Genus Castor, the Beavers
Beavers are large, semiaquatic rodents in the genus Castor native to the temperate Northern Hemisphere. There are two extant species: the North American beaver (Castor canadensis) and the Eurasian beaver (C. fiber). Beavers are the second-largest living rodents after the Capybaras. They have stout bodies with large heads, long chisel-like incisors, brown or gray fur, hand-like front feet, webbed back feet and flat, scaly tails. The two species differ in the shape of the skull and tail and fur color. Beavers can be found in a number of freshwater habitats, such as rivers, streams, lakes and ponds. They are herbivorous, consuming tree bark, aquatic plants, grasses and sedges.
Beavers build dams and lodges using tree branches, vegetation, rocks and mud; they chew down trees for building material. Dams impound water and lodges serve as shelters. Their infrastructure creates wetlands used by many other species, and because of their effect on other organisms in the ecosystem, they are considered a keystone species. Adult males and females live in monogamous pairs with their offspring. When they are old enough, the young will help their parents repair dams and lodges and may also help raise newly born offspring. Beavers hold territories and mark them using scent mounds made of mud, debris and castoreum, a liquid substance excreted through the beaver's castor sacs. Beavers can also recognize their kin by their anal gland secretions and are more likely to tolerate them as neighbors.
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 introduced in South America (Patagonia) and Europe (primarily Finland and Karelia). In Canada and the United States, the species is often referred to simply as "beaver", though this causes 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. The North American beaver is one of the official national wildlife of Canada symbols and is the official state mammal of Oregon and New York.
Before their near-extirpation by trapping in North America, 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. They are widely distributed in boreal and temperate ecoregions, where populations are rebounding from historic over-exploitation. Recently, beaver have been observed colonizing arctic tundra, likely as a result of climate-induced increases in riparian shrubs.
Physician naturalist Edgar Alexander Mearns' 1907 report of beaver on the Sonora River may be the earliest report on the southernmost range of this North American aquatic mammal. However, beavers have also been reported both historically and contemporarily in Mexico on the Colorado River, Bavispe River, and San Bernardino River in the Mexican states of Sonora and Chihuahua.
North American Beaver (Castor canadensis) - seen in many locations in North America, these images are in Washington and Illinois. Note how the below beaver is swimming to distinguish it from Muskrat.
European Beaver (Castor fiber) - seen in Finland and this dam was photographed in Russia at Prioksko-Terrasny Reserve a few hours from Moscow
