The North American beaver (Castor canadensis) is one of two extant beaver species, along with the Eurasian beaver (Castor fiber). It is native to North America and has been introduced in South America (Patagonia) and Europe (primarily Finland and Karelia). The North American beaver is one of the national symbols of Canada and the official state mammal of Oregon and New York.Official symbols of Canada . Government of Canada North American beavers are widespread across the continental United States, Canada, southern Alaska, and some parts of northern Mexico.
In Canada and the United States, the North American beaver is often referred to simply as "beaver;" however, this can cause some confusion because another distantly related rodent, Aplodontia rufa, is often called the "mountain beaver". Other vernacular names, including American beaver and Canadian beaver, distinguish this species from the other extant beaver species, Castor fiber, which is native to Eurasia.
Taxonomy Evolution The first fossil records of beavers are 10 to 12 million years old in Germany, and they are thought to have migrated to North America across the Bering land bridge. During the mid to late Pleistocene epoch, giant beavers weighing up to 200 pounds thrived in central North America. These large early beavers were not yet known to fell trees and build dams. The oldest fossil record (Castor californicus) of beavers in North America are of two beaver teeth near Dayville, Oregon, and are 7 million years old. Early beavers had diverse ways of life, taking on terrestrial and semi aquatic forms. Dam building originated over 10 million years ago by a semi-aquatic variation of beavers.
Subspecies At one time, 25 subspecies of beavers were identified in North America, with distinctions based primarily on slight morphological differences and geographical isolation at the time of discovery. However, modern techniques generally use genetics rather than morphology to distinguish between subspecies, and currently the Integrated Taxonomic Information System (which provides authoritative taxonomic information on plants, animals, fungi, and microbes of North America and the world) does not recognize any subspecies of C. canadensis, though a definitive genetic analysis has not been performed. Such an analysis would be complicated by the fact that substantial genetic mixing of populations has occurred because of the numerous reintroduction efforts intended to help the species recover following extirpation from many regions.
The most widespread (formerly recognized) subspecies, which perhaps are now best thought of as populations with some distinct physical characteristics, are C. c. acadicus (New England beaver), C. c. canadensis (Canadian beaver), C. c. carolinensis (Carolina beaver), and C. c. missouriensis (Missouri River beaver).Baker, B. W. and Hill, E. P. (2003). "Beaver (Castor canadensis)" . 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. The Canadian beaver originally inhabited almost all of the forested area of Canada,Kieffer, Michael Meadows in Mist Bull Run Mountains Conservancy, Inc. Retrieved August 4, 2007. 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.
Description
The beaver is the largest rodent in North America and competes with its Eurasian counterpart, the European beaver, for being the third-largest in the world, both following the South American capybara and lesser capybara. The European species is slightly larger on average but the American has a larger known maximum size. Adults usually weigh from , with being typical. In New York, the average weight of adult male beavers was , while non-native females in Finland averaged . However, adults of both sexes averaged in Ohio.Müller-Schwarze, D., & Schulte, B. A. (1999). Behavioral and ecological characteristics of a "climax" population of beaver (Castor canadensis). In Beaver protection, management, and use in Europe and North America (pp. 161–177). Springer US.Lahti, S., & Helminen, M. (1974). The beaver Castor fiber (L.) and Castor canadensis (Kuhl) in Finland. Acta Theriologica, 19(13), 177–189. The species seems to conform to Bergmann's rule, as northern animals appear to be larger. In the Northwest Territory, adults weighed a median of . The American beaver is slightly smaller in average body mass than the Eurasian species. The head-and-body length of adult North American beavers is , with the tail adding a further . Very old individuals can exceptionally exceed normal sizes, weighing more than or even as much as .The Beaver – Life Tracks . Timberwolfinformation.orgBurnie D and Wilson DE (Eds.), Animal: The Definitive Visual Guide to the World's Wildlife. DK Adult (2005), Boyle, Steve and Owens, Stephanie (February 6, 2007) North American Beaver (Castor canadensis): A Technical Conservation Assessment . USDA Forest Service, Rocky Mountain Region
The beavers are semiaquatic with physical traits suited to this lifestyle. It has a large, scaly, paddle-shaped tail and webbed hind feet. The unwebbed front feet are smaller, have claws, and are highly dexterous. Beavers use their front feet for digging, to feed themselves, and to maneuver small stems as they gnaw bark off trees. The eyes are covered by a nictitating membrane which allows the beaver to see underwater. The nostrils and ears are sealed while submerged. Their lips can be closed behind their front teeth so that they can continue to gnaw underwater. A thick layer of fat under its skin provides insulation.
