 | This articleneeds additional citations forverification. Please helpimprove this article byadding citations to reliable sources. Unsourced material may be challenged and removed. Find sources: "Mountain beaver" – news ·newspapers ·books ·scholar ·JSTOR(April 2017) (Learn how and when to remove this message) |
| Mountain beaver | |
|---|---|
 | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Mammalia |
| Order: | Rodentia |
| Family: | Aplodontiidae |
| Genus: | Aplodontia Richardson, 1829 |
| Species: | A. rufa |
| Binomial name | |
| Aplodontia rufa (Rafinesque, 1817) | |
 | |
Themountain beaver (Aplodontia rufa)[Note 1] is a North Americanrodent. It is the only living member of itsgenus,Aplodontia, andfamily,Aplodontiidae.[2] It should not be confused with trueNorth American andEurasian beavers, to which it is not closely related;[3] the mountain beaver is instead more closely related tosquirrels, although its less-efficient renal system was thought to indicate greater relative antiquity for the species.[citation needed] There are seven subspecies of mountain beavers, six of which are found inCalifornia and three of which areendemic to the state.[4]
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 ofAplodontia to a family name is to drop the -a only and add -idae. Thus, Aplodontiidae is technically correct. This spelling is gaining acceptance in modern texts and is the standard spelling currently recognized by theIntegrated Taxonomic Information System.
Alternate spellings of the genus name have also been reported, with as many as 30 variants historically. These includeHaplodontia,Haplodon,Aploodontia,Apluodontia, andAplodontie, among others. The nameAplodontia ("simple tooth") is in reference to the single large basin comprising the bulk of eachcheek tooth. Thespecific epithet,rufa, means red or reddish.
.JPG)
The mountain beaver is considered to be aliving fossil, due to the presence of a host ofprimitive characteristics, particularly theprotrogomorphous zygomasseteric system. This condition is similar to what is found in most mammal groups, such asrabbits, where no extreme specialization of themasseter muscle has evolved.[citation needed] In the protrogomorphous condition, the medial masseter muscle does not pass through theinfraorbital foramen as it does inguinea pigs andmice. Likewise, the lateral masseter muscle attaches to the base of thezygomatic arch and does not extend to the region in front of the eye as is seen insquirrels and mice.[5]
The mountain beaver is the only living rodent with this primitive cranial and muscular feature (except perhaps theblesmols, which clearly evolved protrogomorphy from ahystricomorphous ancestor). The mountain beaver was once thought to be related to the earliest protrogomorphous rodents, such as the ischyromyids (Paramys). Bothmolecular and morphologicalphylogeneticists have recently suggested a more distant relationship to these animals.
Molecular studies have consistently produced a sister relationship between the mountain beaver and thesquirrels (family Sciuridae). Thisclade is referred to asSciuroidea,Sciuromorpha (not to be confused with thesciuromorphous zygomasseteric system), orSciurida, depending on the author.[citation needed]
According to the fossil record, the Aplodontiidae split from the squirrels in the Middle or LateEocene as indicated by the extinct generaSpurimus andProsciurus.[citation needed] The fossil record for the genusAplodontia extends only to the LatePleistocene of North America.
At present, sevensubspecies ofAplodontia rufa are recognized:
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 500–900 g (18–32 oz), with a few specimens topping 1,000 g (35 oz). Total length is about 30–50 cm (12–20 in), with a tail length of 1–4 cm (0.39–1.57 in). 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.[6] 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.[7]
Theskull isprotrogomorphous;[8] it has no specialized attachments for themasseter muscles as seen in other rodents. It is flattened and lacks apostorbital process.[8] Thebaculum is thin and distinctly forked.[9] Thepenis is about 4.5 cm (1.8 in) in length. The male does not have a truescrotum, but thetestes move into a position called semiscrotal during thebreeding season.[4]
Mountain beavers have an unusual projection on eachmolar andpremolar 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 thelower. The cheek teeth lack the complex folds of other rodents and instead consist of single basins. They arehypsodont and ever-growing. Two upper and one lower premolars are present, along with all the molars, giving adental formula of1.0.2.31.0.1.3[10]
Mountain beavers cannot produceconcentrated urine. They are thought to be physiologically restricted to thetemperate 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.[4] 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.[11]
Mountain beavers are found in theCascade Mountains ofBritish Columbia and southward including the rest of theCascade Range in the United States,[12] theOlympic Mountains and Coast Ranges of Washington and Oregon,[13] plus theKlamath Mountains, and theSierra Nevada,Point Arena andPt. Reyes ofCalifornia and extreme westernNevada.[12] They range from sea level to thetree line.[12] They can be found in bothdeciduous andconiferous forests, but throughout most of the range they appear to prefer the former.
Mountain beavers are capable of climbing trees,[14] but rarely travel far from burrows. Theirthumbs are slightlyopposable 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).[7] They appear to buildhay mounds at some burrow entrances, but whether this behavior is related to water regulation, curing food, or gathering nest materials is debated.[citation needed] Mountain beavers are partially nocturnal, spending more time active at night than the day but still active during both periods.[4] 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.[4] 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,[15] nor do theyhibernate.
They exhibitcoprophagy and eat soft fecal pellets to obtain maximum nutrients; hardfecal pellets are transferred to fecal chambers located within the burrow system.[16][17] Food includes fleshy herbs and young shoots of more woody plants.Ferns probably make up the bulk of their diets. They appear to be strictlyherbivorous.[15] Their consumption of seedling trees has led some to consider them as pests.
Known predators includebobcats,coyotes,weasels,cougars,golden eagles, andowls.[18]
Among the parasites of the mountain beaver is the largestflea known,Hystrichopsylla schefferi. Females of this flea can be 8 mm (0.31 in) long.[19]
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.[11] 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.[11] TheUnited States Forest Service has maintained a program to exterminate the mountain beaver due to beavers causing economic damage to commercial reforestation.[20] Damage control measures taken to protect forest trees by controlling the mountain beaver population include trapping, placing toxic baits, and placing physical barriers (e.g., plastic mesh tubes) around the base of an individual tree.[21]
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.[4] The young are born hairless, pink, and blind. They are weaned at 6 to 8 weeks and leave the burrow soon after.[4]Lifespans are 5 to 10 years – fairly long for rodents. They are not social, though home ranges can overlap.[15] 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.[22]
TheIUCN lists the mountain beaver as a species ofleast concern. However, one subspecies, the Point Arena mountain beaver (A. r. nigra) of California, is consideredendangered by theU.S. Fish and Wildlife Service,[23] andCritically Imperiled byNatureServe.[24] Originally listed in 1991, the Point Arena mountain beaver is distinguished by its black colouration and by characteristic body proportions, including a smaller overall size.[25] The principal threat to this subspecies is habitat loss and fragmentation; its remaining range comprises a disjunct region of 24 square miles (62 km2) in westernMendocino County.[23]
Media related toAplodontia rufa at Wikimedia Commons
