Mountain Beaver: Aplodontidae

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This animal is also commonly called sewellel, named after the Chinook (American Indian tribe) word for a robe made from its pelts. There is only one species of mountain beaver and they are not closely related to the true beaver. These animals are about the same size as a squirrel, with a head and body length of 14.3 inches (36 centimeters), and a tall length of approximately 1.2 inches (3 centimeters).

They have a thickset, heavy body and short limbs. Eyes and ears are small. The head is broad and relatively flat. The neck is short and thick. All the limbs have five well-developed claws. These animals appear nearly tail-less because the tail is so short. They have strong incisors (chisel-shaped teeth at the front of the mouth).

The fur on these animals is thick, short, and typically a grayish, dark brown or reddish brown color, with sparse guard hair, which are coarse hairs that form the outer fur. Lighter, thick fur lies underneath, which is called the underfur. Guard hairs protect the underfur. Its belly is a slightly paler color, a white or chestnut brown. There is a small white patch of short fur at the bottom of its ears.


Mountain beavers are found in North America along the Pacific Coast. They live in southwestern British Columbia to northwestern California, in certain coastal areas as far south as San Francisco Bay, and in the Sierra Nevada of eastern California.


Mountain beavers generally live in moist forests, especially near streams, which are dense with herbs and shrubs. They are found on mountains with deciduous forest to areas at sea level, and also in coniferous forests. Mountain beavers must live in places with deep soils so that they can burrow (dig holes or tunnels).


Mountain beavers are herbivores (plant-eaters) and feed on almost any plant material. These animals eat leaves, branches, bark, and twigs. They also drink large amounts of water.


These animals spend much of their time along the banks of rivers and streams. They frequently wash themselves by dipping their front feet into the water and then scrubbing their body. These animals are strong swimmers.

Mountain beavers live alone or in small colonies. They may live in the same area as other mountain beavers that are sometimes referred to as colonies (groups). The concentration of these animals is most likely due to the fact that the colony sites make good habitats.

These animals have small home ranges, about 0.6 acres (0.25 hectares). Within this range mountain beavers build complex burrows with chambers for food storage, sleeping, and shelter. The burrows are long and close to the surface. The majority of a mountain beaver's time is spent in the underground burrows. They emerge only to forage or during the brief period of time when the young animals leave the nest to establish their own burrow sites. Other animals may also use their burrow system. The tunnels are cleaned and worked on regularly. If a tunnel is flooded by rain, the mountain beaver will swim in it.


The largest flea in the world, the rare Hystricopsylla schefferi, is known from collections plucked from mountain beavers and their burrows. These fleas can grow up to one-third of an inch (9 millimeters) in length!

Mountain beavers are primarily nocturnal, active at night. They are occasionally active for short periods of time during the daytime, especially in the autumn. When foraging for food, they seldom wander more than a few feet (meters) from their burrow. Although food is sometimes eaten above ground, it is generally brought to the burrow. It cuts off the plants desired and drags it to the mouth of the burrow. The food is placed over some logs or some rocks to wilt, then is either stored or eaten. It eats holding its food in its front feet like a raccoon.

While not a great climber, the mountain beaver climbs shrubs and small trees to cut off small limbs and twigs. It cuts off the branches as it climbs. Occasionally, it will let the small limbs and twigs drop to the ground. More typically, the mountain beaver will carry the wood down by climbing down the tree headfirst.

Mountain beavers do not hibernate (slow down their body temperature to conserve energy) and are active year round. In the cooler months they rarely appear above ground and at this time, eat supplies of stored food. In the winter when vegetation is sparse, the beavers will eat bark and small twigs.

Mountain beavers have a brief breeding season. Pregnant females have been collected from late February to early April. Gestation (length of pregnancy) typically lasts twenty-eight to thirty days. Females generally have one litter per year, bearing two or three offspring, and rarely four. Newborns' eyes are tightly closed and may not open fully until about fifty days later. After about eight weeks, offspring are nearly half-grown and able to leave the nest. Offspring reach sexual maturity late in the second year of life.


Humans have caused this family to decrease in population by destroying its natural habitat through development and other activities. In the Pacific Northwest, mountain beavers are considered a pest by many foresters and gardeners because they eat seedlings and young trees. They can also cause damage to trees by peeling off the bark. To prevent damage to their crops and gardens, people may use herbicides (substance used to kill or control plants) and traps, factors that contribute to the decline of mountain beavers.


The World Conservation Union (IUCN) lists the mountain beaver as Near Threatened, not currently threatened, but could become so. Two of the seven subspecies of mountain beavers are listed as Vulnerable (facing a high risk of extinction in the wild) by the IUCN.



Clutton-Brock, Juliet, and Don E. Wilson, eds. Smithsonian Handbooks: Mammals. New York: Dorling Kindersley Publishing, 2002.

Macdonald, David, ed. The Encyclopedia of Mammals. New York: Facts on File Publications, 1984.

Nowak, Ronald M. "Sewellel or Mountain Beaver." Walker's Mammals of the World 5.1 Online. Baltimore: Johns Hopkins University Press, 1997. (accessed on May 3, 2004).


Drew, Lisa. "Creatures that Time Forgot." National Wildlife (June– July, 2002).

Valadka, Andrius. "Meet One of Nature's Survivors: The Mountain Beaver (Aplodontia rufa) is the World's Oldest Living Rodent." Nature Canada (Summer 1988): 6–7.

Web sites:

Newell, T. "Aplodontia rufa." Animal Diversity Web. (accessed on May 17, 2004).

Altig, Ron. "Mountain Beaver: Aplodontia rufa." (accessed on May 15, 2004).

Landes, Charles. "The Mountain Beaver (Aplodontia." Mount Rainier Nature News Notes (Nature Notes). (accessed on May 15, 2004).

"Lewis and Clark Expedition: Scientific Encounters." National Park Service. (accessed on May 13, 2004).

"Mountain Beaver." American Zoo. (accessed on May 13, 2004).

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Mountain Beaver: Aplodontidae

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Mountain Beaver: Aplodontidae