Animal Life in the Yosemite
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SIERRA MOUNTAIN BEAVER. Aplodontia rufa californica (Peters)

Field characters.—Size of small Marmot, with general appearance of Meadow Mouse; tail so short as to appear to be wanting, shorter than hind foot; head blunt, eyes and ears small (pl. 3la). Head and body 11 to 14 inches (280-354 mm.), tail 3/4 to 1-3/5 inches (19-40 mm.), hind foot 2-1/6 to 2-1/2 inches (55-63 mm.), ear from crown 1/2 to 4/5 inch (13-21 mm.); weight 30 to 48 ounces (852-1375 grams). General coloration everywhere plain blackish brown. Workings: Underground burrows or tunnels about 6 to 7 inches in diameter with numerous openings to surface; located usually along brush-covered banks of swift-flowing streams.

Occurrence.—Resident locally in small numbers in Canadian and Hudsonian zones on west slope of Sierra Nevada. Recorded at Aspen Valley, Gentrys, Chinquapin, near Ostrander Rocks, in both forks of Indian Cañon (above Yosemite Valley), near Porcupine Flat, and in head of Lyell Cañon. Altitudinal range, 5800 to 10,000 feet. Lives along swift-flowing streams bordered by willow and creek dogwood. Colonial; nocturnal.

One of the most interesting and at the same time reclusive members of the Yosemite fauna is the Mountain Beaver or Aplodontia. This animal, like the redwood tree and the wren-tit, is peculiar to the west coast of North America, where it occurs scatteringly in the Sierras and northern coast ranges. Although called Mountain Beaver it is in nowise related by structure or mode of life to the true beaver save that both are rodents. The present species has, indeed, no close living relatives anywhere so far as known. Locally we found that some of the workmen on road gangs who knew of the animals called them 'mush-rats' because of their general resemblance to the muskrat. The latter animal does not, to the best of our knowledge, occur anywhere in the Yosemite region.

The Sierra Mountain Beaver is of the size of a small marmot. If one can imagine a meadow mouse grown to fifteen or twenty times its ordinary size, and practically without any tail, one will have a good idea of the mountain beaver. (See pl. 31a). The animal is of stout build, has a short blunt head, small eyes, small nearly naked ears, no obvious neck, a thick body, normal legs and feet, and a mere stub of a tail. The tail is less than the hind foot in length, and in this character the animal is unlike all local small mammals except the rabbits and the cony. The body is covered evenly with a uniform blackish brown pelage of considerable length and of soft texture.

Aplodontia is a timid, retiring animal, practically never seen except when trapped. Its activity is confined to the night-time, and it spends the day in underground retreats. When in captivity the least injury seems sufficient to cause its death; its general resistance seems extremely low. When kept as a captive it may be tamed rapidly, and even at the first its only indication of displeasure is a rapid chattering or grinding with its teeth.

Only once, in our rather extended and intensive work in the habitat of Aplodontia, did any of our party happen to see one of the animals abroad. On the evening of June 23, 1915, at 7:05 P.M., one was seen running along the bank of the creek in Indian Cañon (northeast of Yosemite Falls). It moved very rapidly, at perhaps 5 feet a second, and its gait was like the lumbering gallop of a bear. At our Lyell Cañon camp a month later a specimen of Aplodontia was trapped alive and kept for a while in camp (pl. 31a).

The manner of life of the mountain beaver is, like its general appearance, suggestive of that of the meadow mouse. It frequents, almost without exception, the near vicinity of streams. When the naturalist goes in search of Aplodontia he seeks creek banks bordered by good growths of willow, creek dogwood, and other riparian shrubs and herbs. On the stems of these, marks of gnawings will be in evidence if the animals are present. Also burrows or tunnels in the ground will be found often within but a yard or so of water. These tunnels, like those of meadow mice, are, in general, parallel with the surface of the ground, and have rather frequent openings to the surface. (See fig. 31b). Within these burrows the animals make their nests, in which they remain during the daytime and within which their young are reared.

At Chinquapin a series of Aplodontia workings was laid open and mapped by one of our party on June 21, 1915. The tunnels ran partly through rocky ground and partly through humous soil. Close by was the north fork of Indian Creek in which the stream of water was about 2 feet wide and 3 to 6 inches deep. Water was also running through one of the tunnels. The tunnel system, for the most part, was in the bank, about 3 feet above the level of the stream. The tunnels averaged between 6 and 7 inches (160 mm.) in diameter, the entrances being slightly larger. No nest was found in the series of tunnels opened, but examination of tunnel systems elsewhere has shown the presence of underground nests, so it may be presumed that the animals which made this particular excavation had their nest in some other burrow. The floor of the tunnel system is usually well packed as a result of constant use and is kept clear of debris of every sort so long as the place is occupied by the animals. The set of tunnels at Chinquapin yielded 2 animals, a male and a female, in the several days of trapping prior to the time when the system was dug out.

