Mountain beavers are endemic to North America. There are currently seven subspecies recognized and their distribution ranges south from the southerly edge of British Columbia in Canada down through Washington, Oregon, and into central California. Their west-east distribution stretches from the west coast of the United States east to the Sierra Nevada and Cascade Mountain ranges. There are two isolated subspecies populations, A. rufa nigra which occurs in Point Arena in Mendocino County and A. rufa phaea which occurs in Marin County, both are in California. The historical geographic range has changed very little since the late Oligocene. (Carraway and Verts, 1993; Feldhamer, et al., 2003)
Despite their names, mountain beavers don't live in alpine environments, nor are they aquatic, like beavers. Mountain beavers are found mostly in temperate second growth forests and boreal forests in the northern edge of their distribution. They inhabit densely vegetated areas with high annual precipitation. Mountain beavers are highly dependent on deep moist soils to build their extensive burrow systems. Burrows can have up to 30 exits with many tunnels crossing to form T-shaped junctions. The exits are often covered by vegetation or old tree logs. The den site, that contains the mountain beavers nests, has many passages leading into them. The location they choose to build their burrows must have soil with good soil drainage and slope to prevent flooding. Studies have shown that the slope and temperature (elevation as well in Sierra Nevada populations) of mountain beaver den sites are more important than vegetation type and food availability. Food availability seems to have more of an effect on home range size. Mountain beavers located in areas where food is more scarce will have larger home ranges. The tunnels within the burrows are about 13 to 18 cm high and 15 to 25 cm wide. In most of their distribution mountain beavers are associated with northern facing slopes of hills and mountains. There is evidence that mountain beavers may need access to free flowing fresh water nearby their nest sites because small streams are often found running through their burrow systems. Previously this was thought to be due to the mountain beavers inability to concentrate their urine, but recent studies have shown that mountain beavers can easily obtain their daily water requirements from the metabolic water produced in the food they eat. The presence of water nearby burrows is most likely associated with the moist soil and temperature required by mountain beavers for their burrows. (Arjo, et al., 2007; Beier, 1989; Carraway and Verts, 1993; Crocker, et al., 2007; Feldhamer, et al., 2003; Zielinski, et al., 2010)
Mountain beaver are a small,compact, and thickset rodent, with a cylindrical body form. They grow to be 300 to 500 mm in body length, including a 10 to 25 mm furred tail. They have five equal length digits on each foot. Their pollux is opposable and their foot posture is plantigrade. Mountain beavers have blackish dark fur that is similar in both sexes. The fur is very coarse and dense with longer guard hairs interspersed throughout it. The guard hairs on the back and limbs are darker than those on the ventral side. All mountain beavers have a small white spot on the base of each ear. Females have a nearly circular area of brownish black hair about 15mm in diameter, which surrounds each of the three pairs of mammae, this does not occur in males of the species. Mountain beaver will undergo a molt annually, it begins in July or August and lasts for three months. The skull of mountain beavers is both broad and flat. The postorbital process is incomplete. The most unique features of the skull include a flask shaped auditory bullae, a palate that extends posteriorly to the third upper molar, and a mandible with a greatly inflected angular process and relatively high coronoid process. Teeth are hypsodont and grow continuously. The first premolar is small and peglike. Excluding the first premolar, all the cheekteeth have a unique projection that protrudes from each tooth. For the lower cheekteeth the projection points toward the inside of the mouth and for the upper cheekteeth it points toward the inside of the cheek. Their dental formula is 1/1 0/0 2/1 3/3. (Carraway and Verts, 1993; Feldhamer, et al., 2003)
