Olympic Habitat Development Study


Station 6 -
Mountain Beaver
(Aplodontia rufa)
Family Aplodontiidae

This is a poster child for misleading common names: it’s not a beaver and doesn’t always live in mountains!   Also, it’s a little lonely in its taxonomic neighborhood as the mountain beaver is the only living member of its genus and family.  Considered the most primitive existing rodent, this “living fossil” has shown few changes in the last 40 million years.  Another curiosity about the mountain beaver -- it has the dubious honor of being the exclusive host of the largest flea in the world: (Hystrichopsylla schefferi). This behemoth flea can grow up to 1 cm! 0.4 inches

Although capable of climbing, the mountain beaver (sometimes called a “boomer”) is most often found on the ground, feeding on a great variety of plants and is also known for girdling trees and shrubs, as well as removing newly planted seedlings.   In addition to feeding on many common herbaceous and woody plants, it also feeds bracken fern (Pteridium aquilinum) which is toxic to most other herbivores.

It eats its own feces -in the same fashion as rabbits- and since its kidneys are rather primitive, the mountain beaver has to avoid desiccation, daily consuming up to two thirds its own weight in water, according to some accounts. Probably this is the main reason why this curious animal is found only in the Pacific Northwest and northern California where it is restricted to moist habitats.  It builds extensive borrows, used year around, which are later used by other animal species, for this reason some ecologists consider the mountain beaver a “keystone species”.  Nests are usually located below ground under logs, uprooted stumps, logging slash, or thick vegetation.  Mountain beaver nests are composed primarily of sword fern and salal with the addition of moss to act as a desiccant.

This secretive animal is rarely seen and its presence generally goes unnoticed except for indirect evidence: its tunnels, feeder holes with stacks of vegetation near the entrance (called hay mounds), and damage to vegetation.  These animals can be a pest in gardens and crops. They also can have a significant negative impact in young plantations as they feed on seedlings for several years after planting (climbing small trees and clipping branches and tops of small saplings). Not a social mammal, the mountain beaver is also known for its aggressive behavior when cornered.

Although mountain beavers require moist environments, they can survive forest harvesting as they spend most of their time below ground.  In fact thinning may benefit mountain beavers by providing greater growth of herbaceous vegetation (food) near their burrows.


Mountain beaver. (Image by Wendy Arjo)
Mountain beaver den entrance. Mountain beaver. (Image by Wendy Arjo) Mountain beaver clipping damage on a tree sapling.
Mountain beaver.
(Image by Wendy Arjo)