mountain beaver, stout, short-limbed North American rodent, Aplodontia rufa, not closely related to the true beaver. Also called sewellel beaver after the Chinook word for a robe made from its pelts, it is among the most primitive of the rodents and the only living member of its family. The mountain beaver is about 12 in. (30.5 cm) long, grayish or brownish red in color, and nearly tailless. With small eyes and ears and a blunted muzzle, it resembles a tailless muskrat. Its enlarged claws make it an excellent burrower, and it is also a good swimmer and tree climber. Generally nocturnal, the mountain beaver is found along the Pacific coast from British Columbia to California. Inhabiting damp, wooded country near streams, the rodent eats bark, leaves, and twigs. It builds complex colonial burrows with chambers for food storage, sleeping, and nesting. The location of mountain beavers is in part explained by the fact that they cannot adequately regulate the temperature of their bodies and must therefore live in stable, cool, moist environments. Mountain beavers are classified in the phylum Chordata, subphylum Vertebrata, class Mammalia, order Rodentia, family Aplodontidae.