The Mountain Beaver (Aplodontia rufa) is a primitive rodent that spends the majority of its time underground. Its name is a misnomer as it is not actually a beaver and it prefers lower elevations.
Mountain Beavers are endemic to western North America and extend from southern British Columbia to central California and east to the Cascade and Sierra Nevada Mountains. They are abundant and active year round, yet most people don’t even know that they exist as they are rarely seen out of their burrows.
Did you know? Mountain Beavers are also called "boomers."
Mountain Beavers are stocky burrowing mammals typically between 10 and 18 inches long and weighing from one to three pounds. Their fur is dark brown, they have very short tails and rounded heads with small beady eyes and small ears. Their legs are short but strong and they have long, strong claws on their front feet for digging.
Mountain Beavers have poor hearing and even worse eyesight, but they have well-developed senses of smell and touch.
Did you know? Mountain Beavers have opposable thumbs.
Mountain Beavers prefer dense, moist forests on ferny slopes and are occasionally found in dam ravines in urban areas. They commonly occur shortly after clearcutting of forests and are likely to colonize soft-soiled tracts of land in drainage areas.
Mountain Beavers are powerful diggers, and they build extensive burrow systems with multiple entrances. They dig by scooping soil with the forelimbs and pushing it under their body. They also use their teeth to loosen packed soil and rock. Their burrow system comprises five different compartments: nest, food storage, refuse, fecal pellet and earth ball. The earth balls are thought to be used to plug entrances to the nest and feeding chambers and may serve as a way to sharpen their teeth.
Did you know? Mountain Beaver burrows are used by other species such as moles, voles, weasels, mink and salamanders.
Development and Family Structure
Female Mountain Beavers begin breeding at two years old. Mating takes place in late winter and three to five young are born in the spring. The young stay with their mother throughout the summer, but by autumn they are weaned and set off to establish their own burrows and territories.
An herbivore, the Mountain Beaver feeds on a variety of plants but prefers Sword and Bracken Fern and the cambium of both coniferous and hardwood trees. They are also partial to flower bulbs, tree seedlings and newly sprouted garden plants if the opportunity arises. Food items are usually cut and dragged back to the burrow to be consumed.
Mountain Beavers are predominantly nocturnal, but are occasionally active during the day. They are solitary animals and will defend their burrows from invaders with shrill whistle-like vocalizations and grating of their large incisors.
Did you know? Mountain Beavers are excellent swimmers and tree climbers.
Living with Mountain Beavers
If you live near a moist, forested slope or ravine in Western Washington, there is a good chance that you have Mountain Beavers in your area. Most conflicts that arise between property owners and Mountain Beavers involve the animals burrowing in undesired areas or eating ornamental or garden plants. There are many ways to address conflicts with Mountain Beavers, but exclusion is the most effective way to mitigate the problem in most cases.
To prevent Mountain Beavers from damaging the trunk or branches of young trees, a piece of stovepipe, PVC or sheet metal can be used to encircle the lower trunk. The barrier should be at least 18 inches tall to prevent the animals from climbing over it, and it should be secured in such a way that it can easily be loosened or removed as the tree grows.
Mountain Beavers seem to have a strong affinity for rhododendrons and similar ornamental shrubs. Small, loose wire fencing can be used to prevent Mountain Beavers from accessing these plants. The fence can be constructed using chicken wire, or a similar one-inch mesh wire or heavy plastic attached to metal or wooden stakes. It should be at least two feet high, and it should not be pulled tight between the stakes. If the Mountain Beaver does try to climb the fence, the laxity in the wire mesh will make it much more difficult.
It is also a good idea to build the fence with a slight outward angle (away from the plants) to make it more difficult to climb. A two-foot skirt running along the ground on the outer side of the fence and staked down at the ends will also prevent Mountain Beavers from digging underneath.
It is possible to exclude Mountain Beavers from your yard entirely with a fence like the one described above. It is easiest to do if your yard is bordered on both sides by solid fencing. You simply build the small wire fence between your yard and the Mountain Beavers’ burrow systems and secure to the solid fencing at either end. If your yard already has a traditional wooden or chain-link fence, you can affix the wire or sturdy plastic mesh directly to the existing fencing and extend the two-foot horizontal skirt directly out from the bottom of it to prevent the Mountain Beavers from digging in.
Sharing your space
Simply allowing Mountain Beavers to use your property may be the most desirable course of action. They are interesting, uniquely Northwestern mammals that few people even know exist. If you are fortunate enough to catch a glimpse of these curious little animals as they go about the business of their lives, you may find that they are a welcome addition to the diversity of life in your neighborhood.
Burke Museum. December 10, 2015. Mammals of Washington: Mountain Beaver.
Feldhamer G.A., Thompson B.C. & Chapman J.A. (Eds). (2003). Wild Mammals of North America, Biology, Management, and Conservation (2nd ed.). Baltimore MD: The Johns Hopkins University Press.
Washington Department of Fish and Wildlife. December 10, 2015. Living with Wildlife: Mountain Beavers.