Animal Life in the Yosemite
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SIERRA FLYING SQUIRREL. Glaucomys sabrinus lascivus (Bangs)

Field characters.—Body size about 2/3 that of House Rat; body flattened, with a broad fur-covered extension of the skin along each side between fore and hind feet (fig. 32); tail heavily furred, flat; eyes large; pelage dense and soft, silky in texture. Head and body 5-1/2 to 6-1/2 inches (142-166 mm.), tail 4-1/2 to 5-3/4 inches (116-145 mm.), hind foot about 1-1/2 inches (36-39 mm.), ear from crown 3/4 to 1 inch (18 to 26 mm.); weight 3-1/2 to 5-3/4 ounces (103.5-164.5 grams). General color above dark leaden gray; under surface of body dull white; both surfaces of tail dark gray. Voice: A low whurr.

Occurrence.—Moderately common resident in Transition and Canadian zones on west slope of Sierra Nevada. Recorded from Smith Creek, 6 miles east of Coulterville, and from Sweetwater Creek, eastward to Merced Lake and Porcupine Flat. Inhabits trees, chiefly black oaks and red firs, dwelling in holes in daytime, coming forth at night.

The Sierra Flying Squirrel is relatively common in the main coniferous belt of the Sierra Nevada, and a considerable population of the species lives right in Yosemite Valley. Yet, because it comes forth only under cover of darkness and then goes about in a very quiet manner, its activities and even its presence are known to very few persons.

The use of the word 'flying' in connection with this squirrel is not strictly accurate. The animal is unable to course about freely in the air, in the way of a bat or a bird. It can only volplane from a high perch to a lower one and is therefore a 'glider,' rather than a 'flyer.' The structure of the Flying Squirrel is modified importantly in several respects to ensure success in this mode of progression. Yet it has not, of course, reached an extreme specialization of structure anywhere near that of a bat. Its feet are nearly normal in form, and its toes are provided with claws like those of other squirrels, so that it is able to run about on all fours. It has a broad brush-like tail, roughly similar to that of other arboreal squirrels.

The body of the Flying Squirrel is somewhat flattened and along each side there is a fur-covered double layer of skin extending between the fore and hind legs and out to the 'wrist' and 'ankle.' (See fig. 32.) This sort of membrane, when extended, about doubles the area of the lower surface of the animal and thus contributes to the success of the squirrel's passage through the air. The tail is broad, due to a thick, close covering of long hairs at the sides, and is remarkably thin, reminding one, in toto, of a single tail feather of a bird. The fur everywhere is dense, of even length, and of a silky texture. The under surface of the body, which is of course exposed during 'flight,' has a proportionately heavier covering of fur than is present on other squirrels. Indeed, this heavy furring covers even the feet, save for the sole pads. This type of pelage, besides serving to keep the animal warm during the cold Sierran nights when it is abroad, also makes for passage quietly through the air, a necessary precaution in a region where owls are abundant.

Fig. 32. Sierra Flying Squirrel. Photographed from fresh specimen trapped in Yosemite Valley, December 24, 1914; about 2/5 natural size.

The head of the Flying Squirrel, and particularly the eyes, are proportionately large as compared with the head and eyes of other members of the squirrel tribe. Indeed the combination of large head and eyes and soft body covering reminds one strongly of the condition found in such nocturnal birds as the poor-wills and nighthawks.

Information concerning the Flying Squirrel is much more difficult to obtain than concerning the species of squirrels which are abroad in the daytime. Our own knowledge of it was gained partly by sleeping out at night under the trees, where the animals might be expected to occur, and partly by trapping. A large number of traps was set at likely places, such as the mounds at the bases of black oaks and red firs and the tops of fallen logs adjacent to standing trees.

At Merced Grove on the night of June 15, 1915, at about 10 P.M., one of us while in his sleeping bag was aroused by a soft thud made by a Flying Squirrel which alighted on the base of a white fir close by. It scuttled up the tree, and in the dim light the observer was able to note that this particular animal carried its tail above its back like other tree squirrels. In climbing up the bark it made less noise than a Red Squirrel. In Yosemite Valley, one night in June, one of us heard the low chuckling notes of a Flying Squirrel and the scratching of the animal's claws on the bark of a big yellow pine.

At Gentrys, Flying Squirrels were heard on several nights in October of 1915. Their voices reminded one of the low vibrant whurr of a cord suddenly whipped through the air. Needles and other small debris kept falling from the trees, shaken down by the squirrels. At 8 P.M. on October 22 they were heard in at least three places, all within 150 feet of the location of our camp amid some sugar pines and white firs.

