Mountain Beaver

The Mountain Beaver (Aplodontia rufa) is a primitive rodent unrelated to beavers and not usually found in mountain areas. It has several common names including Aplodontia, Sewellel, Boomer, Ground Bear, and Giant Mole. This species is the only member of its genus, Aplodontia, and family, Aplodontiidae.


Mountain Beavers are brown in color, but fur can range from slightly more reddish or blackish depending on subspecies. There is a light patch under each ear. The animals have distinctively short tails. Adults weigh from 17.64 to 31.75 oz (500-900 g), with a few specimens topping 35.27 oz (1,000 g). Total length is about 11.81 to 19.69 inches (30-50 cm) with a tail length equal to .39 to 1.57 inches (1-4 cm).

The skull is protrogomorphous. This means that it has no specialized attachments for the masseter muscles as seen in other rodents. It is flattened and lacks a postorbital process. The baculum is thin and distinctly forked. The penis is about 1.77 in (4.5 cm) long. They do not have a true scrotum, but testes move into a position called semi scrotal during the breeding season.

Mountain Beavers have an unusual projection on each molar and premolar. This is unique among mammals and allows for easy identification of teeth. This projection points toward the cheek on the upper tooth row, but points towards the tongue on the lower. The cheek teeth lack the complex folds of other rodents and are instead comprised of a single basin. They have high crown teeth, which are ever growing. Two upper and one lower premolars are present. All three upper and lower molars are also present.

Mountain Beavers cannot produce concentrated urine. They are thought to be physiologically restricted to the temperate rain forest regions of the North American Pacific coast. This is due to their inability to obtain sufficient water in more arid environments.

Habits and distribution

Mountain Beavers are found in coastal areas of the Pacific Northwest of North America. These are usually low elevation regions, but they can occasionally be seen as high as tree line. They can be found in both deciduous and coniferous forests. These animals appear to be physiologically limited to moist regions with minimal snowfall and cool winters. They do not appear to be able to conserve body heat or warmth as efficiently as other rodents. They do not hibernate.

Mountain Beavers build elaborate burrow systems with chambers devoted to fecal and food caches. They eat soft fecal pellets to obtain maximum nutrients. Hard fecal pellets are transferred to fecal chambers using their incisors. Food includes fleshy herbs and young shoots of more woody plants. Ferns probably make up the bulk of the diet. They appear to be strictly vegetarian. Their consumption of seedling trees has led some to consider them a pest. They appear to build hay mounds at some burrow entrances.

A host of other animals have been documented within the burrow system of Mountain Beavers. Because of their effect on such a wide variety of plants and animals, some ecologists consider Mountain Beavers to be keystone species.

Known predators include Bobcats, Coyotes, Cougars, Golden Eagles, and Owls. Among the parasites of the Mountain Beaver is the largest flea known to modern science, Hystrichopsylla schefferi.

The breeding season is between January to March, with 2-3 young born February to April. The young are born hairless, pink, and blind. Longevity is 5-10 years, fairly long as rodents go. They are not social, though home ranges can overlap.

Mountain Beavers are capable of climbing trees, but rarely travel far from burrows. The thumb is slightly opposable. The animals will sit on their hindquarters and manipulate food with their forelimbs and incisors.