Mountain beaver

The mountain beaver (Aplodontia rufa) 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. Mountain beavers are gray or brown, but their fur can range from slightly more reddish 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 consume tree seedlings as food – mountain beavers do not fell adult trees (though such trees may be killed by 'girdling'), build dams, live in lodges, or communicate by tail slappings. They are predominantly nocturnal in above ground activities. 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 which are toxic to other animals. 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 4.5 cm (1.8 in) 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 1.0.2.31.0.1.3 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. Their karyotype is 2n = 46. Mountain beavers are found in the Cascade Mountains of British Columbia and southward to include the rest of the Cascade Range in the United States, the Olympic Mountains and Coast Ranges of Washington and Oregon, plus the Siskiyous, and the Sierra Nevada mountain range, 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 appear to prefer the former. These animals appear to be 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. Mountain beavers are asocial and do not live in shared burrows. Burrows usually consist of a network of underground 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 mountain lions and owls (though skunks and weasels that also occupy mountain beaver burrows and tunnels may take nestlings as food). 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. 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.