Mountain Beaver

"one of the most remarkable animals
discovered by Lewis and Clark"

--Elliott Coues

That "small animal about the size of a squirrel" with the fur of which Lewis wished his "Tiger Cat" capote to be lined, may have been the one that he soon learned was locally called a sewelel. At Fort Clatsop on February 26, 1806, he wrote the following comments on this "new" species of mammal:

Sewelel [suh-WELL-ul] is the Chinnook and Clatsop name for a small animal found in the timbered country on this coast.    the natives make great use of the skins of this animal in forming their robes, which they dress with the fur on them and attatch together with sinews of the Elk or deer. I have never seen the animal and can therefore discribe it only from the skin and a slight view which some of our hunters have obtained of the living animal.    the skin when dressed is from 14 to 18 inches in length and from 7 to 9 in width; the tail is always severed from the skin in forming their robes I cannot therefore say what form or length it is.    one of the men informed me that he thought it reather short and flat.    that he saw one of them run up a tree like a squirrel and that it returned and ran into a hole in the ground.    the ears are short thin pointed and covered with short fine hair.    they are of a uniform colour, a redish brown; tho' the base of the long hairs, which exceed the fur but little in length, as well as the fur itself is of a dark colour for at least two thirds of it's length next to the skin.    the fur and hair are very fine, short, thickly set and silky.    the ends of the fur and tips of the hair being of the redish brown that colour predominates in the ordinary appearance of the animal.I take this animal to be about the size of the barking squirrel of the Missouri.1    and beleive most probably that it is of the Mustela genus,2 or perhaps the brown mungo itself.3 I have indeavoured in many instances to make the indians sensible how anxious I was to obtain one of these animals entire, without being skined, and offered them considerable rewards to furnish me with one, but have not been able to make them comprehend me. I have purchased several of the robes made of these skins to line a coat which I have had made of the skins of the tiger cat.    they make a very pleasant light lining.

Figure 1

Mountain Beaver

Aplodontia rufa Rafinesque (1817)

Hand-tinted lithograph by John James Audubon
from Quadrapeds of North America (1845-48),
Vol. III, No. 25, Plate CXXIII

Our human impulse to name everything has made an imposter of the "mountain beaver." First–and last–it is not a beaver. It doesn't confine itself to mountainous habitats. It doesn't build dams on streams to create ponds, or dwell in partially subaqueous lodges made of sticks. It is a sluggard compared with the nimble and active squirrel. In dry areas it favors the proximity of small streams, but it is strictly a land animal, living in underground burrows much like the prairie dog's. The subterranean domicile of a single A. rufa may occupy an area 100 yards in diameter. In rainy climates it shelters the entrances to its burrows with little tents of sticks covered with leaves. The mountain beaver is so attached to an underground existence that it is rarely to be seen in the light of day, and consequently is hard to capture–which might explain why the natives around Fort Clatsop seemed not to understand Lewis's request. The only place on earth where it can ever be seen, day or night, is in the Pacific Northwest of North America.

This secretive, primitive little rodent, which somewhat resembles the woodchuck and the muskrat, belongs to the same mammalian order, Rodentia (ro-DEN-tee-uh), as the more familiar and ubiquitous flat-tailed beaver, Castor canadensis, but otherwise they have nothing in common. The order consists of 39 families, 389 genera, and 1,702 species. The so-called mountain beaver is the only species, rufa (ROO-fuh; reddish),4 in the only genus, Aplodontia (ap-lo-DON-ti-uh), in the singular family Aplodontiidae (AP-lo-don-TEE-ih-dee). Aplo is a Latin word for simple; dontia is Latin for teeth. The sewellel must keep chewing throughout its entire 5- to 10-year life–fleshy and woody plants, principally–in order to keep its incisors from growing too long. It also grooms its teeth on "beaver baseballs"–chunks of stone or clay of about that size which it encounters in digging its dens. Its Indian name, sewellel, is said to have come from the Chinookan word swal·l, which actually refers only to a robe made of the animal's skins.5

The mountain beaver is less of a show-off than its larger and more seductive cousin the genuine beaver–Castor canadensis–partly because it doesn't have a flat tail with which to spread its alarms. In fact, it has only a stumpy vestigal tail, less than an inch and a half long on a mature male. Perhaps its best attribute has been that despite the old appeal of its pelt in Northwest-coast Indian clothing, no one else–or since–has found it to have any merchantable value.

1. Lewis's standard for his comparison of the sewellel with the "barking squirrel of the Missouri" or prairie dog, which he had first seen on September 7, 1804, was close; the latter averages one or two inches longer than the mountain beaver.

2. This is one of the few times we find Lewis venturing to identify a new species in scientific terms. Understandably, he was wrong. The genus Mustela belongs to the the family Mustelidae, which includes weasels, ferrets, minks, otters, and badgers, but not the mountain beaver.

3. Mungo was a name for the mongoose, a native of Nigeria and the Congo. The mountain beaver bears a strong resemblance to the mongoose, the most conspicuous difference being the latter's much longer tail. The mongoose is native to Asia, Africa and southern Europe. Where or when the captains might have seen one, or even a picture of one, is not known. Furthermore, they never saw a live mountain beaver nor, evidently, did they even see the carcass of a dead one So how could they have made the comparison? Perhaps one of the hunters who got a "slight view" of one or more in the flesh, recognized it.

4. In giving it the specific epithet rufa, Rafinesque relied on Lewis's description of the fur as reddish brown, although it also appears as a dark brown or black. Adrian Forsyth, Mammals of North America (Buffalo, New York: Firefly Books, 1999), 80.

5. Coues's detailed annotation on the history of the sewellel's common and Latin names illustrates the progress of the scientific discipline of classification and nomenclature between 1806 and 1892:

Fortunately [Lewis and Clark] gave it a name by which it could be called, and which has passed into our language. . . . It seems by the later researches of George Gibbs into the unspellable jargon of the Columbia River Indians, that "sewellel" is their name for the robes, mistaken by Captain Lewis for the name of the animal which furnishes the skin, and that the latter is "show'tl" in Nisqually. . . . It was first technically named Anisonyx rufa by Rafinesque (American Monthly Magazine, II. 1817, p. 45). In 1829 Sir John Richardson renamed it Aplodontia leporina in the Zoˆlogical Journal, IV, p. 335; and this naturalist described and figured it fully in the Fauna Boreali-Americana, 1829, p. 211, pl. 18 C, figs. 7 to 14. Correcting the faulty orthography of this generic name, and coupling it with the prior specific name given by Rafinesque, I called the animal Haplodon rufus, in the Monographs of N. Am. Rodentia, 1877, p. 557, where its anatomy, as well as external character is, is given at length, with all that was then known of its history. This name is the one by which it has since been known to naturalists. I understand that the whites on the Columbia call it the "mountain boomer" – a queer name, which I hear applied to the red squirrel (Sciurus hudsonius) by the natives of the mountains of North Carolina, where I happen to be penning this note (Aug. 9th, 1892). There is a second species of sewellel in California, Haplodon major.

Coues (1842-99; pronounced cowz), the most prominent naturalist of the later 19th century added copious annotations to Nicholas Biddle's paraphrase (1814) of the journals of Lewis and Clark, and published it as History of the Expedition Under the Command of Lewis and Clark (4 vols, 1893; reprint, 3 vols.; New York: Dover, 1965), 847.


The Encyclopedia of Mammals. Edited by David Macdonald. New York: Facts on File, Inc., 1984.

National Audubon Society Field Guide to North American Mammals. Edited by John O. Whitaker, Jr. Rev. ed., New York: Knopf, 1996.