Natural History of the Ground Squirrels of California/California Ground Squirrel
CALIFORNIA (OR BEECHEY) GROUND SQUIRRELS.
Other names.—Digger Squirrel, part; Beechey Ground Squirrel; Beechey's Marmot; Beechey Spermophile; Spermophilus beecheyi, part; Arctomys beecheyi; Spermophilus grammurus beecheyi, part; Citellus variegatus beecheyi; Citellus grammurus beecheyi; Otospermophilus beecheyi.
Field characters.—A large ground-dwelling squirrel, with long bushy tail, good-sized ears, and general brownish coloration; dull whitish area on side of neck and shoulder, and fine dappled pattern of coloration on back and sides, to be seen in close view. Length of body alone about 10½ inches, with tail (without hairs) about 6½ inches more.
Description.—Adults in summer pelage: Top of head, stripe down middle of hind neck, whole back, sides, and rump, of a general wood brown tone of coloration, but variegated in fine pattern on back, rump and sides by mottlings of snuff brown and buffy white; these mottlings usually line up in transverse rows, the rows being most distinct across the rump; a large area centering on side of neck and involving
Fig. 1. Ears of ground squirrels to show characters of size and shape in different species. a, California Ground Squirrel; b, Fisher Ground Squirrel; c, Oregon Ground Squirrel; d, Stephens Soft-haired Ground Squirrel; e, Mohave Ground Squirrel; f, Yuma Round-tailed Ground Squirrel; g. Sierra Golden-mantled Ground Squirrel; h. Desert Antelope Ground Squirrel. All natural size and drawn direct from specimens. Note: a and b are extreme examples; the average difference existing between the California and the Fisher ground squirrels is much less; individuals of each race can be found which will overlap some individuals of the other in size and shape.
shoulder, and a faint stripe backward a short distance from upper margin of this area, dull white. Cheeks dirty white, changing to wood brown color between eye and ear; eyelids white; whiskers black. Ears tall and conspicuous, finely haired; color of ear inside, pale pinkish buff; back of ear, front half, black, becoming dull cinnamon buff at base and on hinder margin; fine black hairs extending above rim at tip of ear sometimes so numerous and long as to form a small tuft; whole lower surface of body, inner sides of fore and hind legs, and upper sides of feet, pinkish buff; hairs of breast and belly gray at base, this resulting in a darker tone on this area. Palms of fore feet naked; soles of hind feet thinly haired behind tubercles, or else wholly naked, due apparently to wearing away of the hairs altogether; claws brownish black, horn-color toward tips. Tail bushy, though not nearly so much so as in the tree squirrels, flat haired, parallel-sided, and square or round ended; hairs along sides of tail about 41 mm. (1⅝ inches) long, at end of tail the same; general color of tail both above and below buffy grizzled gray, in other words mixed black and buffy white in fine pattern; the buif tone is deeper below than above; close inspection shows the hairing of the tail to be concentrically banded, three black bands and four light ones, the outermost black band being broadest, and the outermost light one constituting a peripheral whitish fringe.
Color variations.—As far as we can see, the two sexes are identical in coloration, save as caused by the greater rate of wear to which the pelage of the female is subject during the season when the young are being reared. Wear progresses in some cases until most of the colored ends of the hairs are gone, and a dingy light brown color is acquired, including also the tail. Molting begins anteriorly and progresses backward.
The material we have studied seems to show but one decided molt in adults each year, and this takes place during July and August. Young, however, seem to undergo two molts in the first six months of their lives. When one-third grown their pelage is characterized by a fluffy texture and a yellowish tone of color, but the general pattern is closely similar to that of adults; when nearly full grown the young are smooth-coated and show rather brighter tones of brown and clearer white shoulder patches that even fresh-pelaged adults.
There are not infrequent special, or "sport" variations, in the Beechey Ground Squirrel, such as albinos, either complete or partial, which have been reported from time to time. We have been told of "black" ground squirrels; and there is in the Museum of Vertebrate Zoology an adult male, from Stanislaus County, taken by W. C. Jacobsen, January 30, 1918, which is of a curious light pinkish-buff tone of coloration, save for the outermost concentric black band on the tail and for the whitish shoulder patches and a suggestion of dappling on the back.
The color description given above was taken from specimens from the vicinity of San Francisco Bay. Specimens from other parts of the general range of the Beechey Ground Squirrel depart from this slightly in different respects. Two specimens at hand from Marysville Buttes, Sutter County, are of paler, grayer tone of general color. A series of skins from the western slope of the central Sierra Nevada averages whiter underneath and darker brown on middle of back. Examples from the southern San Joaquin Valley are paler in tone of coloration and exhibit whiter shoulder patches, thus evidently constituting intergrades towards the Fisher Ground Squirrel. A series from the coast district of southern California, from Santa Barbara to San Diego, shows darker brown back, but whiter under surface of body, and the white shoulder patches are more conspicuously contrasted. Some San Diego County examples in rather worn pelage show a curious reddish tinge on the rump.