Male mountain beavers bodies will begin to prepare for the breeding season in late December or early January. The testes of the male may become visible and semi scrotal. During the rest of the year the testes are abdominal. Females can enter estrus as early as mid-February or as late as early May. Within populations mating is fairly synchronous. Yearling females can enter estrus but they don't conceive. Parous females (ones that have given birth) can be differentiated from nulliparous females by long, pendant-like nipples. After giving birth placental scars about 1 cm in diameter are visible on the uterus. Young are altricial and can emerge head or tail first. At birth vibrissae, pinnae, and tail are all visible. The eyes open after 7 to 8 weeks. Offspring grow rapidly and are weaned after 2 months but may begin eating vegetation as early as 5 weeks old. (Feldhamer, et al., 2003)
Little is known about mortality rates. Numerous predators have been reported. Habitat management activities such as controlled burns have been reported as an anthropogenic source of mortality. (Feldhamer, et al., 2003)
Mountain beaver are generally solitary creatures but may form groups due to suitable habitat and food resources being in a small area; population densities vary widely because of this. There is some evidence of populations having a sex ratio biased toward males but the reason is unknown. Mountain beavers are nocturnal but may be active during any time of day. Mountain beavers are very faithful to nest sites and have been known to stay at the same one for several years before relocating; they rarely venture far from their nests. Mountain beavers are largely fossorial. Burrows are built by scooping soil with forefeet and pushing it underneath its body; the soil is expelled by scraping with the hindfeet. Most burrowing occurs during summer. They can also climb trees and swim. Haystacking has been observed by mountain beavers. While eating mountain beavers will grasp food with their opposable pollex and squat with hind feet positioned ventrally and soles facing forward. Mountain beavers are autocoprophagous so after feeding they will reingest soft pellets from their feces. The hard fecal pellets are about 1/4 the size of the soft pellets and are thrown into a pit rather than reingested. (Carraway and Verts, 1993; Feldhamer, et al., 2003)
Males have slightly larger home ranges than females and juveniles. Average range sizes are 0.3 hectares for males, 0.2 for females, and 0.1 for juveniles but can be much larger depending on food availability. Ranges may overlap but they defend their nest sites. Dispersing subadults travel far to establish home ranges, sometimes over 500 meters. (Feldhamer, et al., 2003)
Ability to see and hear is limited though tactile and olfactory senses are well developed. The only sounds produced are a soft whining and sobbing sound when in pain, a grating sound with the teeth, and a high pitched squeal when fighting. (Carraway and Verts, 1993)
Mountain beavers are strictly herbivorous and they have a functional cecum. The majority of their diet are ferns. In addition to ferns they have been known to eat other types of vegetation such as conifers, grasses, forbs, hardwoods, mosses, and shrubs. The type of vegetation they consume is mostly tied to their condition and the season. Diets change slightly during the summer with males and juveniles consuming more grasses to gain more protein to grow. Females will switch to eating more conifers and forbs while they are lactating. Mountain beavers consume many plants that are toxic to other animals and they may specialize on this. One of the main plants eaten by mountain beavers, bracken fern, is toxic to many animals. Other toxic plants consumed include: rhododendron, devil's club, and stinging nettle. Non-toxic plants eaten are: vine maple, salmonberry, blackberry, red huckleberry, red alder, bigleaf maple, cascara, hazel, ocean spray, and willow. They will collect more vegetation than they actually consume, storing the rest in piles. Much of the vegetation decays before it is eaten. (Carraway and Verts, 1993; Feldhamer, et al., 2003)
There are many predators of mountain beavers. Coyotes are noted as a significant predator. (Feldhamer, et al., 2003)
Many animals will use the old burrow systems left by mountain beavers for shelter. The extensive digging to build the burrows helps to aerate soils which promotes plant growth. Mountain beavers are important host species for many parasites. 29 different species of parasite have been known to use this species as a host. Many of the ectoparasites are mites and fleas, with one beetle. Interestingly mountain beavers are the specific host for the world's largest flea, the mountain beaver flea. The flea's distribution and life history is entirely dependent on the mountain beaver. Endoparasites include two parasitic flatworms of class Cestoda. (Carraway and Verts, 1993; Feldhamer, et al., 2003)
There are no known positive effects of mountain beavers on humans.
