The mountain beaver ( Aplodontia rufa)[Other names include boomer, mountain boomer, ground bear, giant mole, gehalis, sewellel, suwellel, showhurll, showtl, and showte, as well as a number of other Native American terms. "Mountain beaver" is a misnomer as the animal is not a true beaver ( Castor) and it is not restricted to mountains. "Boomer" refers to the loud vocalizations that these usually-solitary animals make when in social situations, but this has not been recorded nor verified. Lewis and Clark originally called the animal "sewellel", a misunderstanding of the Chinook word "she-wal-lal", the name for garments made from the skin of the creature. See Borrecco and Anderson, 1980.] is a North American rodent. It is the only living member of its genus, Aplodontia, and family, Aplodontiidae.[Piaggio, A. J., B. A. Coghlan, A. E. Miscampbell, W. M. Arjo, D. B. Ransome, and C. E. Ritland. 2013. Molecular phylogeny of an ancient rodent family (Aplodontiidae). Journal of Mammalogy 94(3): 529-543.] There are seven subspecies of mountain beaver, six of which are found in California and three of which are endemic to the state.
Spelling and etymology
The name
Aplodontia ("simple tooth") is in reference to the single large basin comprising the bulk of each
cheek tooth. The specific epithet,
rufa, means red or reddish.
Most references use the spelling " Aplodontidae" for the family name. This has been deemed incorrect due to the technical rules of converting a genus name into a family name. The proper conversion of Aplodontia to a family name is to drop the - a only and add -. Thus, Aplodontiidae is technically correct. This spelling is gaining acceptance in modern texts and is the standard spelling currently recognized by the Integrated Taxonomic Information System.
Alternate spellings of the genus name have also been reported, with as many as 30 variants historically. These include " Haplodontia", " Haplodon", " Aploodontia", " Apluodontia", and " Aplodontie", among others.
Taxonomy and phylogeny
Originating in the late Oligocene, the mountain beaver has retained a number of primitive characteristics that make it a
living fossil. In particular, it is the only surviving species to retain the
Eocene-era protrogomorph chewing mechanism (zygomasseteric system).
In the protrogomorphous condition, the medial masseter muscle does not pass through the infraorbital foramen as it does in and mouse. Likewise, the lateral masseter muscle attaches to the base of the zygomatic arch and does not extend to the region in front of the eye as is seen in and mice.
In addition to its skull and jaw, the mountain beaver's kidneys are notably primitive. Their poor renal function is considered one of the key limits on the species' geographic distribution.
The mountain beaver was once thought to be related to the earliest protrogomorphous rodents, such as the ischyromyids ( Paramys). Both molecular and morphological phylogenetics have recently suggested a more distant relationship to these animals.
Molecular studies have revealed the Sciuridae (squirrels) clade as the mountain beaver's closest living relative.
Subspecies
At present, seven
subspecies of
Aplodontia rufa are recognized:
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A. r. californica (Peters, 1864): distributed throughout the Sierra Nevada in Northern California and extreme western Nevada
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A. r. humboldtiana Taylor, 1916: restricted to the far northwestern coast of California
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A. r. nigra Taylor, 1914: restricted to a small region in southern Mendocino County, California
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A. r. pacifica Merriam, 1899: distributed across coastal Oregon
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A. r. phaea Merriam, 1899: found mostly in Point Reyes, northwest of San Francisco, California
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A. r. rainieri Merriam, 1899: found across the Cascade Range from southern British Columbia to southern Washington
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A. r. rufa (Rafinesque, 1817): found along coastal Washington, all the way down through Oregon to Northern California
Description
Mountain beavers are usually dark gray or brown, but their fur can range from slightly more reddish (rufa) to more blackish depending on subspecies, with a light patch under each ear. The animals have distinctively short tails. Adults weigh about , with a few specimens topping . Total length is about , with a tail length of . Their superficial similarity with true beavers reflects only their relatively large size (for rodents), strong odor, preference for living in extremely watery / moist habitats, and propensity to gnaw bark and cut branches. Mountain beavers do not fell trees, build dams, live in lodges, or communicate by slapping their tails (their tail is minute). They are predominantly nocturnal and crepuscular in above ground activities.
[Godwin, A. J. 1964. A review of the literature on the mountain beaver. USDI, Fish and Wildlife Serv., Washington, D.C. Spec. Sci. Rep. Wildl. No. 78. 52 pp.] They are known to climb trees a few meters to acquire food in the form of branches and leaves, but otherwise their diet consists mostly of ferns, especially species that are toxic to other animals.