Measurements.[1]—Average and extreme measurements, in millimeters, of twenty full-grown specimens from west-central California are as follows: Ten males: total length, 435 (405–475); tail vertebræ, 164 (150–175); hind foot, 57 (52–60); ear from crown, 20.5 (17–24); greatest length of skull, 59.1 (56.9–61.1); zygomatic breadth, 36.9 (35.0–39.2); interorbital width, 14.1 (13.3–15.0). Ten females: total length, 423 (400–460); tail vertebræ, 162 (150–175); hind foot, 57 (55–58); ear from crown, 18.4 (16–20); greatest length of skull, 56.2 (53.8–59.5); zygomatic breadth, 35.8 (34.3–37.6); interorbital width, 13.9 (13.0–14.7).
It will be seen from the above figures that females are decidedly smaller bodied than males though in tail length they are about the same. The skulls of the oldest individuals, particularly males, show greatest general size, greatest zygomatic breadth (as compared with total length of skull), broadest jugals, stoutest postorbital processes, highest developed sagittal crest, most nearly approaching parietal ridges (these meeting also farthest forward), and broadest frontal region (which also shows a concave or "dished" upper surface). In other words, old animals have skulls which are more massive and angular than those of young ones. We find that relative age of an individual can be recognized approximately by the relative degree of development of the above characters. Amount of wear on the crowns of the molariform teeth and of advance in coalescence of the contiguous bones along certain sutures also give criteria for determination of age.
Weights.—Average and extreme weights, in grams, of twenty full-grown specimens from west-central California are as follows: Ten males, 696 (600–923); ten females, 592 (491–774). Averages in ounces: males, about 24 (1½ pounds); females, about 20 (1¼ pounds).
It appears that males are 17 per cent heavier than females. The heaviest specimen out of a total of 36 weighed, was an old male tipping the scales at 923 grams, or 32½ ounces, or a trifle over two pounds. This animal was shot June 12, 1918, in a slaughter yard at Mendota, and was exceedingly fat.
Type locality.—"Neighborhood of San Francisco and Monterey" (Richardson, 1829, p. 170).
Distribution area.—The greater part of central and southern California west of the desert divides. Altitudinally, ranges from sea-level up regularly to about 6,000 feet, and locally and sparsely to as high as 8,200 feet (in Yosemite National Park). As regards life-zone, the California Ground Squirrel is most abundant in the Upper Sonoran zone, less so in the Lower Sonoran and Transition, and but relatively rare and local in Canadian (see fig. 23).
More in detail, this squirrel is limited to the northward in the coast belt abruptly at the south sides of the Golden Gate, San Francisco Bay, Carquinez Strait, and Suisun Bay. Its range extends northward over the eastern half of the Sacramento Valley to the Marysville Buttes and its limits thence swing northeastwardly through the Feather River country to the southern border of Lassen County. From this last point south it covers both slopes of the Sierras nearly to the Yosemite region, but thence south to Tulare County, only the western slope. In the vicinity of Lake Tahoe it gets a little way into the state of Nevada. To the southward it covers most of the San Joaquin Valley, and the coast belt south throughout the San Diegan district to the Mexican border, and beyond this even to the San Pedro Martir mountains.
Along the eastern border of its range, from Tulare County south to Riverside County, the race beecheyi grades into the race fisheri. The dotted line on the map (fig. 17) marks approximately the center of the area of intergradation between the two races. As will be seen, the limits of beecheyi swing west across the southern San Joaquin Valley and thence around south so as to exclude the Bakersfield region and the Tehachapi, Tejon, San Bernardino and San Jacinto mountains.
Specimens examined.—A total of 149 specimens from the following localities in California: San Francisco County: Ingleside Race-track, 1. Alameda County: vicinity of Berkeley, 9. Contra Costa County: Walnut Creek, 12; west side Mount Diablo, 2. San Mateo County: Sierra Morena, 1; Pescadero Creek, 1. Santa Clara County: Palo Alto, 1. San Benito County: Cook, 2. Monterey County: Monterey, 6. Sutter County: Marysville Buttes, 2. Stanislaus County: Claribell Station, 1. San Joaquin County: eight miles southwest of Tracy, 1. Sierra County: near Sierraville, 1. Placer County: Dutch Flat, 1; Blue Canyon, 1; Cisco, 2. El Dorado County: Fallen Leaf Lake, 1; Kyburz Station, 2. Tuolumne County: Aspen Valley, 6,400 ft., 1. Mariposa County: Merced Grove, 1; Crane Flat, 6,300 ft., 1; Indian Creek, 6,100 ft., 1; Yosemite Valley, 3; Merced Lake, 7,500 ft., 2; Mono Meadow, 7,300 ft., 1; Chinquapin, 6,200 ft., 1; El Portal, 2,000 ft., 2; Coulterville, 2; Pleasant Valley, 1. Merced County: Snelling, 1; Los Baños, 1. Madera County: Raymond, 2. Fresno County: Mendota, 4; Panoche Creek, at 502 ft., 1; Friant, 1; Kings River, 5,000 ft., 2. Ventura County: Matilija, 4; Ventura, 3. Los Angeles County: vicinity of Pasadena, 7; near Azusa, 2. San Bernardino County: near Colton, 4. Riverside County: Thomas Mountain, 6,800 ft., 1. San Diego County: Warner Pass, 3; Grapevine Spring, 1; Witch Creek, 8; Julian, 8; Cuyamaca Mountains, 4; San Diego, 2; Point Loma, 10; Chula Vista, 3; near mouth Tiajuana River, 1; Dulzura, 7; Campo, 1; Jacumba, 1; Mountain Spring, 4.
