Nature Notes

Volume XII No. 1 - October, 1946

Brown Mountain Beavers
By O. L. Wallis, Ranger-Naturalist

The brown mountain beaver (Aplodontia rufa rufa Rafineque), furnishes one of the most individual forms of animal life in Crater Lake National Park, interesting not only because it is one of the most primitive mammals in existence but also because of its limited range, its lack of close relatives, and its intensive food storing activity.

mountain beaver

Confined to the Pacific Northwest, mountain beavers are grouped into the single rodent family Aplodontiidae, into one genus Aplodontia and into one species rufa, divided into nine subspecies. The mammal has several other names: sewellel (Indian name), boomer, and mountain burrower. Of these, the last is the most appropriate; for Aplodontia is only distantly related to the American Beaver (Castor canadensis), the builder of dams and lodges. Differing in habitat, the mountain beaver lies in a series of underground tunnels, six to seven inches in diameter, with many external openings that are connected, above ground, by a network of runways. There are two types of openings: The one for general use is kept fairly clean and is used as an entrance and exit. The other type of burrow is employed as a refuse opening, through which the rocks and dirt of excavation are pushed forth.

Burrows are usually located in a moist area, on a hilly slope where the soil is relatively loose, where springs exist, and the vegetation is dense. During my summer's observations, I have noted the (inner) workings near Vidae Falls, in Castlecrest Gardens, in the springy area below the Gardens, and on the south slope of Munson Ridge near the Park Headquarters. In all of these workings, water was found to be trickling through at least a section of the tunnel system throughout the entire summer. For such a timid animal, water supply of this type proves to be protective as well as convenient. The creature is able to drink in perfect safety within the confines of its dwelling; also, the running water offers a means of disposal of waste vegetation material which readily floats out of the lower end of the burrow.

Aplodontia is a mammal which is rarely seen, because it is naturally shy and is mainly nocturnal in habit. On the early morning of August 26, I saw one of the animals loping along its runway and sliding into its burrow in the Castlecrest Gardens.

Ranger-Naturalist R. R. Huestis reports: "During August 1941, a brown mountain beaver took temporary refuge just above the ramp leading to the Sinnott Memorial, using the lower part of a burrow system which has been occupied by a golden mantled ground squirrel for the last three seasons. During two days the mountain beaver made a number of emergences to gather forage or to run the few feet between two entrances to the burrow system, both of which had been somewhat enlarged to better fit the new occupant. The Sinnott Memorial ramp is one of the most traveled walks in the park, so many visitors had the chance of observing and photographing a mountain beaver's feeding technique, an activity that many experienced naturalists have never observed. The animal itself largely ignored the presence of the gallery except when someone moved close to it. Most of the visitors had never heard that there was such an animal, and, being duly advised of its rarity in public places, were enchanted at their opportunity and stood hopefully for considerable periods of time with their cameras at the ready."

On August 27, at the Munson Ridge location, I caught a male in a Verbail wire loop live trap. This capture was made to find out what foods the mammal would eat and to study its habits. Measurements of the individual were: total length: 326 mm; tail: 23 mm; hind foot: 59 mm; ear: 23.5 mm.

Characterized by short legs, tail and ears; exceeding small eyes; large feet with naked soles and strong claws; long and stiff whiskers; the animal is very primitive in appearance and resembles a Mazama pocket gopher, (Thomomys monticola mazama Merriam), which has been enlarged fifteen times. Coarse, shiny dark guard hairs extend above the softer rich brownish or chestnut fur. Below each ear is a white spot. Its stout body and broad, blunt head makes it especially adapted to the burrowing life.

Mountain beavers are strictly vegetarian in food habits, eating most types of herbage which grow near their burrow. I found the vegetation cut for drying in neat piles around the entrance holes or upon nearby rocks or logs. All the stems of the material in any one pile pointed in one direction. Around the hole would be several piles; each contained the same species of plant and may have indicated the size of the load carried by the individual. As a rule, the material constituted plants which grew in the immediate vicinity; on one occasion, however, red elderberry cuttings were observed 45 yards from the nearest shrub.

Foliage collected and eaten by mountain beavers covers a wide variety of plants. Availability seems to be the important factor in the type of food selected. In each of the locations the materials collected varied in direct proportion to the species growing nearby. Although the area adjacent to the burrows at Vidae Falls and Castlecrest Gardens supported great numbers of Lewis' monkeyflower, in no case, was I able to find cuttings of this species. But my captive animal ate this plant quite readily. At the location on the south slope of Munson Ridge, three of the burrow entrances were located under a green-leaved alder; but no traces of this shrub were identified in the bunches of cuttings.

Although F. Lyle Wynd reports that false green hellebore is fatal to live stock (Nature Notes Volume 2, No. 2 August 1929) and that most species of Aconitum are poisonous to animals and human beings (Nature Notes Volume 2, No. 3 September 1929), both monkshood (Aconitum columbianum), and false green hellebore, (Veratrum viride Ait.), were eaten by the captured beaver without apparent maleffect. Monkshood was identified, also, in the bunches of drying vegetation at Castlecrest.

