Dec. 10, 2012

The Beaver With the Stumpy Tail

by Kara Rogers

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“A very distinctive feature of the mountain beaver’s external anatomy is a cylindrical stump of a tail.”
That description, from a 1989 report on the animal in California, is not perhaps the most eloquent one the mountain beaver (Aplodontia rufa) could have hoped for, but it is nonetheless accurate. While the more familiar North American beaver (Castor canadensis) sports a spanking big tail, which serves variously as a rudder, a prop, a fat store, and a communication device, the mountain beaver gets along just fine with its stunted and furry rear appendage.
The mountain beaver is only a “beaver” in name, presumably a name that was derived from the creature's tendency to gnaw on tree bark, a behavior known to beavers (but common to a number of other rodents as well, including certain species of voles and mice). In fact, while the mountain beaver and the North American beaver are both classified as rodents, they are unrelated. The latter animal, its European cousin (Castor fiber), and the extinct beaver ancestors together make up the family Castoridae, whereas the mountain beaver is all alone in family Aplodontidae. That taxonimic division leaves the mountain beaver more closely aligned with squirrels than with beavers.
The distinctions in physical features, behavior, and geographic distribution of mountain beavers and North American beavers are many. The mountain beaver is a much smaller animal -- measuring about a foot in length and weighing around two or three pounds -- compared with the North American beaver, which is more than three feet in length and weighs between 30 and 65 pounds. And while the latter species is found across much of North America, the mountain beaver is found only in the Pacific Northwest, specifically in various mountain ranges running between southwestern British Columbia, Canada, and the Sierra Nevada in California.
The mountain beaver is considered to be one of the world's most primitive living rodents. Its scientific name Aplodontia means “simple tooth,” which refers to the uncomplicated and somewhat antiquated structure ("a puddle of dentine") of its cheek teeth. Its teeth are appropriate for its diet, which consists mainly of the leaves and bark of various plants, including bracken fern, poison oak, and thistle. The archaic nature of the mountain beaver's anatomy is also seen in its kidneys, which lack the structures needed to efficiently concentrate urine. Because the animal loses relatively large amounts of water in its urine daily, it must have a reliable water source nearby and depends on well-hydrated plants for its survival.
The mountain beaver is also known for its talent at digging. It carves out burrows, usually a few inches beneath the ground surface, and creates extensive systems of tunnels, with multiple openings. This is in stark contrast to the North American beaver's semi-aquatic lifestyle and reputation as a lumberjack, felling trees and chewing them into portable pieces that are then reassembled into a dam in a lake or stream.
There are seven subspecies of mountain beaver, each of which is distinguished by certain physical traits (such as cranial anatomy and coat color) and geographical occurrence. Two subspecies, the Point Reyes mountain beaver (A. rufa phaea) and the Point Arena mountain beaver (A. rufa nigra), are vulnerable to extinction. The Point Arena subspecies is listed as endangered by the U.S. Fish and Wildlife Service.
The vulnerability of the Point Reyes and Point Arena mountain beavers comes with the extreme isolation of their populations in coastal California. A fire that swept through Point Reyes in the mid-1990s nearly decimated mountain beavers in the area. With protection and monitoring efforts, however, the outlook for the animals is positive.
About Kara Rogers

Kara is a freelance science writer and senior editor of biomedical sciences at Encyclopaedia Britannica, Inc. She is the author of Out of Nature: Why Drugs From Plants Matter to the Future of Humanity (University of Arizona Press, 2012).

The views expressed are those of the author and are not necessarily those of Science Friday.

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