Beavers are semi-aquatic rodents native to North America and Europe. They are the only living members of the family Castoridae, which contains a single genus, Castor. Genetic research has shown the European and North American beaver populations to be distinct species and that hybridization is unlikely.
Beavers are best known for their natural trait of building dams in rivers and streams, and building their homes (known as beaver lodges) in the resulting pond. They are the second-largest rodent in the world (after the capybara). They are also known for their "danger signal": when startled or frightened, a swimming beaver will rapidly dive while forcefully slapping the water with its broad tail. This creates a loud 'slap', audible over large distances above and below water. This noise serves as a warning to other beavers in the area. Once a beaver has made this danger signal, all nearby beavers will dive and may not reemerge for some time.
Beavers have webbed hind-feet, and a broad, scaly tail. They have poor eyesight, but a keen sense of hearing, smell, and touch. Beavers continue to grow throughout life. Adult specimens weighing over 25 kg (55 lb) are not uncommon. Females are as large as or larger than males of the same age, which is uncommon among mammals.
The word beaver is descended from the Proto-Indo-European name of the animal, cf. Sanskrit babhru's, brown, the great ichneumon, Lat. fiber, Ger. Biber, Swed. bäver, Russ. bobr'; the root bhru has given "brown," and, through Romanic, "bronze" and "burnish."
Beavers are closely related to squirrels (Sciuridae), agreeing in certain structural peculiarities of the lower jaw and skull. In the Sciuridae the two main bones (tibia and fibula) of the lower half of the leg are quite separate, the tail is round and hairy, and the habitats are arboreal and terrestrial. In the beavers or Castoridae these bones are in close contact at their lower ends, the tail is depressed, expanded and scaly, and their habitats are aquatic. Both European and American beavers grow to about 2 feet long (plus 10 inches of tail). They are essentially aquatic in their habits, never travelling by land unless driven by necessity. They are crepuscular rather than nocturnal, which means they are active at dawn and dusk, and subsist chiefly on bark and twigs or the roots of water plants.