The California Ground Squirrel is probably known by sight to more people than any other one of our four hundred kinds of native mammals. It inhabits open ground in well-settled territory and it forages abroad during the daylight hours when its movements are most likely to attract attention. Numbers are to be seen from the windows of passing trains, and the traveller by automobile is often thrilled by the narrow escapes of those heedless individuals which dash across the road immediately in advance of him, not infrequently to their own undoing. Then, too, this squirrel has, perhaps, been more widely advertised than any of our other mammals. A few years ago it came into prominence as a proven disseminator of the dreaded bubonic plague, and it has become notorious for its exceeding destructiveness to cultivated crops.
The term "Digger Squirrel" is often applied to this species, more especially in the foothill and mountain regions, in recognition of its burrowing habits, to distinguish it from the tree-inhabiting gray and red squirrels. The book name, Beechey Ground Squirrel, much used in the literature relating to it, is derived from the accepted scientific name Citellus beecheyi. This name, beecheyi, was bestowed upon the animal by its original describer (Richardson, 1829, p. 170) "in honour of the able and scientific Commander of the Blossom," Captain F. W. Beechey. The British ship "Blossom" cruised the Pacific Ocean northward even to Bering Strait during the years 1825 to 1828. Collections of specimens were brought back from many localities visited, including San Francisco and Monterey; among these specimens was one or more of the squirrels in question. These were evidently preserved for the most part by Mr. Collie, surgeon of the ship, who is quoted by Richardson as stating that "this kind of Spermophile 'burrows in great numbers in the sandy declivities and dry plains in the neighbourhood of San Francisco and Monterey, in California, close to the houses. They frequently stand up on their hind legs when looking round about them. In running, they carry the tail generally straight out, but when passing over any little inequality, it is raised, as if to prevent it being soiled. In rainy weather, and when the fields are wet and dirty, they come out but little above ground.'" And further information is given, according, for the most part, with what anyone can see for himself today in the same general region. This attests to the acuteness of observation of Mr. Collie, and also shows how the squirrels had already, some ninety years ago, begun to impress people with their numbers and boldness.
The California Ground Squirrel may be distinguished from other members of the squirrel family by the combination in it of the fallowing characters: essentially ground-dwelling habits, relatively large size, long bushy tail, tall pointed ears, and generally grayish coloration with a three-cornered silvery white patch on each shoulder. Close inspection discloses a finely dappled pattern of coloration (see Fig. 2) such as is not shown in any tree squirrel or in any of our other ground squirrels except its near relatives, the Douglas, Fisher, Rock and Catalina Island squirrels. The detailed descriptions, measurements, etc., as given in the accompanying small-type paragraphs, should be studied for further particulars in this connection.
Fig. 2. A typical example of the California Ground Squirrel, the rodent of chief economic importance in the state. Note the tall, rather pointed ears, the whitish eyelids, the grizzled-white shoulder patch, the mottled back and rump, and the bushy tail. Photographed from life by J. Dixon.
The species now under discussion is restricted in its distribution mainly to the state of California. It extends a little ways south into Lower California; and to the eastward it barely crosses the Nevada line in the vicinity of Lake Tahoe. To the northward in the coast belt it is cut off sharply by the Golden Gate and San Francisco Bay, but in the interior it extends to the headwaters of the Feather River. Southeastwardly toward the deserts the race beecheyi blends into the race fisheri, which in turn is wholly cut off by the hottest deserts beyond. (See map, fig. 17.) In the coast belt north of San Francisco Bay and from the upper Sacramento Valley northward the Beechey Squirrel is replaced by the Douglas Ground Squirrel.
Altitudinally the California Ground Squirrel ranges from sea level, as on the shores of Monterey Bay, up to an altitude of at least 8200 feet, as in the Yosemite National Park. It is most abundant on the plains of the San Joaquin and in the Coast Ranges and Sierra foothills. As regards life-zone, the metropolis of the species lies in the Lower and Upper Sonoran (see fig. 23). It is less numerous in the yellow pine belt (Transition zone), and is but rarely or sparingly represented in the Canadian zone, still higher on the mountains. Its preferences as to local conditions are not closely limited, except that it avoids dense chaparral and thick woods. It frequents pasture lands, grain fields, orchards, sparsely tree-covered slopes, small mountain meadows, rock outcrops on the tops of ridges, and even granite talus slopes. It is always most abundant, however, in the open situations, and its decided preferences are such that it thickly populates much of the best farming and grazing lands in the state, to the great reduction of their producing value from the human standpoint.
