Natural History of the Ground Squirrels of California/Nelson Antelope Ground Squirrel
NELSON ANTELOPE GROUND SQUIRREL.
Ammospermophilus nelsoni nelsoni (Merriam).
Other names.—Nelson Spermophile; Nelson Ground Squirrel; Antelope Chipmunk, part; Spermophilus nelsoni; Citellus nelsoni.
Field characters.—A small yellowish-brown ground squirrel with one narrow white stripe on each side of body, and with a short flat tail nearly always held curled up over the rump so as to show the creamy white under side. Length of body alone about 6¼ inches, tail about 2⅘ inches more.
Description.—Adult in summer pelage: General color of upper surface from nose to base of tail light clay color, brightening toward pinkish cinnamon on shoulders, flanks, and outer sides of fore and hind limbs; a narrow white stripe on each side of body from shoulder to side of rump. Eyelids, ears and sides of head dull buffy white; whiskers black. Whole lower surface of body white, the hairs white to bases; soles of feet densely white-haired forward to tubercles, thence to balls of toes naked; upper surfaces of feet white, buff tinged; claws blackish brown with pale horn-colored tips. Tail as in leucurus, but upper side near base clay color; under surface creamy or buffy white centrally. Adult in winter pelage: Coat softer, the hairs being longer and more silky than in summer. General coloration as in summer, but tone of upper surface a little darker, and with a fine grizzling due to more variegated color pattern on the individual hairs. White hairs of lower surface with extreme bases lead-color.
Color variations.—-Sexes alike, as far as we can see. Young colored as in the summer adults, but pelage finer in texture.
Measurements.—Average and extreme measurements, in millimeters, of twenty full-grown specimens from the vicinity of Bakersfield, Kern County, are as follows: Ten males: total length, 226 (210–242); tail vertebræ, 70 (61-76); hind foot, 38.6 (36–40); ear from crown, 5.3 (5–6); greatest length of skull, 39.9 (39.0–41.6); zygomatic breadth, 23.2 (21.4–25.0); interorbital width, 9.9 (9.5–10.4). Ten females: total length, 221 (203–238); tail vertebræ, 68 (64–74); hind foot, 38.4 (35.0–41.0); ear from crown, 5.7 (5.0–6.0); greatest length of skull, 39.4 (37.8–41.3); zygomatic breadth, 22.6 (21.5–24.4); interorbital width, 9.7 (9.0–10.6).
Males will be seen from the above figures to average slightly larger than females. The decidedly greater size of nelsoni as compared with leucurus is at once apparent.
Weights.—Three adult females were found to weigh 141.8, 142.6, and 179.0 grams, respectively; average, 154.5 grams, or 5½ ounces. In bulk nelsoni is thus about 50 per cent larger than leucurus.
Type locality.—Tipton, Tulare County, California (Merriam, 1893, p. 129).
Distribution.—Occupies the floor of the southern end of the San Joaquin Valley, and adjacent arid hills and included valleys immediately to the westward (see fig. 24). Life-zone Lower Sonoran. More specifically, extends from vicinity of Bakersfield (8 miles northeast) and Poso, in central Kern County, west to the Carrizo Plains and Cuyama Valley, in southeastern San Luis Obispo County, south to the mouth of San Emigdio Creek, 12 miles due east of Maricopa, Kern County, and north to Huron, Fresno County, and Tipton, Tulare County.
Specimens examined.—A total of 43, from the following localities in California. Kern County:eight miles northeast of Bakersfield, 32;twelve miles due east of Maricopa, 5;McKittrick, 3. San Luis Obispo County:Carrizo Plains, 3.
The Nelson Antelope Ground Squirrel is called Antelope Chipmunk by many of the people who live in the southern San Joaquin Valley. No distinction is made by them between the present species and the Antelope Squirrel of the Mohave and Colorado deserts. This is not surprising when we consider the rather close general resemblance between the two. When specimens of the two animals are in hand, tone of coloration alone suffices for distinguishing them. The ground color of the Nelson Squirrel is distinctly clay color, while that of the Desert Antelope Ground Squirrel is pinkish buff. In the former, too, the under side of the tail is creamy buff, while in the latter it is pure white. In bulk the Nelson is approximately 50 per cent the larger.