The beaver's fur consists of long, coarse outer hairs and short, fine inner hairs, and is usually dark brown. Scent glands near the genitals secrete an oily substance known as castoreum, which the beaver uses to waterproof its fur. There is also another set of oil glands producing unique chemical identifiers in the form of waxy esters and fatty acids.
The beaver, like all rodents, possesses continuously growing incisors, and is a hindgut fermenter whose cecum, populated by symbiotic bacteria, helps to digest plant-based material. The beaver is specialized for efficient digestion of its lignocellulose-heavy diet.
Distribution Beavers were practically ubiquitous and lived from south of the arctic tundra to the deserts of northern Mexico, and from the Atlantic to the Pacific Oceans, but were trapped across the continent for economic gain and political competition, leading to regional extirpation in many regions like the Great Plains. Certain areas saw near-extirpations and declining numbers, but not extirpation, partially due to various conservation measures taken, with mixed results, by the Hudson's Bay Company. By the 1900s and 1910s, beavers existed largely in isolated pockets, but where those pockets existed is not well documented. However, despite these difficulties, reintroduction and natural spread has restored the species back into most of its natural range. They are widely distributed in boreal and temperate ecoregions, where populations are rebounding from historic over-exploitation. Recently, beavers have been observed colonizing arctic tundra, likely as a result of climate-induced increases in riparian shrubs.
Records of beavers at the southern extent of their range have historically been somewhat sparse. Nevertheless, they have been reported in the North American southwest as early as the mid-19th century. Historical and contemporary reports of their presence have been made in the Sonora River, the Colorado River, Bavispe River, and the San Bernardino River in the northern Mexican states of Sonora and Chihuahua (state).
Some scientists believe the species may have been present in Eastern Eurasia historically in regions such as the Anadyr, Amur, and Lena River Basins as opposed to C. fiber as species like wolves and moose in these regions are genetically closer to North American populations than Eurasian populations, although this is unconfirmed.Duncan John Halley (2022). Population history and distribution of Eurasian beavers. https://hdl.handle.net/11250/3529150
Behavior
Beavers are active mainly at night. They are excellent swimmers and may remain submerged up to 15 minutes. More vulnerable on land, they tend to remain in the water as much as possible. They use their flat, scaly tail both to signal danger by slapping the surface of the water and as a location for fat storage.
They construct their homes, or "lodges", out of sticks, twigs, rocks, 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. Beavers are well known for building dams across streams and constructing their lodges in the ponds which form. When building in a pond, the beavers first make a pile of sticks and then eat out one or more underwater entrances and two platforms above the water surface inside the pile. The first is used for drying off. Towards winter, the lodge is often plastered with mud which, when it freezes, has the consistency of concrete. A small air hole is left in the top of the lodge.
Dam-building The purpose of the dam is to create a deepwater refuge enabling the beaver to escape from predators. When deep water is already present in lakes, rivers, or larger streams, the beaver may dwell in a bank burrow and bank lodge with an underwater entrance. The beaver dam is constructed using branches from trees the beavers cut down, as well as rocks, grass, and mud. Where naturally occurring woody material is limited, beavers may build their dams largely of rocks. Beavers cut down trees using their strong incisor teeth, and use their front feet for digging, carrying, and placing materials. 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. However, beaver dams are not permanent and depend on the beavers' continued presence for their maintenance. Beavers generally concentrate on building and repairing dams in the fall in preparation for the coming winter. In northern areas, they often do not repair breaches in the dam made by otters, and sometimes breach the dam themselves and lower the water level in the pond to create more breathing space under the ice or get easier access to trees below the dam. In a 1988 study in Alberta, Canada, no beavers repaired "sites of water loss" during the winter. Of 178 sites of water loss, beavers repaired 78 when water was opened, and did not repair 68. The rest were partially repaired.Donald G. Reid, Stephen M. Herrero and Thomas E. Code, "River Otters as Agents of Water Loss from Beaver Ponds," Journal of Mammalogy, February 1988.