This squirrel secures shelter for itself and young, and safety from its enemies, by burrowing in the ground. Where possible it chooses to excavate its retreats in hillsides or in low earth banks. Here some at least of the necessary digging can be done in a horizontal direction. But, of course, those members of the species which live on the plains or on small flats or meadows in the foothills or mountains must dig down vertically for considerable distances to gain the requisite protection. Many of the squirrels which live in the granite country make their homes under large boulders or in rock taluses where a minimum of burrowing is necessary to insure safe retreats. On wooded hillsides special safety from enemies that dig is secured by location of the burrows under tree-roots or old stumps.
In the foothill region at the southern end of the San Joaquin Valley there seems to be a decided tendency on the part of the ground squirrel to select alluvial fans for home sites. This preference may be accounted for by the fact that the plants upon which the squirrel feeds make a better growth in the deep alluvial soil there than they do on the adjoining hillsides which are often steep and with but shallow cover of soil. For the same reason the alluvial fans afford easier digging to considerable depth and hence better protection. In seven burrows in different localities, in which the squirrels were gassed and then dug out (J. Dixon, MS), the extent, diameter and depth were found to vary and to depend largely upon the nature of the soil. In shallow adobe or clayey soil, underlaid by broken rock, the burrows were found to be short, of 
Fig. 3. Four characteristic poses of a California Ground Squirrel. Photographed by J. Dixon at Berkeley in August, 1918.
small diameter, and not reaching to any considerable depth. Those in alluvial or sandy soil were found to be of large diameter, of greater extent, and to reach to much greater depths.
The most conspicuous signs of activity on the part of ground squirrels in any locality are the large mounds of earth that have accumulated in the course of excavating the burrows. This earth is commonly thrown out in a fan-shaped pile directly in front of, and to the sides of, the main entrance to the burrow (see fig. 4). These mounds of earth are often three or four feet in diameter and from six to ten inches above the general level. They vary greatly in size, but average larger in sandy soil than in clayey or rocky ground. The size of the mound is, however, no reliable index to the length or size of the burrow except in those cases where the burrow is of a straight or simple pattern. In
Fig. 4. Mound and burrow entrance of a "digger" squirrel, in sandy ground. Mounds of earth such as these are often three or four feet in diameter and rise from six to ten inches above the general level. The route taken to the feeding grounds being used at this particular time is indicated by the numerous tracks at the left-hand side of the entrance. Photographed at Tipton, Tulare County, May 23, 1918.
colonial or intercommunicating burrows the dirt is not always thrown out at those entrances which allow of the shortest possible "haul."
Most of the work of tunnel excavation is carried on during the spring months, as is shown by the mounds of fresh, soft earth accumulated at the mouths of the burrows in that season. In the lowlands, where there is a large crop of wild oats in the springtime, this newly excavated earth supports a ranker growth than the surrounding parts of the field, so that, as one of our party wrote in his field notes, "the plain looks like a cemetery overgrown with grass," with these taller stands of oats about the squirrel holes suggesting grave mounds.
To some extent the ground squirrels, like the pocket gophers, thus act on wild land as natural cultivators of the soil, and may thus serve a useful purpose. On the other hand, their burrows are frequently the cause of much destructive erosion on hillsides during heavy rainstorms. Numerous small landslides have been noted on steep hillsides on the campus at Berkeley, that were plainly caused by the presence of squirrel burrows which had concentrated and conducted the water in narrow channels instead of permitting it to spread out and soak in or run off in the natural way. The presence of squirrels along irrigation canals results in the embankments becoming undermined by their burrows, with ensuing disastrous breaks in the canals, especially at times of high water.
Digger Squirrels are firm believers in the daylight saving plan. Their activities above ground are restricted to the hours between sunrise and sunset. They love the warm sunshine and may often be seen sprawled upon the summits of stumps, rocks or other points which afford safety as well, basking in the morning or later afternoon sunshine. During spring and summer they come out of their burrows soon after sun-up. They are at those seasons most active during the middle of the forenoon and again during the late afternoon, but avoid the intensest heat of midday. During midwinter those squirrels which do not remain underground altogether make their appearance only late in the forenoon of bright sunny days. Light and warmth seem to be essential to their successful existence aboveground.
The observer afield often comes upon ground squirrels which are some distance from their holes. Such animals usually run, with bodies and tails undulating and closely paralleling the ground, to the near vicinity of their burrows, where they then post themselves in upright position. They can then watch the intruder, yet be in readiness to dart down into their holes at an instant's warning. While thus on watch a squirrel is wont to repeat, at regular intervals of from two to five seconds, its characteristic "bark." This note is really a double one, and may be indicated by the syllables, clink-sup. The second syllable, however, is not audible for any great distance, while the first is loud, staccato, and of decided metallic quality, calling to mind the sound produced by the blow of a light hammer on an anvil. The impression is enhanced by the regularity and frequency of its utterance, and this will be kept up five minutes at a time. Sometimes, when a squirrel is startled, it gives a more prolonged note, clink-sup-sup-sup-sup, the last syllables running together as a sort of chuckle. In any event, it is the clink which is the metallic syllable, and which one hears a long ways, more or less mellowed by distance.