The Tehachapi Mountains on the south and the Greenhorn Mountains to the east help to form a continuous barrier between the range of these two near-related squirrels. We find the Nelson Squirrel to be restricted for the most part to the Lower Sonoran life-zone in that portion of the San Joaquin Valley which lies south of Tulare Lake. It is notably numerous in the oil districts. The Nelson Squirrel may be distinguished from all other rodents that occur in this region by having a very short, flat-haired tail (less than 3 inches in length) and by the presence of a single white stripe on each side of the body. The name borne by this species was given to it by its original describer (Merriam, 1893, p. 129) as a recognition of the contributions to mammalogy of Edward W. Nelson, now chief of the United States Bureau of Biological Survey.
During the rainy season many of the smaller streams in the southern and western foothills around the San Joaquin Valley cut deep and narrow channels, but when the flood waters of such streams reach the upper plains of the great valley their course is marked by broad washes with low, perpendicular banks. The silt-bearing waters finally spread out, forming broad, alluvial fans, and often sink into the thirsty soil before they reach the lower alkaline plains. The lower reaches of such stream courses, dry and desertlike most of the year, are the preferred haunts of the Nelson Ground Squirrel. Here burrows are easily dug between the hard layers of the stratified banks of the washes, affording safe retreats from such predators as the coyote and badger. A luxurious though brief-lived growth of vegetation results from the thorough natural irrigation of the rich soil of the alluvial fans and the squirrels are thus afforded an abundant food supply the rest of the year. Individuals and even colonies are to be found along the little gullies and ridges of the upper slopes between the mouths of the streams and the foothills, while a few may be found along the edges of the alkali ground of the lower levels; but the metropolis of the species is in the middle region among the salt-bushes (Atriplex) which thickly dot these plains.
This ground squirrel is soon driven out when fields come under cultivation. It clings closely to the wild land and apparently rarely if ever invades adjoining ground which may happen to be under cultivation. Thus in May, 1918, at the mouth of San Emigdio Creek, Kern County, we found these squirrels quite abundant on three sides of an extensive alfalfa field which was entirely surrounded by virgin tracts of the salt-bush (Atriplex), yet during our stay of nearly two weeks, not a single Nelson Squirrel was observed to enter this field or even to touch a leaf of the alfalfa.
As usually encountered, the Nelson Squirrels are seen scurrying rapidly across open places between clumps of salt-bushes, or else, more rarely, standing straight up to their full height in true "picket-pin" fashion just before they disappear down their burrows. The normal mode of travel is by a series of short rapid jumps of from 6 to 12 inches. When approaching a hole leisurely, or when foraging about, the animals sometimes slow down into a walk.
These squirrels are not early risers, being rarely found abroad until well after sun-up. At the mouth of San Emigdio Creek, during the second week in May, Nelson Squirrels began to appear at the entrances to their burrows in the south-facing overhanging bank of a wash, between 8 and 8:15 in the morning. They appeared earlier in the day at this point than elsewhere in the vicinity, doubtless because this bank first received the full force of the early morning sun. Ten o'clock marked the period of greatest activity. The squirrels under observation disappeared each day between 11:30 and 12 o'clock and were rarely seen again until 2:30 in the afternoon, when they began to reappear in the shade of certain dense-foliaged salt-bushes that grew on the brink of the wash. Although they were frequently seen to bask in the rays of the early morning sunshine, these squirrels shunned the direct sunlight at noonday. As early even as 10 o'clock in the morning one female was seen repeatedly to seek shelter in the shade of a fencepost (J. Dixon, MS).
