap problem beavers for relocation to less built-up areas.
KEY TERMS
Crepuscular— Refers to animals that are most active in the dim light of dawn and dusk, and sometimes at night as well.
Overharvesting— The unsustainable exploitation of a potentially renewable natural resource, such as hunted animals or trees. Overharvesting eventually leads to a collapse in the abundance of the resource.
The Eurasian beaver
The natural range of the Eurasian beaver (Castor fiber ) extends through most of Europe and through much of northwestern Asia. However, the modern range of the Eurasian beaver has become rather restricted and fragmented, and the species is now much less abundant than it used to be. The population declines are due to the conversion of the species’ natural wetlands habitat into agricultural lands as well as to overhunting. The European beaver has long been exploited as a natural resource, for both its thick, lustrous fur, and also for a musky oil called castoreum, which is extracted from the anal or castor gland of this animal.
The mountain beaver
The mountain beaver (Aplodontia rufa, family Aplodontidae) occurs in the Cascade Mountains from east-central California north through Oregon and Washington to southwestern British Columbia. Other than being a large rodent, the mountain beaver is not particularly closely related to the true beavers, which are in the family Castoridae. In fact, mountain beavers are the only species in their family, and they not closely related to any other rodents. Because of their ancient evolutionary history, mountain beavers are sometimes considered to be living fossils. In body form, the mountain beaver looks like a tailless muskrat or large vole, with small ears, short legs, and grizzled, brownish fur. Mountain beavers are terrestrial animals, digging long, complex burrows in moist, workable soil near streams, with numerous entrances and exits located in concealed places. These animals live in loose colonies, but they are not very social animals, preferring to avoid frequent, direct contact with each other. Mountain beavers eat a wide range of plant foods, including herbaceous plants and fruits, young twigs of woody plants, and conifer foliage and shoots in the winter. Mountain beavers store food for the winter in underground haystacks.
Resources
BOOKS
Banfield, A.W.F. The Mammals of Canada. Toronto: University of Toronto Press, 1974.
Long, Kim. Beavers: A Wildlife Handbook. Boulder, CO: Johnson Books, 2000.
Nowak, Ronald M. Walker’s Mammals of the World. 6th ed. Baltimore: The Johns Hopkins University Press, 1999.
Ryden, H. The Beaver. London: Lyons & Burford, 1992.
Wilson, D.E., and D. Reeder. Mammal Species of the World. 3rd ed. Baltimore: Johns Hopkins University Press, 2005.
Bill Freedman