If closely pressed the squirrels drop down at once into the protection of their subterranean retreats. Ordinarily when thus frightened down they do not reappear at the surface of the ground for some minutes, five to twenty-five minutes in tested cases, as if to give the suspected enemy a chance to tire of his waiting and depart. Occasionally, when surprised at a distance from its burrow, a squirrel will crouch motionless, it may be almost at the feet of the observer, as if to escape detection by the "freezing" ruse. Extreme fear also on occasion may be part of the basis for this mode of behavior.
In some respects the California Ground Squirrel is much "wiser" than is generally supposed. This has been forcibly impressed upon the junior author during his endeavors to secure photographs. Living squirrels were then observed at close range in their various activities under natural conditions. Several species of chipmunk, as well as the Golden-mantled Ground Squirrel and the Nelson Antelope Ground Squirrel, encountered under the same circumstances, soon became accustomed to the camera so that the photographer, himself at some little distance, was able to release the shutter when the lens of the camera was less than thirty inches from the animal photographed. Compared with this, the Digger Squirrels proved exceedingly wary, refusing to show more than their heads even though the camera was disguised and placed six feet distant. The confidence of the Digger Squirrel could no doubt be gained, given sufficient time; but when an approach was attempted by the methods that had proven successful with the other squirrels the results were nil. It looks as though the reactions of the Digger Squirrel had been adjusted to meet that category of enemies which lie in wait at jumping distance.
Fig. 5. Typical "hog-wallow" land showing trail and burrows of the California Ground Squirrel. Photographed ten miles north of Fresno, April 10, 1911.
Ground squirrels traveling to and fro, between their holes and their feeding grounds frequently traverse the same courses until regular radiating trails 2½ or 3 inches wide are worn through the grass (see fig. 5). This is particularly well seen on many hillsides, and on the rolling "hog-wallow" lands along the eastern side of the San Joaquin Valley, where, in the fall, when the grass and weeds are dry, the trails show most distinctly. In the spring, when the new growth is just appearing, the trails are still conspicuous, as the vegetation is slower in starting there than in the adjacent unbeaten tracts. Soon, however, the trails are entirely obliterated, save as the animals renew them by further use.
In foraging for seed-pods, grain, or fruits, the ground squirrel does not usually eat the food on the spot where it is gathered, but he stuffs it into his capacious cheek pouches (see fig. 13) or else, if it is too large for this, carries it in his mouth nipped between the incisor teeth. He then repairs to some point of vantage such as a rock pile or to the mound at the entrance of the burrow. Here he proceeds to hull and devour the food at leisure and at the same time is near enough to shelter so that he can quickly duck in should an enemy suddenly appear at short range. These "husking places" are conspicuously marked by the hulls of seeds and by the rinds and pits of cultivated or wild fruits. Examination of these "kitchen middens" will sometimes give a pretty accurate idea of the character of the squirrels' rations in any locality. A great many of the matured seeds, however, are carried directly below ground to the permanent storehouse.
Droppings, or feces, of the Ground Squirrel are to be observed widely scattered rather than deposited in piles. They may be found about the "husking places" or along the trails or paths which lead from the burrow to the feeding grounds. In the burrows they are accumulated in special places evidently set aside for the purpose. The feces are generally of a cylindrical shape, rounded at the ends, but are quite variable in diameter and volume. In April when green food is abundant fresh feces are of a greenish hue and are often soft and flattened. During the drier portions of the year the droppings are covered with a dark brown coating, while the interior is composed of a dry mass consisting of hulls of weed seeds and finely chopped and shredded vegetable fiber, from 3 to 10 millimeters long. A typical dry dropping measured 16 millimeters (⅝ inch) in length, with a diameter of 6 millimeters (¼ inch), and weighed 1⁄10 of a gram.
The California Ground Squirrels do not dwell in thickly populated "colonies" of sharply restricted extent, as is the case with the prairie dogs of the Middle West. Still there is with our rodent a tendency to occupy certain definite tracts in a general territory to the exclusion of intervening places and this without obvious reason as regards food
Fig. 6. Plot (plan and elevation) of used burrow of a male "digger" squirrel, as excavated by J. Dixon on an alluvial talus in the foothills near San Emigdio, Kern County, April 28, 1918.
Main entrance at a; refuse sump in old nest-cavity at b; "blind" exit in thick grass at c. Unusual depth of burrow, as shown in profile, was due to thick rock-filled overlying stratum, beneath which the squirrel had found easy digging horizontally after having once penetrated the less resistant part of the layer at the edge of the talus.
