Mountain Beaver (also called Sewellel, and Boomer), a chunky, burrowing rodent of the North American Pacific uplands. It is not a beaver, but is distantly related to the squirrels. The mountain beaver is probably the most primitive living rodent, first appearing some 10,000,000 years ago. It is about 12 inches (30 cm) long and has rough, dark-gray fur; small eyes and ears; short legs; and a very short tail. Its home is a complex system of tunnels dug in a damp forest. The animal feeds on vegetation, chiefly at night. It breeds in early spring, two to five furry young being born about seven weeks later.

When Is a “Beaver” Not a Beaver?

A mountain beaver is not a beaver. A mountain beaver doesn’t have a tail, as a beaver does. And a mountain beaver doesn’t build dams, as a beaver does. But a mountain beaver is a rodent. In fact, the mountain beaver is the oldest kind of rodent in the world.

Mountain beavers are found in North America along the Pacific coast and in nearby mountains. There they roam the mountain forests.

Mountain beavers live together in a colony, or large group. Like beavers, mountain beavers love water. They make their homes in tunnels near streams.

The mountain beaver is Aplodontia rufa of the family Aplodontidae.