At Chinquapin at about 6 P.M. on May 19, 1919, one of us, while searching for occupied woodpecker holes, tapped the side of a dead bole. At this, a Flying Squirrel put its head out of a hole about 12 feet above the ground, gazed down at the disturber for a few seconds, and then drew back inside again.

In Yosemite Valley a young Flying Squirrel was captured by a cat on July 13, 1915. Another young squirrel was brought to us by a Valley resident on September 16, 1915. It had fallen from the nest hole high in a black oak near the government stables adjacent to the old presidio.

At Merced Grove in June, 1915, the cook of a construction crew complained that some animal had been getting into a box of crackers in the cook tent. This was close by our own camp. A rat trap was set on the box and at about 8 P.M. it was heard to go off. Investigation showed a Flying Squirrel caught across the back. It was still alive and so was taken out of the trap and placed in an improvised cage. Another squirrel was taken similarly at about 2 A.M. a few nights later. It was placed with the first. Neither of these squirrels seemed harmed by being caught in the rat trap. Possibly the heavy pelage and broad body so distributed the blow that it was not as serious as it might otherwise have been. One squirrel was in the trap half an hour before being rescued. The first squirrel was active on the morning following its capture, probably because of its unusual surroundings. Thereafter it spent most of the daytime in sleep, becoming active at night. When asleep the broad furry tail was wrapped over the face. Some redwood bark was put into the cage and the squirrel soon began to fashion a nest with it. Various items from the camp food supply were offered. Oatmeal was taken by preference, then bread crusts. Dried prunes, put in at the same time, were not touched until later. Once during the day one of the squirrels washed its face by licking its forepaws and then rubbing them over its face.

At Porcupine Flat a red fir stub was found to be inhabited by Flying Squirrels. Two of the animals were trapped there and evidence of a third obtained, after which the trunk was cut down and examined. The stub was about 40 feet high and 5 feet in diameter, hollow at the base, and well rotted interiorly. Inside, just beneath the bark, at a height of 10 feet above the level of the ground, was a nest which was at least one year old. It was composed chiefly of shredded bark. In a cavity in the center of the tree, at the same level, was a new nest, evidently incomplete. This was made of twig ends from the red fir, rolled into a spherical mass about 5-1/2 inches (140 mm.) in diameter. The twigs used were 1/8 inch or less in diameter and from 1 to 4 inches in length. They had been cut off neatly from the extreme ends of the smaller branches of the fir, and green needles were still adhering to the twigs. There was as yet no internal cavity in this nest. Below the newer nest was a large mass of fir twigs. Various cavities in the stump below the two nests contained droppings, suggesting extended occupancy. Lumbermen near Chinquapin told of cutting down a tree in which a nest containing two young was found.

A pure albino Flying Squirrel, with pink eyes and pink claws, was found in Yosemite Valley in August, 1918. It had been drowned in a water bucket in Camp 17. This specimen was mounted and on exhibition in the Superintendent's Office in 1919.

The young of the Flying Squirrel are produced during the summer season. The broods are small, and evidently but one brood is reared each season. Four females containing embryos were collected, as follows: June 11, Merced Grove Big Trees, 2 small embryos; June 18, Mono Meadow, 4 embryos; June 29, Porcupine Flat, two specimens, one with 2 small embryos, the other with 4 large embryos. A young individual, scarcely half-grown, was taken in Yosemite Valley on July 13. A quarter-grown youngster, which fell out of its nest in a black oak, was obtained in the Valley on September 16; this represented an exceptionally late brood. Two individuals, obtained at Aspen Valley on October 17 and at Sweetwater Creek on October 31, were about half-grown. All these young animals were well furred and all, including the smallest one, had obvious 'flight membranes.'

The enemies of the Sierra Flying Squirrel are not known with certainty. Several individuals which had come to grief in different ways were noted by our party. In Yosemite Valley the dried remains of one was found on the ground. On the Yosemite Falls trail, November 19, 1915, one was seen partly buried in the snow and minus its head; some predatory animal was probably responsible. At Smith Creek, Mr. Donald D. McLean once found a Flying Squirrel hanging on a barbed wire fence. It had probably sailed against the wire in the dark and received a mortal wound.


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

grinnell/mammals68.htm — 19-Jan-2006