Total length of burrow, 34 feet; average diameter, 4½ inches; greatest depth reached, 5½ feet; volumetric contents of entire burrow, 44⁄5 cubic feet.
supply and kind of soil. It would seem that centers of population may arise through the historical circumstance of original settlement by first-comers. This would be particularly the case in fields newly invaded, where descendants would establish their burrows in the near vicinity of their pioneer parents.
Digging operations were carried on by us during the breeding season of the ground squirrel, in quest of all obtainable facts in regard to their habits underground. Three general types of burrows were encountered. The male squirrels were usually found in short, shallow, simple burrows at the outskirts of the "colony." The burrow belonging to a male herewith illustrated (fig. 6) proved to be longer than usual with males,
Fig. 7. Plot (plan and elevation) of used nesting burrow of a female "digger" squirrel, as excavated by J. Dixon and G. R. Stewart on a west slope in Strawberry Canyon near Berkeley, April 6, 1918.
Entrance at left; old nest chambers at a; refuse sump at b; used nest at extreme right, which was found to contain the mother and four small young.
Total length of burrow, 22 feet; average diameter, 4½ inches; greatest depth reached, 30 inches; volumetric content, 44⁄5 cubic feet.
and reached to a greater depth, but its simplicity is characteristic for that sex. We failed to secure a single male squirrel in any burrow found to be occupied by a female with young. It is believed that at least during the breeding season the male squirrels live altogether by themselves in their own individual burrows.
A burrow from which a female and four young with eyes still unopened were secured is shown in fig. 7, It will be seen from this illustration that the nest burrow of the female is relatively complicated. This particular burrow was extremely difficult to follow on account of the many turns and "blind alleys."
The third type of burrow (see fig. 8) might well be called a "colonial burrow," as it is used by both sexes and also by the young after these leave the nest burrow and begin to forage for themselves. Colonial burrows are used largely as "safety zones." They afford convenient places for the squirrels to duck into when danger unexpectedly appears. These burrows are often from 100 to 200 feet in length and form a communicating system of underground runways connecting from six to twenty entrances or surface openings. The nests in the colonial burrows were old and had the appearance of having been used by many
Fig. 8. Plot (plan and elevation) of a "colonial" burrow-system of "digger" squirrel in sandy ground in irrigated section near Bakersfield; excavated by J. Dixon and H. G. White, May 3, 1918.
Various entrances at h; food store at a; "back-door" exit for emergency purposes at b; nest cavities as indicated.
Total length, 138 feet; average diameter, 4¾ inches; greatest depth reached, 4 feet; volumetric content, 174⁄5 cubic feet.
individuals at various times. These colonial burrows were not found to be in any case used as breeding burrows. It is possible that they may have consisted of one-time breeding burrows, now connected or linked together.
The relative extent of any one burrow system is thus dependent not only upon kind of ground—in other words, upon the difficulties encountered in digging—but also upon the estate of the individual or individuals directly concerned. Table I gives data in regard to the three types of burrows. In the seven burrows which were dug out and of which careful record was kept, the shortest occupied burrow was five feet long and the longest 138 feet. The average was 35.2 feet. The average diameter varied from 3½ to 5 inches, with a mean of 4.3 inches. The cubic air content was found to vary from 1.03 to 17.8 cubic feet, the average being 5.2 cubic feet.
Table I. Data relative to burrows of the California Ground Squirrel.
| Type of burrow | Locality | Date (1918) | Length of burrow (in feet) | Greatest depth of burrow (in inches) | Average diameter of burrow (in inches) | Cubic content of burrow (in cubic feet) |
| Male | Strawberry Canyon, Berkeley | April 3 | 5 | 13 | 3½ | 1.03 |
| Male | Strawberry Canyon, Berkeley | April 4 | 8 | 30 | 4 | 1.40 |
| Male | Strawberry Canyon, Berkeley | April 4 | 14 | 30 | 4½ | 2.40 |
| Male | 12 miles west of Fresno | May 27–28 | 26 | 45 | 5 | 4.6 |
| Female | Strawberry Canyon, Berkeley | April 6 | 22 | 30 | 4½ | 4.8 |
| Male | San Emigdio Creek, Kern Co. | April 27–28 | 34 | 66 | 4½ | 4.8 |
| Colonial | 12 miles south, 5 miles west of Bakersfleld | May 3 | 138 | 48 | 4¾ | 17.8 |
| Average | ----------- | 35.2 | 38.1 | 4.3 | 5.2 |
In illustration of the fact of variability in depth and extent of burrow system with nature of soil, some actual instances as revealed, by excavation may be described. The layers of alkali hardpan in the Fresno region were found to have a very decided influence on the course of the burrows. In most cases where the hardpan was near the surface, the burrows were found to extend through the hardpan to the soft ground that is often to be found just beneath. No evidence was found to indicate that the squirrel had dug through even thin layers of solid hardpan except at points where natural cracks or openings through it occurred. Slight cracks in the hardpan were sometimes enlarged, this apparently having been done during wet weather, to sufficient size to enable the squirrel, but not such an enemy of the squirrel as a coyote or badger, to readily pass through. In following the various cracks and openings through and between the strata of hardpan, the burrows were found to twist about in very erratic fashion. The sudden elevation in a burrow of sometimes as much as two feet was found to form a very effective barrier to the flow of any gas such as that of carbon bisulphid, which is heavier than air; such a gas would gather into the low places (see Stewart and Burd, 1918).
