Creature Feature: Mountain Beaver

April, 2017


Aplodontia rufa

The first thing you should know about the mountain beaver (Aplodontia rufa) is that it is not a true beaver. The mountain beaver is the sole surviving member of the superfamily Aplodontoidea and is a distant relative of the squirrel (Sciuridae) family. Like their namesake, however, the mountain beaver gnaws on bark, limbs, and roots, often killing young trees near their nests. Like rabbits, the mountain beaver also often eats its nutrient rich feces, a practice known as coprophagia. 

The mountain beaver is considered a living fossil and is the world’s most primitive rodent, having undergone very little morphological change in several million years. Its ancient lineage suggests that the rodent is still perfectly suited for our forests. It is small and stout, about 13 inches from its whiskered head to its stubby tail and weighs between two and three pounds.

Mountain beavers are found from the Cascade Mountains of British Columbia all the way to the southern Sierra Nevada mountains. While they range from the coast to the tree lines, the mountain beaver appears to prefer areas without much snowfall or cold temperatures, as the critter does not hibernate and does not appear to conserve body heat as well as other rodents. 

Mountain beavers are seldom seen due to their largely subterranean lifestyle. Equipped with long, sharp claws for digging, mountain beavers construct a complicated burrow system throughout their territory, which can be two acres or larger. Tunnel systems tend to radiate from a nest with multiple specialized chambers (used for storing food and nesting) and have as many as ten exits.  

Also known as the mountain boomer for its loud bark-like chattering, the mountain beaver is bold and brave. Researchers are careful in handling them because of their sharp bite. Mountain beavers often live a solitary life, procreating through brief trysts rather than prolonged romances.  

The mountain beaver is also host to a notable parasite, the mountain beaver flea (Hystrichopsylla schefferi). The mountain beaver flea, living only on the poor mountain beaver, is also an ancient relic. The flea is unique for its massive size, up to a half-inch long and violates general rules of evolution. Cope’s Rule, while much disputed, suggests that more ancient groups tend to be smaller. Berggman’s Rule, also disputed, suggests that bigger bodies tend to be associated with colder habitat. However, the flea is both ancient and at home on the warm bodies of mountain beavers in the temperate forests of the west, violating both of these rules. 

Though the mountain beaver is not a protected species, the Point Arena subspecies, which exists solely in a 24-square-mile area in western Mendocino County, has been listed as endangered since 1991. Humboldt County has its own subspecies, Aplodontia rufa humboldtiana, although reports generally consider this subspecies to be more stable than its Mendocino cousin. The timber industry considers mountain beavers pests due to their snacking on young saplings and regularly trap or poison them to control populations. 

If you are trying to spot a mountain beaver, look for wet conifer forests. Try to find a somewhat younger forest or one that was recently logged and replanted with delicious young saplings. Next, look for damage to young saplings, herbs, and ferns. Mountain beavers, like other rodents, typically gnaw off growth at a 45-degree angle (compared to the relatively flat cuts of deer and elk). You can also look for tracks—little rodent footprints with long toes.

Burrows are often camouflaged under stumps or in patches of ferns. Once you find a potential burrow, settle down and watch. Mountain beavers are sensitive to vibrations and they likely felt you approaching. Be silent and still, and you may see one.

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