The skull is protrogomorphous;[Druzinsky, Robert E. " Functional anatomy of incisal biting in Aplodontia rufa and sciuromorph rodents–Part 1: masticatory muscles, skull shape and digging." Cells Tissues Organs 191.6 (2010): 510-522.] it has no specialized attachments for the as seen in other rodents. It is flattened and lacks a postorbital process.
Mountain beavers have an unusual projection on each molar and premolar tooth, which is unique among mammals and allows for easy identification of teeth. This projection points toward the cheek on the upper tooth row, but points toward the tongue on the mandible. The cheek teeth lack the complex folds of other rodents and instead consist of single basins. They are hypsodont and ever-growing. Two upper and one lower premolars are present, along with all the molars, giving a dentition of [citation needed]
Mountain beavers cannot produce concentrated urine. They are thought to be physiologically restricted to the temperate rain forest regions of the North American Pacific coast and moist microenvironments inland due to their inability to obtain sufficient water in more arid environments.
Habitat and distribution
Mountain beavers are found in the Cascade Mountains of
British Columbia and southward including the rest of the
Cascade Range in the United States,
the Olympic Mountains and Coast Ranges of Washington and Oregon,
plus the Klamath Mountains, and the Sierra Nevada, Point Arena and
Point Reyes of
California and extreme western
Nevada.
["Mountain Beaver" in The New Encyclopedia of Mammals. David MacDonald (ed). Oxford University Press, 2001. Pages 596–597.] They range from sea level to the
tree line.
They can be found in both
Deciduous forest and coniferous forests, but throughout most of the range they appear to prefer the former.
Behavior
Mountain beavers are capable of climbing trees,
[Lloyd Ingles. (1960). Tree Climbing by Mountain Beavers. Journal of Mammalogy, vol. 41, no. 1, (Feb., 1960), pages 120–121.] but rarely travel far from burrows. Their
are slightly
opposable thumb and the animals will sit on their hindquarters and manipulate food with their forelimbs and incisors.
Mountain beavers are asocial and generally do not live in shared burrows. Burrows usually consist of a network of tunnels built in deep soil. The entrances to these burrows often contain clumps of wilted vegetation which the animal likely uses as a kind of food cache as well as a source of nesting material. Mountain beavers seldom travel more than a few meters from their burrow entrances, taking advantage of the protection such burrows offer from predators, such as cougars and owls (though skunks and weasels that also occupy mountain beaver burrows and tunnels may take nestlings as food).
[ They appear to build hay mounds at some burrow entrances, but whether this behavior is related to water regulation, curing food, or gathering nest materials is debated. Mountain beavers are partially nocturnal, spending more time active at night than the day but still active during both periods.]
Diet
They exhibit coprophagy and eat soft fecal pellets to obtain maximum nutrients; hard feces pellets are transferred to fecal chambers located within the burrow system.[ Their consumption of seedling trees has led some to consider them as pests.
]
Ecology
Known predators include , , , cougars, , and .
Among the parasites of the mountain beaver is the largest flea known, Hystrichopsylla schefferi. Females of this flea can be long.
Mountain beavers are considered pests in areas of the Pacific Northwest because of the extensive damage they cause to forest trees due to basal-grinding (removal of bark), branch cutting, or clipping small sapling and seedlings.
Breeding
The breeding season is between January and March, with two or three young born February to April. The ovulation period lasts a few weeks and gestation lasts for a month.[ While mountain beavers are generally solitary creatures, they may form groups due to suitable habitat and food resources being in a small area; population densities vary widely because of this.]
Status and conservation
The IUCN lists the mountain beaver as a species of least concern. However, one subspecies, the Point Arena mountain beaver ( A. r. nigra) of California, is considered endangered by the U.S. Fish and Wildlife Service,[
]
Notes
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Adkins, R. M., E. L. Gelke, D. Rowe, and R. L. Honeycutt. 2001. Molecular phylogeny and divergence time estimates for major rodent groups: Evidence from multiple . Molecular Biology and Evolution, 18:777–791.
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McKenna, Malcolm C., and Bell, Susan K. 1997. Classification of Mammals Above the Species Level. Columbia University Press, New York, 631 pp.
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Nowak, R. M. 1999. Walker's of the World, Vol. 2. Johns Hopkins University Press, London.
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