The deepest burrow system uncovered was situated in an alluvial talus in the foothills near San Emigdio, Kern County. The maintenance of the great depth (from four to five and a half feet for a distance of twenty feet) was clearly due to the squirrel having followed a soft layer at the margin of the talus down to below the level of the four-foot rock-filled surface layer. Beneath this the squirrel had progressed easily through the soft soil as long as he kept beneath the rocks—which he was practically forced to do (see fig. 6).
There seems to be little or no evidence to support the rather widespread notion that ground squirrels burrow down until they reach water. A colony burrow was unearthed in an irrigated section near Bakersfleld, where the water level was known to be only five or six feet below the surface of the ground. No part of the burrow (see fig. 8) was found to extend deeper than four feet and hence not down to the water level. While ground squirrels do not absolutely require water, where surface water is to be had they often go considerable distances to secure it, going across the country sometimes as far as a quarter of a mile. In many places squirrels are found thriving where it is known that it is over 100 feet to ground water and miles to surface water.
It is quite likely that California Ground Squirrels construct new burrows from time to time, or, what is more probable, that each young individual as it approaches maturity leaves the parent burrow and digs a home for itself. In any event, in places there are many more burrows than individual squirrels present at any one time. Some of these tunnels, especially in the plains and foothill country, are joined together below ground to a greater or less degree and constitute the colonial burrows already described. When hurriedly seeking safety a squirrel will pitch down into the nearest one of a number of holes in the vicinity of the one about which it was first seen. The commonly uninhabited burrows may thus serve in extremity as temporary refuges.
The burrows of the squirrels are often inhabited by species of animals other than the rightful owners. Ground owls habitually make their homes in squirrel holes, probably deserted ones; and, to a less extent,
Fig. 9. Nest and male of "digger" squirrel as dug out after burrow was treated with carbon-bisulphid. The spherical shape of the nest-cavity and the structure of the nest itself is well shown.
the holes are frequented by California toads, Western gopher snakes and Pacific rattlesnakes. It is unlikely that the presence of the latter two animals is congenial to the squirrels, as both of these snakes are known to eat ground squirrels in numbers. Regularly communal occupants of squirrel burrows are scorpions, centipedes and mole crickets. Mole crickets were found to serve as reliable indicators of the efficiency of the gas when squirrels were fumigated in their burrows. If the gas had not killed the crickets it was found that the squirrels had not succumbed.
California Ground Squirrels are accustomed to furnish their underground quarters comfortably. Special nests are constructed and maintained in good order, where the individual may sleep or rest in warmth, free from contact with the damp earth. Each burrow occupied by a single squirrel was found to contain at least one well-made nest. In some eases there were two, one obviously older than the other. In the colonial burrow that was dug out, three nests were found, of which two were new. The nests were always placed well back in the burrows (see figs. 6, 7), where they would have maximum protection from digging enemies such as coyotes and badgers. The cavities in which the nests were placed were short globular chambers and were usually situated slightly above and to one side of the main run, so that the drainage was away from rather than into the nest. The cavity in which a nest containing a female and four small young was found measured 10 inches in length, 9 inches wide and 7 inches high. The nest cavity used by a male squirrel was 12 × 10 × 7 inches (in the same dimensional order), while the two nest cavities in the colonial burrow measured 12 × 10 × 8 and 12 × 12 × 7 inches, respectively.
All of the nests found were of similar composition and construction. Finely shredded dry grass blades and roots, and fine stems of foxtail and needlegrass, formed the bulk of the constituent material. The nests were spherical in shape and deeply cupped. The walls were from
Fig. 10. Nest and female of "digger" squirrel as uncovered after the burrow had been gassed. Excavated on the University of California Campus, Berkeley, April 6, 1918.
two to two and one-half inches thick. The walls of the nest which contained the young squirrels were arched over and met at the top, forming a sort of a canopy. Entrance to this nest was gained through a hole near the top. The material in the walls had been compressed or felted into a thick, warm fabric. The outside dimensions of this nest were 10 × 9 × 7 inches, while the inside cavity measured 6 × 4 × 5 inches. Compared with that of the female just described, the nests of male squirrels were smaller, had lower walls and were more loosely constructed (see fig. 9). The nests of the males did not completely fill the cavities in which they were placed, as did the nest of the female. A nest occupied by a male measured 8 × 10 × 7 inches outside and 4 × 5 × 4 inches inside. The three nests in a colony burrow excavated were large and evidently of considerable age, since the foxtail blades and stems composing the nests were old and broken up into short bits. One of these nests measured 12 × 10 × 8 inches outside and 6 × 5 × 4 inches inside, while the other measured 12 × 12 × 7 inches outside and 6 × 4 × 5 inches inside. The third nest was old, being merely a flat mat of trampled down bits of foxtail stems.
