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 beaver is a large animal, with the biggest animals 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 dextrous 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 wintertime 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.