Mountain beavers can cause extensive damage to conifer seedlings and saplings, resulting in great economic losses for timber companies. Over 300,000 acres of land in western Washington and Oregon have been damaged, with these lands estimated to have a timber value of over $10,000 an acre. Increased need for weeding and maintenance can add to the costs. The damage can have effects lasting up to 20 years. Consequently mountain beavers have now become a pest to many forest managers because of their interference with forest restoration. The best method implemented to limit damage is a physical barrier around the tree saplings to prevent mountain beavers from cutting them. Poisoning of mountain beaver populations is another common method for controlling damage to forests. Other methods include using repellents sprayed on the plants or physically removing the effect plants. Occasionally they may cause damage to gardens and crops. (Campbell, 1994; Carraway and Verts, 1993; Feldhamer, et al., 2003)
The subspecies Aplodontia rufa nigra is protected under the Endangered Species Act as of 1991. (Fellers, et al., 2008)
The genus name Aplodontia is derived from the Greek roots haploos, which means simple, and ondontos, which means tooth. Therefore the genus name roughly translates to "simple tooth". The species name rufa is derived from the Latin rufus, which means reddish. Other common names for the mountain beaver include: boomer, sewellel, farmer, gehalis, and ground bear. The family that the mountain beaver belongs to, Aplodontiidae, is one of only two families endemic to the Nearctic ecoregion, the other being pronghorns Antilocapridae. (Carraway and Verts, 1993; Cole, et al., 1994)
Benjamin Hurlock (author), University of Alaska Fairbanks, Laura Prugh (editor), University of Washington, Tanya Dewey (editor), University of Michigan-Ann Arbor.
living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.
young are born in a relatively underdeveloped state; they are unable to feed or care for themselves or locomote independently for a period of time after birth/hatching. In birds, naked and helpless after hatching.
having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.
uses smells or other chemicals to communicate
having markings, coloration, shapes, or other features that cause an animal to be camouflaged in its natural environment; being difficult to see or otherwise detect.
animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.
parental care is carried out by females
an animal that mainly eats leaves.
forest biomes are dominated by trees, otherwise forest biomes can vary widely in amount of precipitation and seasonality.
Referring to a burrowing life-style or behavior, specialized for digging or burrowing.
An animal that eats mainly plants or parts of plants.
offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).
having the capacity to move from one place to another.
the area in which the animal is naturally found, the region in which it is endemic.
active during the night
communicates by producing scents from special gland(s) and placing them on a surface whether others can smell or taste them
breeding is confined to a particular season
reproduction that includes combining the genetic contribution of two individuals, a male and a female
digs and breaks up soil so air and water can get in
places a food item in a special place to be eaten later. Also called "hoarding"
uses touch to communicate
that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).
Living on the ground.
reproduction in which fertilization and development take place within the female body and the developing embryo derives nourishment from the female.
Arjo, W., Huenefeld, Nolte. 2007. Mountain beaver home ranges, habitat use, and population dynamics in Washington. Canadian Journal of Zoology, 85: 328-337.
Beier, P. 1989. Use of habitat by mountain beaver in the Sierra Nevada. Journal of Wildlife Management, 53: 649-654.
Campbell, D. 1994. "Mountain Beavers and Control of Mountain Beaver Damage" (On-line). Internet Center for Wildlife Damage Management. Accessed November 30, 2014 at http://icwdm.org/handbook/rodents/mountainbeaver.asp.
Carraway, L., B. Verts. 1993. Aplodontia rufa. Mammalian Species, 431: 1-10.
Cole, F., D. Reeder, D. Wilson. 1994. A synopsis of distribution patterns and the conservation of mammal species. Journal of Mammalogy, 75: 266-276.
Crocker, D., N. Kofahl, G. Fellers, N. Gates, D. Houser. 2007. High Rates of Energy Expenditure and Water Flux in Free-Ranging Point Reyes Mountain Beavers. Physiological & Biochemical Zoology, 80: 635-642.
Feldhamer, G., B. Thompson, J. Chapman. 2003. Wild Mammals of North America: biology, management, and conservation. Baltimore, MD: John Hopkins University Press.
Fellers, G., W. Lidicker Jr., A. Linzey, D. Wiliams, G. Hammerson. 2008. "Aplodontia rufa" (On-line). The IUCN Red List of Threatened Species. Accessed December 04, 2014 at http://www.iucnredlist.org/details/1869/0.
Zielinski, W., J. Hunter, R. Hamlin, K. Slauson, M. Mazurek. 2010. Habitat characteristics at den sites of the Point Arena mountain beaver (Aplodontia rufa nigra). Northwest Science, 84: 119-130.