Mountain Beavers (Aplodontidae)
Medium-sized burrowing rodent with a stocky body, blunt nose, and short stubby tail; small ears, short legs, and long claws; dark brown pelage with sparse coarse guard hairs, and small white patch below the ears
Body length average 14.3 in (364 mm), range 9.3–19.7 in (238–500 mm); tail length average 1.2 in (29.8 mm), range 0.4–2.2 in (10–55 mm); weight average 2.6 lb (1,158 g), range 1.8–4.0 lb (800–1,800 g)
Number of genera, species
1 genus; 1 species
Temperate forest with dense understory of herbs and shrubs
Lower Risk/Near Threatened
Western North America
Evolution and systematics
The mountain beaver, or sewellel (Aplodontia rufa), is not related to the beaver but is in its own distinct family. Classification of suborders of rodents is based, in part, on the anatomy of the jaw muscles. In early classification schemes, three suborders are recognized. Alternative schemes recognize two suborders, with no assignment given to several families whose lineage is uncertain. Regardless of which scheme is used, Aplodontidae are substantively different from other families of rodents, and considered the most ancient of living rodents. Their origins trace back 50 million years to the Eocene. They and related groups flourished through to the Miocene and once occupied parts of Asia, Europe, and western North America. Today, the family is represented by a single species, mountain beaver, confined to a small portion of western North America.
The taxonomy for this species is Aplodontia rufa (Rafinesque, 1817), Oregon, United States.
The mountain beaver has a robust, cylindrical body and is approximately the size of a muskrat without its tail. It is adapted for burrowing with a rounded head, blunt nose, long vibrissae, short ears and small eyes. The furred tail is so short that at first glance the animal appears to be tailess. The pelage is dark brown with coarse but sparse guard hair and lighter brown thick underfur. There is a small white patch of short fur at base of the ears. The underparts are slightly lighter than the back, except on pregnant and lactating females where three pairs of mammae are surrounded by zones of darker fur. The legs are short and strong for digging. The front and hind feet have five toes terminating in long strong claws, except the pollex (thumb), which has a short stubby claw and is somewhat opposable.
Mountain beavers inhabit forestlands along the Pacific Coast of North America, from sea level to as high as 7,000 ft (2,000 m) in subalpine forests. They range south from 50°N in southwestern British Columbia through western Washington and Oregon to northern California where inland they extend further south to 36°40'N along the Sierra Nevada Mountains. Two disjunct populations occur on the coast of California north of San Francisco Bay.
Habitats used by mountain beavers vary, but are typically forests with dense patches of herbs and shrubs. This vegetation supplies not only forage, but also security and thermal cover. On forested slopes, mountain beaver are frequently found in thickets of diverse vegetation near streams. In closed canopy forests, mountain beaver tend to be associated with canopy gaps created by disturbances such as root-rot and windfall. These openings allow light to penetrate the forest and promote growth of herbs and shrubs. The fallen trees form the main structures beneath which mountain beaver construct tunnels and runways to access foraging sites in their home range. Mountain beavers readily occupy early seral stages after logging as long as there is sufficient cover and food.
Mountain beavers do not hibernate and so are active throughout winter. They are primarily nocturnal but are frequently active for short periods during the day. They occur in concentrations sometimes referred to as colonies. However, they are not colonial but rather solitary, and the concentrations are likely simply sites of suitable habitat that form population foci. Mountain beaver have small home ranges (average males 0.7 ac [0.3 ha]; females 0.5 ac [0.2 ha]) and, within them, the animal excavates a burrow system with extensive runways. Generally, the burrow system is associated with structures like fallen logs, root wads, or large rocks. A log can act as the main route for an arm of the tunnel system. From this main arm, accessory tunnels extend to favored feeding sites or other structures. These traveling tunnels are generally near the surface and occasionally the roof collapses, exposing the subterranean runway. The burrow system leads deeper to separate underground chambers containing the nest, food larders, and refuse piles. The 1.6-ft (0.5-m) diameter nest chamber contains dry vegetation for insulation. Nearby are smaller feeding chambers where mountain beaver store food plants collected during forays aboveground. Smaller refuse chambers contain decaying vegetation left over from feeding. Other small chambers contain fecal pellets. Besides these, an earth ball chamber can occur containing stones or balls fashioned from soil, thought to be used to seal portions of the living quarters.
Feeding ecology and diet
Mountain beavers are herbivores with a large cecum, a diverticulum from the junction of the small and large intestine, in which microbes digest much of the vegetation. They are
also copraphagic, consuming their excreted soft pellets, a type of feces. This behavior allows digestion of microbial protein and other nutrients remaining in the soft pellets. Mountain beavers travel along their tunnels where they exit and gather food on the surface. During spring and summer, various ferns, grasses, and herbs are eaten along with some leaves and shoots of shrubs. During winter, their diet shifts to ferns, shrubs, and conifers. Mountain beaver readily climb 6.5 ft (2 m) or higher in trees to clip small branches; some saplings appear as if they have been severely pruned. Also during winter, mountain beavers will feed on the bark of young conifers, sometimes girdling trees. Most of their moisture is supplied in their food, but additional water is generally required because mountain beaver have inefficient kidneys and cannot concentrate urine to the extent as other rodents. The tunnels are strategically located so the foraging animal need move only short distances from the safety of their burrows. They cut vegetation and place it at their burrow entrances. Although sometimes food is eaten on the surface, more often it is transported underground for later consumption.
In the central portion of the mountain beaver's range, males are in breeding condition from January through March, whereas most females are in estrus during mid-February. Only females two years or older breed and, after a 28- to 30-day gestation period, parturition occurs in the nest chamber from
late March through early April. Only one litter is produced per year and, although females have three pairs of mammae, typically between two to three young are born, though litter size can vary up to six. Young are weaned at 6–8 weeks old and, a week or two later, they leave the nest chamber to begin surface activities.
The mountain beaver is listed as Lower Risk/Near Threatened as a species. However, two of its seven subspecies (A. r. phaea and A. r. nigra) are listed as Vulnerable by IUCN, in part because of habitat loss and also because these are small isolated populations in California disjunct from the principal range of the species. The U. S. Fish and Wildlife Service lists A. r. nigra as Endangered. Elsewhere, the mountain beaver is secure, but local extirpations can occur. From a jurisdictional perspective in Canada, A. r. rufa could be considered threatened because it has been lost from most of its range in British Columbia due to urbanization and agriculture. However, from a broader geographic perspective, this could be regarded as little more than an emargination of its range in the Pacific Northwest.
Significance to humans
The mountain beaver is significant because it is the most ancient living rodent. Its peculiar kidney is physiologically interesting because of its inability to concentrate urine. At times, the mountain beaver can be a garden pest, but they are more commonly forest pests. They cut newly planted conifer seedlings and remove bark, thus girdling stems and causing root damage on trees up to 20 years old. Such damage can be economically substantial and can diminish forest productivity for timber values.
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Alton S. Harestad, PhD
David M. Shackleton, PhD