It is found 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.)
This beaver is the largest rodent in North America and the second or third largest rodent in the world, after the South American capybara. The species’ Eurasian counterpart, the European beaver reaches similarly large sizes. Adults usually weigh from 11 to 32 kg (24 to 71 lb), with 20 kg (44 lb) being a typical mass. The head-and-body length is 74–90 cm (29–35 in), with the tail adding a further 20–35 cm (7.9–14 in). Very old individuals can exceptionally exceed normal sizes, weighing more than 40 kg (88 lb) or even as much as 50 kg (110 lb).
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 colours 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.
Before their near extirpation by trapping in North America, beaver were practically ubiquitous and lived from the arctic tundra to the deserts of northern Mexico, and from the Atlantic to the Pacific Oceans. Physician naturalist Edgar Alexander Mearns’ 1907 report of beaver on the Sonora River may be the southernmost extent of the range of this North American aquatic mammal. However, beaver have also been reported both historically and contemporaneously in Mexico on the Colorado River, Bavispe River and San Bernardino River. Explorer David Thompson, after crossing much of North America in 1784, stated that “this Continent…from the Atlantic to the Pacific Ocean, may be said to have been in the possession of two distinct races of Beings, Man and the Beaver.”