The tail of the Nelson Antelope Squirrel, as with the Desert Antelope Squirrel, is the most conspicuous feature about the animal. When running, the tail is curved forward over the back, in which position the creamy under surface is most effectively displayed so that at a distance one receives the impression that merely a bit of thistledown is blowing along over the sand. The body of the animal, with its ground-like tone of color, practically disappears. This illusion is furthered by the twitching of the tail and by the momentary pauses of the animal which correspond closely with the usual interrupted flight of a tuft of thistledown.
When the squirrel is foraging about on all four legs, or else sitting up, the tail is held curved forward over the back; in fact, one rarely sees the tail held in any other position (see fig. 28). At such times the tip of the tail is often curved slightly upward or outward. When excited or frightened the tail of the animal is twitched rapidly fore and aft, but rarely or never sideways. One individual observed at a distance of ten feet was seen to vibrate its tail intermittently with exceeding rapidity, there being half-minute intervals between the periods of vibration. There were from four to six of these periods of vibration in a series. The tip of the tail would travel only a short distance, less than a couple of inches, as it was never seen to reach a vertical position above the animal's back when the squirrel was standing on all fours.
These squirrels are notably cautious about coming out of their holes; they were never seen to come out hastily. First the nose and then the eye of an animal would stealthily appear, and then a thorough look around for possible danger taken, before a squirrel considered it safe to leave the shelter of its burrow. However, when they do move, their actions are very sudden, as though they had been undecided just what to do, but having once made up their minds are off in a whirl of dust. These rodents are more easily alarmed by sound than by sight. The
Fig. 28. Young Nelson Antelope Ground Squirrel, photographed by J. Dixon, May 9, 1918, on lower San Emigdio Creek, Kern County. Note how well the general color of the animal blends with the tone of the background, and yet how strikingly the white of the under side of the tail shines forth.
cracking of a twig would send them hot-footed to their burrows. Yet the observer was able to walk up to within thirty feet of them in plain sight in the open as long as he made no violent motions. By approaching slowly and directly towards the squirrels, it was frequently possible to get within ten feet when they were sunning themselves at the entrances to the burrows under the overhanging banks.
The sense of smell seems to be extensively used in the daily life of these animals. At the close approach of the observer the noses of these little squirrels were seen to twitch constantly as if in effort to catch the scent of the stranger. The sense of smell also plays an important part in locating food. Then again it is used socially. When following each other about, in and out of the burrows, they often stop and sniff to see which way the other has gone. Smell serves as a means of identifying the other members of the same family. Outsiders are quickly detected and promptly driven away. The members of one family of squirrels which was closely watched were found to be very sociable, never quarreling among themselves. The parents were often seen sitting side by side feeding in perfect harmony (see fig. 29), while at other times this pair would sit together and rub noses in a very affectionate way. The only time that they were seen to show fight was when a strange male squirrel attempted to enter their burrow, and then the male of the pair promptly put the intruder to flight.
Fig. 29. Adult male and female Nelson Antelope Ground Squirrels, feeding in harmony side by side under wholly natural conditions. Photographed by J. Dixon, May 9, 1918, on lower San Emigdio Creek, Kern County. The openings under the horizontal layers of the wash banks afforded these squirrels safe retreats. Note the characteristic Ammospermophilus pose of body and tail.
The alarm note of the Nelson Antelope Ground Squirrel is much subdued as compared with the clear penetrating trill of the Desert Antelope Ground Squirrel. In fact, the former is much less frequently heard at all. While the junior author was watching a family of Nelson Squirrels at play on the morning of May 8, 1918, an old female was seen to disappear into one of the numerous holes in the bank. About two minutes later his attention was attracted by the low, inquisitive chirr of this same squirrel, which was standing motionless less than ten feet behind him. This alarm note was repeated five or six times at intervals of from 30 to 45 seconds. In uttering this note the mouth was opened, but the effort was not convulsive nor was the thorax greatly contracted. The note was subdued in tone and probably not audible to human ears at a distance exceeding one hundred feet. The confidential quality of the call note reminded the observer very much of the clucking note of the female valley quail when keeping her young together.
