SIERRA FLYING SQUIRREL. Glaucomys sabrinus lascivus
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.
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
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
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.