Mountain beaver

The mountain beaver (Aplodontia rufa) is a rodent unrelated to beavers and that is not usually found in mountainous areas. It has several common names including aplodontia and sewellel. This species is the only member of its genus, Aplodontia, and family, Aplodontiidae.

Habits and Distribution

Mountain beavers are found in coastal areas of the Pacific Northwest of North America. These are usually low elevation regions, but they can occasionally be seen as high as treeline. These animals appear to be physiologically limited to moist 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.

Mountain beavers build elaborate burrow systems with chambers devoted to fecal and food caches. They exhibit caprophagy and will remove fecal pellets directly using their incisors. Food includes fleshy herbs and young shoots of more woody plants. They appear to be strictly vegetarian.

These chunky animals weigh about 800+ grams and are 300–460 mm long with a tail length equal to 10–40 mm.

The breeding season is between January-March with 2–3 young born February-April. Longevity is 5–10 years, fairly long as rodents go.

Spelling the Family Name

Most references use the spelling Aplodontidae for the family name. This has been deemed incorrect due to the technical rules of convertinga genus name into a family name. The proper conversion of Aplodontia 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.

Closest Relatives

The mountain beaver is considered a living fossil by many researchers due to the presence of a host of primitive characters, particularly the protrogomorphous zygomasseteric system. The mountain beaver is the only living rodent with this primitive cranial and muscular feature (except perhaps the mole rats who clearly evolved protrogomorphy from a hystricomorphous ancestor). The mountain beaver was once thought to be related to the earliest protrogomorphous rodents such as the ischyromyids like Paramys. Both molecular and morphological phylogeneticists have recently suggested a more distant relationship to these animals.

Molecular results have consistently produced a sister relationship between the mountain beaver and the squirrels (family Sciuridae). This clade is referred to as Sciuroidea, Sciuromorpha (not to be confused with the sciuromorphous zygomasseteric system), or Sciurida depending on the author.

According to the fossil record, The Aplodontoidea split from the squirrels in the Middle or Late Eocene as indicated by the extinct genera Spurimus and Prosciurus. The fossil record for the genus Aplodontia itself extends to the Late Pleistocene of North America.

Mountain beavers have an unusual projection on each molar and premolar which is unique among mammals and allows for easy identification of teeth.

References

• 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 genes. Molecular Biology and Evolution, 18:777–791.

• MacDonald, D. ed. 1987. The Encyclopedia of Mammals. Facts on File Publications, New York.

• McKenna, Malcolm C., and Bell, Susan K. 1997. Classification of Mammals Above the Species Level. Columbia University Press, New York, 631 pp. ISBN 0–231–11013–8

• Nowak, R. M. 1999. Walker's Mammals of the World, Vol. 2. Johns Hopkins University Press, London.