Another colony was noted along the East Fork of Indian Cañon (above Yosemite Valley). Here the workings occupied, in 1915, practically all available locations from the crossing of the trail to North Dome northward to the headwaters of the creek. In one place, even during June, the creek practically disappeared from view so great was the amount of water running through the tunnels. Other colonies were found on creeks near Porcupine Flat; and finally a colony of Aplodontia was discovered at an altitude of 10,000 feet on the slope of Kuna Crest in the head of Lyell Cañon. (See fig. 21.) The lowest record, altitudinally, was made at Gentrys, 5800 feet, where, in the fall of 1915, tunnels were found in fair numbers though none of the animals was obtained.

The 'colonies,' or at least the series of workings so called, are in many cases of considerable extent. In one place an area estimated at 50 by 100 yards was occupied; other colonies were of somewhat less extent. The number of holes in a colony is large, 20 to 30 being noted in one locality on Snow Creek. The population in any one limited series of burrows consists usually of not more than two adults, comprising a pair. If these are trapped out, several days intervene before animals from neighboring burrows move in, to occupy the deserted ones.

The colonies are in most cases fairly well sheltered from view by the vegetational cover of the stream banks. But in early spring, just after the snow has melted off and before the willows and dogwood are leaved out, the burrow openings may be readily seen.

Aplodontia seems to be active, even at the higher levels, throughout the year. There are no data to indicate that the animals hibernate, while much circumstantial evidence points toward regular active life throughout the winter season. In many places we saw willow branches and small coniferous trees which showed signs of beaver activity as high as 5 feet above the ground. This animal is not known to climb to any extent, so the conclusion seems justified that it comes up through the snow, even out on the surface of the snow, and nibbles at the twigs then within easy reach. Along the west fork of Indian Cañon (above Yosemite Falls) a quantity of 'hay' was observed, consisting of a narrow-leaved lupine (Lupinus longipes) which had been cut green and piled and cured on dry masses of drift material. This 'hay' when seen on October 30, 1915, was nearly dry. In each pile the butt ends of the stems usually lay in one direction, toward the entrance of the adjacent burrow. Whether this material was for winter food, as with the cony, or for a dry and warm winter nest below-ground, was not ascertainable.

Aplodontia feeds upon most of the plants growing in the vicinity of its burrows. At Chinquapin the following plants gave evidence of being used by the animals: Azalea (Rhododendron occidentale), the commonest plant and used very much; hazel, common but little used; Sierran currant (Ribes nevadense), common, and many cuttings seen; creek dogwood (Cornus pubescens), common, many cuttings; wild cherry, fairly common, a few cut twigs seen; snow-bush (Ceanothus cordulatus), abundant at edges of thickets and occasionally used; chinquapin, abundant at edges of inhabited thickets and much used in places; incense cedar, few young trees much used; white fir, many young trees, but rarely used; sugar pine, young trees common but only occasionally used; brake fern (Pteris aquilina), fairly common, used slightly.

From the azalea, snow-bush, hazel, and cherry, sticks 1/4 inch in diameter and 6 to 8 inches long were cut; the pines and cedars had the smaller twigs pruned off. One azalea stem 1-1/2 inches in diameter had been cut through but had not been carried away. Chinquapin stems which were taken had the leaves still in place.

Elsewhere in the region still other plants showed signs of having been used as food. In one place young aspens had been eaten; and Labrador tea (Ledum glandulosum) and another currant (Ribes viscosissimum.) had been cut by the animals. In one instance a 'whole bush' of creek dogwood had been cut off at about 18 inches above the ground.

Although living in a damp environment, in some places where it must of necessity enter the water at times, there is no evidence that Aplodontia does so by preference. It is not nearly so aquatic in habits as the muskrat or the true beaver. When the fur of Aplodontia is touched by water it wets about as readily as that of other less aquatic animals.

The breeding season of Aplodontia seems to occupy the summer months. Females containing embryos are very seldom taken. We did not secure a single one in the Yosemite region. A quarter-grown youngster weighing about 6 ounces (182 grams) was trapped in Lyell Cañon, July 20, 1915.

Other animals frequent Aplodontia burrows to some extent. Several Sierra Chickarees were caught in traps set in Aplodontia burrows and well out of view from above. One Mountain Weasel was taken in a similar setting. The contrast in vitality between these animals and Aplodontia is marked. Aplodontia even when held lightly by the trap was usually dead when found. The squirrels and weasels had survived, doubtless for several hours.


Animal Life in the Yosemite
©1924, University of California Press
Museum of Vertebrate Zoology

grinnell/mammals53.htm — 19-Jan-2006