Twice daily at 7:00 A.M. and 10:00 P.M., from August 20 to August 26, examinations were made of the food piles at the Munson Ridge location to determine if the mountain beaver was active during the daytime. These observations showed that most of the activity of cutting and stacking occurred during the night; only once, on August 24, were new cuttings found at the night examination.

Each morning from August 11 through August 27, the materials appearing at the eleven holes at the Munson Ridge location were counted and marked. The marking was done by writing the date on a prominent leaf with pen and ink. By means of this counting and marking, it was possible to note the number of pieces of vegetation cut and stacked and the length of time required for the material to dry before it was taken into the burrow. During this period, the mountain beaver collected over 3246 pieces of vegetation. Dicentra stems amounted to nearly 25% of the material gathered and Valeriana followed with nearly 24%. On August 22, alone, 488 pieces were piled. Note Table I.

Number of
of Total



Table I - Material gathered at the Munson Ridge location by one mountain beaver during the period of August 11 through August 27. These figures list the number of stems of each of the species and not the bulk of the plant cut by the mammal.

The length of time the cuttings were left to dry depended upon the species, the weather, and the location of the pile. Specimens of Sambucus were not dry after five days. On August 25, four out of twenty-seven cuttings of Smilacina marked on August 21, and five out of twenty-one stems marked on August 22 were still present revealing the length of time some cuttings were left to dry. Dicentra and Valeriana stems were usually removed the next day after they were counted and marked; these have thinner leaves. In locations situated in the dense shrubbery, the drying of the material took twice as long as in open situations. On cloudy or rainy days, the vegetation scarcely wilted, and therefore, remained piled for a longer period.

At the Munson Ridge location the mountain beaver harvested its material as soon as it became completely dried, while the individual at the Vidae Falls site allowed the material to stay out until it became so dry that it was brittle.

In captivity the mountain beaver consumed 497 plants from August 30 to September 5. This count was taken in the number of plants eaten, therefore, indicates a greater amount of food than the number applies when compared with the number of stems counted in the field survey. Twenty-one species of vegetation are represented. See Table III. This range could be greatly increased by more intensive experimenting.

Although the mountain beaver is considered to be quite stupid and primitive, from these food studies, it can be assumed that he is quite a diligent and systematic worker.

Plants Collected and Eaten by Mountain Beaver
Aconitum columbianum Nutt.Monkshood
Anaphalis margaritacea (L.) Benth. and Hook.Pearly Everlasting
Aquilegia formosa Fisch.Columbine
Aster englemanni var. ledophyllus Gray.Engelman Aster
Dicentra formosa (Andr.) DCBleeding Heart
Epilobium angustifolium L.Fireweed
Lupinus latiffolius ligulatus (Greene) C. P. SmithKlamath Lupine
Salix eastwoodae CockerellEastwood's Willow
Sambucus racemosa calicarpa (Greene) JepsonRed Elderberry
Senecio triangularis Hook.Tall Ragwort
Smilacina amplexicaulis glabra MacBrideFat Solomon
Valeriana sitchensis Beng.Varlerian
Veratrum viride Ait.Corn Lily; False Green Helebore

Table II - Vegetation identified in piles stacked by mountain beavers in all of the locations noted in Crater Lake National Park.

Rubus lasiococcus Gray.Creeping Raspberry
Vaccinium membranaceum Dougl.Big Huckleberry
Erigeron salsuginosus Gray.Fleabane
Ribes erythrocarpum Coville & LeibergCrater Lake Currant
Castilleja miniata Dougl.Paint Brush
Mimulus Lewisii Pursh.Lewis's Monkey Flower
Phacelia heterophylla Pursh.Phacelia
Sorbus cascadensis G. N. JonesMountain Ash

Table III - Plants, in addition to those found in Table II, which were eaten by the mountain beaver in captivity.

Activity Of Park Beaver
By O. L. Wallis, Ranger-Naturalist

Beaver activity in Crater Lake National Park at present is at a low ebb. A survey of the old workings of the American beaver (Castor canadensis pacificus), in the Copeland Creek region reveals that there has not been any renewal for many years. Several of the dams observed in 1931 by the park naturalist, (Nature Notes Volume 4 No. 1, July 1931) still exist and are strong enough to retain a great quantity of water.

Observations in the Annie Creek basin near the South Entrance show that beaver have been active here to a limited extent. Cuttings estimated from the weathering to be no older than four months occur in the quaking aspens and the poplars along the stream bed. Older cuttings were seen on white fir, aspen, and poplar trees. One tree, 15 inches in diameter, remains standing although almost completely cut through.

Wildlife expert Robert S. Evans in his 1934 "Survey of Animal Life of Crater Lake National Park" states: "Since last year when an estimated number of 12 was reported for beaver living along the streams of the west slope, fresh beaver work has been found along Annie Creek in the southeast corner of the park;" the 1934 estimate was 18, but that exceeds the numbers which exist in the park today.

The beaver migration from the areas on Copeland Creek and Annie Creek probably resulted from one or both of two main causes: lack of sufficient food to support a colony and freezing of the water too deeply to permit winter activity.

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