The North American Beaver (Castor canadensis) is the only species of beaver in the Americas, native to North America and introduced to South America. In the United States and Canada, where no other species of beaver occurs, it is usually simply referred to as beaver. Its other vernacular names, including American beaver and Canadian beaver, distinguish this species from the one other extant beaver, Castor fiber, native to Eurasia. ("Canadian beaver" also refers to the subspecies Castor canadensis canadensis.)

Description

This beaver is the largest rodent in North America and the third largest rodent in the world, after the South American capybara and the Eurasian beaver. Adults usually weigh 15 to 35 kg (33--77 lbs), with 20 kg (44 lbs) a typical weight, and measure around 1 m (3.3 ft) in total body length. Very old individuals can weigh as much as 45 kg (100 lbs).

Like the capybara, the beaver is semi-aquatic. The beaver has many traits suited to this lifestyle. It has a large flat paddle-shaped tail and large, webbed hind feet reminiscent of a human diver's swimfins. The unwebbed front paws are smaller, with claws. The eyes are covered by a nictitating membrane which allows the beaver to see underwater. The nostrils and ears are sealed while submerged. A thick layer of fat under its skin insulates the beaver from its cold water environment.

The beaver's fur consists of long, coarse outer hairs and short, fine inner hairs (see Double coat). The fur has a range of colors but usually is dark brown. Scent glands near the genitals secrete an oily substance known as castoreum, which the beaver uses to waterproof its fur.

Behavior

Beavers are mainly active at night. They are excellent swimmers but are more vulnerable on land and tend to remain in the water as much as possible. They are able to remain submerged for up to 15 minutes. The flat, scaly tail is used to signal danger and also serves as a source of fat storage.

They construct their homes, or "lodges," out of sticks, twigs, and mud in lakes, streams, and tidal river deltas. These lodges may be surrounded by water, or touching land, including burrows dug into river banks. They are well known for building dams across streams and constructing their lodge in the artificial pond which forms. The entrance to the lodge or burrow is located underwater. In the event of danger, a beaver slaps its tail on the water to warn other family members.
The dam is constructed using sections of deciduous trees, especially birch, aspen, willow and poplar. The inner bark, twigs, shoots and leaves of such trees are also an important part of the beaver's diet. The trees are cut down using their strong incisor teeth. Their front paws are used for digging and carrying and placing materials. Some researchers have shown that the sound of running water dictates when and where a beaver builds its dam. Besides providing a safe home for the beaver, beaver ponds also provide habitat for waterfowl, fish, and other aquatic animals. Their dams help reduce soil erosion and can help reduce flooding. During the summer, beavers eat grasses, cattails, water lilies and other aquatic plants.

Beavers usually mate for life. The young beaver "kits" typically remain with their parents for up to two years.

Common natural predators include gray wolves, coyotes, and mountain lions. Less significant predators include bears, which can dig into a lodge, wolverines, river otters, Canadian lynx, bobcats, and mink.

Subspecies

There are 25 subspecies of beaver in North America, but different subspecies have been reintroduced to areas with previously geographically isolated subspecies, following population decline or extirpation of the indigenous subspecies. This has led to very substantial mixing of the subspecies gene pools, and some subspecies may have disappeared entirely.

The most widespread subspecies are C. c. acadicus, C. c. canadensis (Canadian beaver), C. c. carolinensis (Carolina beaver), and C. c. missouriensis (Missouri River beaver).Baker, B. W., and E. P. Hill. . G. A. Feldhamer, B. C. Thompson, and J. A. Chapman, editors. Wild Mammals of North America: Biology, Management, and Conservation. Second Edition. The Johns Hopkins University Press, Baltimore, Maryland, USA, pp. 288-310. 2003. Retrieved on 2007-08-04. The Canadian beaver originally inhabited almost all of the forested area of Canada, and because of its more valued fur, was often selected for reintroductions elsewhere. The Carolina beaver is found in the southeastern United States, the Missouri River beaver, as its name suggests, is found in the Missouri River and its tributaries, and C. c. acadicus is found throughout the New England area in the northeastern United States.

