Mountain beaver
The mountain beaver is a North American rodent. It is the only living member of its genus, Aplodontia, and family, Aplodontiidae. It should not be confused with true North American and Eurasian beavers, to which it is not closely related; the mountain beaver is instead more closely related to squirrels. 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 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.In the protrogomorphous condition, the medial masseter muscle does not pass through the infraorbital foramen as it does in guinea pigs and mice. 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 squirrels and mice. Although modern blesmols also share the mountain beaver's primitive, protrogomorphous chewing mechanism, this trait appears to have newly re-evolved in that family.
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. Both molecular and morphological phylogeneticists have recently suggested a more distant relationship to these animals.
Molecular studies have revealed the Sciuridae clade as the mountain beaver's closest living relative. According to the fossil record, the Aplodontidae clade split from the squirrels in the Middle or Late Eocene as indicated by the extinct genera Spurimus and Prosciurus.
Subspecies
At present, seven subspecies of Aplodontia rufa are recognized:- A. r. californica : distributed throughout the Sierra Nevada in Northern California and extreme western Nevada
- A. r. humboldtiana Taylor, 1916: restricted to the far northwestern coast of California
- A. r. nigra Taylor, 1914: restricted to a small region in southern Mendocino County, California
- A. r. pacifica Merriam, 1899: distributed across coastal Oregon
- A. r. phaea Merriam, 1899: found mostly in Point Reyes, northwest of San Francisco, California
- A. r. rainieri Merriam, 1899: found across the Cascade Range from southern British Columbia to southern Washington
- A. r. rufa : found along coastal Washington, all the way down through Oregon to Northern California
Description
The skull is protrogomorphous; it has no specialized attachments for the masseter muscles as seen in other rodents. It is flattened and lacks a postorbital process. The baculum is thin and distinctly forked. The penis is about in length. The male does not have a true scrotum, but the testes move into a position called semiscrotal during the breeding season.
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 lower. 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 dental formula of.
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. It is thought that Aplodontia prefer vegetation high in water content due to their poor ability to concentrate urine which makes it necessary to consume large amounts of water daily.
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 Pt. Reyes of California and extreme western Nevada. They range from sea level to the tree line. They can be found in both deciduous and coniferous forests, but throughout most of the range they appear to prefer the former.Behavior
Mountain beavers are capable of climbing trees, but rarely travel far from burrows. Their thumbs are slightly opposable 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. 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. They actively feed in periods of up to 2 hours and 45 minutes throughout the day with 6 or 7 periods of feeding per day. In total they forage for up to 9 hours per day. These animals are physiologically limited to moist microenvironments, with most subspecies occurring only in regions with minimal snowfall and cool winters. They do not appear to be able to conserve body heat or warmth as efficiently as other rodents, nor do they hibernate.
Diet
They exhibit coprophagy and eat soft fecal pellets to obtain maximum nutrients; hard fecal pellets are transferred to fecal chambers located within the burrow system. Food includes fleshy herbs and young shoots of more woody plants. Ferns probably make up the bulk of their diets. They appear to be strictly herbivorous. Their consumption of seedling trees has led some to consider them as pests.Ecology
Known predators include bobcats, coyotes, weasels, cougars, golden eagles, and owls.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, branch cutting, or clipping small saplings and seedlings. This damage often poses a problem for forest management and reforestation when smaller seedlings are buried or uprooted during mountain beaver feeding and can result in tree deformities, growth suppression, and mortality. The United States Forest Service has maintained a program to exterminate the mountain beaver due to beavers causing economic damage to commercial reforestation. Damage control measures taken to protect forest trees by controlling the mountain beaver population include trapping, placing toxic baits, and placing physical barriers around the base of an individual tree.