Numerous holes in the sides of gullies and in the banks of washes form the most conspicuous signs of the presence of the Nelson Squirrel. Tracks and mounds at the entrances to burrows are not as noticeable as one would expect from the number and size of the animals. Trails from the burrows to the feeding grounds of this species are usually not well defined. In one place near the mouth of San Emigdio Creek these squirrels were obliged to cross a large dusty area in traveling back and forth between their burrows and their feeding grounds. Numerous tracks in this dusty spot showed that the squirrels in this colony did not follow definite trails, but that each individual chose his own route. In passing around a projecting bank, however, the tracks were found to converge for a short distance into broad, well-beaten paths six to eight inches wide.
The favorite location for burrows of this squirrel is, as already stated, in the sides of banks at the edges of washes or gullies. The burrows usually enter near the bottom of the bank. The entrances to the burrows vary from 1½ to 8 inches in diameter. The largest holes are in soft ground beneath horizontal hard strata in the bank and their large diameter is due at least in part to the weathering or caving in of the soft earth. The burrows of smallest diameter are found in hard, level ground at the roots of Atriplex bushes. The entrances to burrows in the banks are from six inches to fifty feet apart. These bank burrows run back in a generally horizontal direction and are interconnected to a considerable extent under the bank. This was proven by seeing a certain squirrel, known by sight to the observer, disappear into one hole and then in a few minutes reappear at the mouth of an adjoining burrow twenty or more feet distant.
All of the eight burrows dug into by various persons from this museum have proven to be simply refuge burrows, with only one entrance; that is, not of the intercommunicating type. Squirrels were found to reappear in from 5 to 15 minutes after they were chased into such burrows. These refuge burrows were found to be short, less than twelve feet in length; shallow, less than three feet in depth; and of small size, less than two inches in diameter. In three cases the squirrels were gassed in these burrows and an hour later dug out. Two of the animals were found dead near the wasteball at the entrance of the burrow, where they had evidently been overcome by the gas when attempting to make their way out.
No nests were found in any of the burrows that were dug out, although nesting burrows were particularly sought for. Consequently we have no information to offer regarding this phase of the animal's life history.
The Nelson Squirrels found near the mouth of San Emigdio Creek were found to be moderately infested with fleas. The squirrels were often seen to stretch out to their full length and roll over and wallow about in the fine powdery alkaline dust which in many places was an inch deep under the overhanging banks along the edges of the washes. Such dust baths, which were frequently indulged in with considerable evident satisfaction by the squirrels, are likely to serve in keeping the pelage of the animal clean as well as to discourage the fleas.
Our data regarding the breeding of Ammospermophilus nelsoni is scanty. The breeding season appears to be much earlier in this species than in the case of the Fisher Ground Squirrel in the same region. The young of nelsoni are apparently all born before the last of April. None of the numerous females taken in the vicinity of Bakersfield between April 27 and May 12 were found to contain embryos. A male one-fourth grown was taken eight miles northeast of Bakersfield on May 7, 1911, and a half-grown young one was taken at McKittrick on May 18, 1911. On May 9, 1918, two immature individuals, weighing 100 and 103.3 grams, respectively, were taken at the mouth of San Emigdio Creek. These, male and female, were thus over two-thirds grown and were at this time foraging for themselves. These two youngsters were extremely playful and on several occasions they were seen to stand upon their hind legs and with their front paws braced against one another's shoulders they wrestled and pushed each other about energetically. The parents of these young squirrels had evidently cast them off to shift for themselves.
Because the California and Fisher Ground Squirrels have increased and profited by the farming activities of man in parts of the San Joaquin Valley, it has been suggested that possibly this increase has tended to crowd out Ammospermophilus nelsoni and therefore restrict the range of the latter (Taylor, 1916, p. 20). At the mouth of San Emigdio Creek both nelsoni and fisheri were found in numbers. Old colonies of fisheri, as shown by their extensive workings, were found between colonies of nelsoni. The Antelope Squirrels occupied the sandy washes and the areas covered by the salt-bush, while the Digger Squirrels occupied the more open tracts which were covered by a low growth of foxtail. While there must be a certain degree of competition as regards food supply between the two kinds of squirrels, this competition is probably no greater than that which exists between the Nelson Antelope Ground Squirrel and, for example, each of the two species of kangaroo rats which forage at night for similar food over the same ground used by the Nelson Squirrels in the daytime. We found no evidence, in this case, that the Digger Squirrels were driving out the Antelope Squirrels.