Differences from European beaver

Although superficially similar to the European beaver (Castor fiber), there are several important differences between the two species. North American beavers tend to be smaller, with smaller, more rounded heads, shorter, wider muzzles, thicker, longer and darker underfur, wider, more oval-shaped tails and have longer shin bones, allowing them a greater range of bipedal locomotion than the European species. North American beavers have shorter nasal bones than their European cousins, with the widest point being at the middle of the snout for the fomer, and in the tip for the latter. The nasal opening for the North American species is square, unlike that of the European race which is triangular. The foramen magnum is triangular in the North American beaver, and rounded in the European. The anal glands of the North American beaver are smaller and thick-walled with a small internal volume compared to that of the European breed. Finally, the guard hairs of the North American beaver have a shorter hollow medulla at their tips. Fur colour is also different. Overall, 50% of North American beavers have pale brown fur, 25% are reddish brown, one fifth are brown and 6% are blackish, while in European beavers 66% have pale brown or beige fur, 20% have reddish brown, nearly 8% are brown and only 4% have blackish coats..

The two species are not genetically compatible. North American beavers have 40 chromosomes, while European beavers have 48. Also, more than 27 attempts were made in Russia to hybridize the two species, with one breeding between a male North American beaver and a female European resulting in one stillborn kit. These factors make interspecific breeding unlikely in areas where the two species' ranges overlap.

Conservation

These animals are considered pests in some parts of their range because their dams can cause flooding in nearby areas. Because they are persistent in repairing any damage to the dam, they were historically relocated or exterminated. However, non-lethal methods of containing beaver-related flooding have been developed. One such device has been utilized by both the Canadian and U.S. governments, called "Beaver Deceivers," or levelers, invented and pioneered by wildlife biologist, Skip Lisle.

In 1999, Washington, D.C.'s annual Cherry Blossom Festival was plagued by a family of beavers who lived in the Tidal Basin. The offenders were caught and removed, but not before damaging 14 cherry trees, including some of the largest and oldest trees.

These animals are often trapped for their fur. During the early 19th century, trapping eliminated this animal from most of its original range. The beaver furs were used to make clothing and beaver hats. Much of the early exploration of North America was driven by the quest for this animal's fur. Native peoples and early settlers also ate this animal's meat. The current beaver population has been estimated to be 10 to 15 million; there may have originally been 100 to 200 million North American beaver before the days of the fur trade.

As invasive species

Beaver damage on the north shore of Robalo Lake, Navarino Island, Chile

In the 1940s, beavers were brought to the island of Tierra Del Fuego in southern Chile and Argentina, for commercial fur production. However, the project failed and the beavers, a few pairs, were released into the wild. Having no natural predators in their new environment, they quickly spread throughout the island, and to other islands in the region, reaching a number of 100,000 individuals within just 50 years. They are now considered a serious invasive species in the region, due to their massive destruction of forest trees, and efforts are being made for their eradication. The drastically different ecosystem has led to substantial environmental damage, as the ponds created by the beavers have no ecological purpose (wetlands do not form there as they do in the beavers' native territory) and there are no native, large predators. They have also been found to cross saltwater to islands northward; a possible encroachment on the mainland has naturalists highly concerned.

As food

Beaver meat is similar tasting to lean beef, but care must be taken to prevent contamination from the animal's strong castor (musk) gland. It is usually slow-cooked in a broth, and was a valuable food source to Native Americans.

Despite their name, the fried pastries found in parts of North America called beaver tails contain no beaver.

Symbolism

One of the national symbols of Canada, the beaver is depicted on the Canadian five-cent piece and was on the first Canadian postage stamp, the Three Penny Beaver. It is also the state animal of Oregon and New York, and a common school emblem for engineering schools, including the California Institute of Technology and the Massachusetts Institute of Technology, as well as the mascot for Oregon State University. The beaver also appears in the coats of arms of the University of Toronto, Wilfrid Laurier University and the London School of Economics.