The badger is one of the chief enemies of the Nelson Ground Squirrel. Many squirrel burrows were found that had been dug out by this animal, and, since in their refuge burrows these squirrels have no back door of escape, capture in such cases must be more or less certain.
On May 19, 1918, a pair of Nelson Squirrels was observed to remain out in the open and watch a pair of Golden Eagles go through a series of aerial evolutions overhead, in which, with talons tightly locked together, the birds looped the loop three times. The eagles were obviously seen by the squirrels, and the swish of their wings was plainly heard by the observer, yet the squirrels were not in the least alarmed. However, when a Red-tailed Hawk flew over, the squirrels hustled at once into their holes. It was therefore inferred that the hawks and not the eagles were the active enemies of these small squirrels. Coyotes and kit foxes also dig out the squirrels in their burrows and probably pounce upon a few individuals during the daytime.
In foraging, these squirrels slip along close to the ground, often stopping in the shelter of a bush or pausing in the open and searching quietly, with body extended, for small seeds upon which they feed. On May 6, 1918, an old female which was nursing young was observed for several minutes at a distance of fifty feet. During this time the squirrel stood at her full height with body erect, busily munching a green head of alfilaria and keeping a careful watch at the same time upon the intruder. A few days later a squirrel of this species was seen gathering dry seeds of the alfilaria. It is difficult in the field to be sure just what kind of seeds the squirrels are seeking out, since the seeds are too small to be seen at any great distance even with the aid of binoculars, and the stomach contents are so finely chewed that it is impracticable to identify the food constituents.
On one occasion an individual was seen to eat the dried flesh from the hind leg of a dead kangaroo rat. This sort of provender had been secured from a near-by meat-baited steel trap. From this incident we conclude that this species of squirrel is not altogether vegetarian in its food preferences.
Very little information is at hand regarding the food carried in the cheek-pouches of this rodent. A specimen taken at McKittrick, Kern County, on May 19, 1911, had 744 seeds of the alfilaria (Erodium cicutarium) in its cheek-pouches. No food stores of any kind were found in the few burrows excavated.
The Nelson Antelope Squirrel is distributed unevenly. It occurs in abundance at only a few localities. At one of these favored localities, eight miles northeast of Bakersfield, squirrels of this species were found scattered over the low hills in little colonies of six or eight individuals (H. S. Swarth, MS). It is believed that there were certain small areas here that supported at least twenty-five of the squirrels to the acre. However, they were present to this extent on only a small per cent of the total acreage inhabited. At San Emigdio Ranch ten squirrels represent the greatest number found on any one acre. At McKittrick the number per acre was thought to be not over five. Taking the entire range of the species into consideration, there is probably about one squirrel to every two acres.
Our impression is that on the east side of the San Joaquin Valley the range of this squirrel is now being rapidly restricted by farming activities. In 1911, and again in 1918, no Antelope Squirrels whatever could be found in the vicinity of the type locality, Tipton, in Tulare County, where it was common in June, 1893 (Merriam, 1893, p. 129). The first Nelson Squirrel was noted in the 1911 search thirty miles south of Tipton. The gradual settling up of the country, and the cultivation of the kind of ground inhabited by this squirrel, has resulted in the crowding out of the species over much of the eastern part of its original range. It seems only a question of time when continued reclamation will gradually restrict and eventually exterminate this species over the arable portions of the San Joaquin Valley.
The Nelson Antelope Ground Squirrel is at the present time of little or no economic importance. It inhabits barren situations, apart from cultivated land. Our opinion is that this squirrel is not likely ever to become a pest.