The nest of the female (fig. 10) was 30 inches below the surface of the ground. The nest of a male was 28 inches below the surface, while the two used nests in the colony burrow were 20 and 24 inches underground. The female and the male nests were in clay ground and the two colonial in sandy soil. The average depth of nests below the surface of the ground, taking into account all of the nests found, was 30 inches.
Contrary to general belief, we have found ground squirrels to be very cleanly animals about their nests and burrows. No feces (droppings or dung) were found in any nest. Such material was found
Fig. 11. Nest and small young of "digger" squirrel after removal of the female. Same nest as shown in Fig. 10.
heaped up in piles in special chambers usually just off the main run, but within easy reach, 18 to 24 inches, from the nest. These sumps were lower than the nests and were sometimes nothing but old nest cavities which had been dug somewhat deeper than they had formerly been. In the burrow of a male squirrel, a pile six inches in diameter and two inches high, of feces soggy with urine was found in a sump slightly below and fourteen inches distant from the nest. Female squirrels appear to be more particular in this regard than the males, in that the sump is farther removed from the nest.
The nest that contained the four young squirrels (see fig. 11) was alive with fleas, which swarmed over the helpless young. These fleas persisted in remaining in the nest for three days after the young squirrels had been removed. Other nests were found to be infested with fleas, though at least one-half of the nests examined were free from these parasites. In certain localities squirrels were taken that were to casual appearance absolutely free from fleas, while in other localities squirrels taken were invariably infested to a greater or less extent. The species of flea that infests ground squirrels is not the species that commonly attacks human beings. While ground squirrels are their preferred hosts, we found that the former did not object to human society when their squirrel hosts had died. At least two methods are used by the squirrels to rid themselves of these uninvited guests. The first, or dust-bath method, is that of suffocation of the fleas which hide in their fur by thorough wallowing in especially dusty places. The second is by digging a new burrow and making a new nest, thereby leaving the bulk of the fleas behind.
Some years ago it was discovered that the fleas harbored by the California Ground Squirrel carried the bacillus of bubonic plague. A vigorous campaign of extermination was waged against the squirrels by the United States Public Health Service and they were practically eliminated from many areas, locally, in the San Francisco Bay region.
Fig. 12. Diagram showing extent and height of breeding season in the California Ground Squirrel. Heavy line shows percentage of females pregnant, for weekly periods from January to May. Based on record of embryos found in over 10,000 females examined by the United States Public Health Service (McCoy, 1912, p. 1070).
Soon after the efforts against the squirrels were relaxed the latter began to "spill in" from adjacent areas until now in places they are as numerous as ever. Nevertheless the prime object was attained, that of eliminating the foci of dissemination of the disease.
It is extremely important to know definitely the season and rate of breeding of any economically important rodent. Fortunately for our purpose, there is available for the California Ground Squirrel abundant data, supplied through the records of the United States Public Health Service (see McCoy, 1912, p. 1070). As will be seen from the accompanying diagram (fig. 12) based on over 10,000 females examined, the breeding season is restricted to a comparatively brief period of the year. Pregnant female ground squirrels have been taken in the Bay region as early as the first week in January, but the main breeding season does not begin until February, and it is practically concluded by the middle of April. The largest per cent of pregnant females is to be found during the last week in February. By June 4 only two-tenths of one per cent (0.2) of females examined contained embryos. At higher altitudes, where warm weather comes on much later and more abruptly, these breeding dates would be correspondingly later and the breeding season still more restricted.
While males and females occur in practically equal numbers, mating seems to be promiscuous; there is no permanent pairing off.
The number of young per litter, as ascertained from counts of embryos, varies from 4 to 11. The average, from very extensive records kept by the United States Public Health Service (McCoy, 1912, p. 1070), may be inferred to be very close to 7.2. The same records serve further to show that there is some variation in size of litter from month to month. The average for February is 6.9; for March, 7.3; for April, 7.5; for May, 6.8. The tendency seems to be toward slightly larger litters in April, which is beyond the date of maximum number of pregnant females (see fig. 12). Number of mammæ (nipples), which is usually six pairs in this species, occasionally but five, is no criterion for number of young per litter. All the evidence at hand indicates that each female raises but one litter each year. A female ground squirrel was taken on the University campus at Berkeley, on March 13, 1918, which contained eleven embryos each of which measured three-fourths of an inch long. Eight of these were contained in one branch of the uterus and three in the other. Another female taken at the same time contained eight embryos, each of which measured five-eighths of an inch long. Five of these were in one branch of the uterus and three in the other.
On April 6, 1918, G. R. Stewart and the junior author dug out two female ground squirrels which had been previously gassed in their burrows. One of these females was found in a nest with four small young which we took to be about ten days old, since their eyes were not yet open. These baby squirrels averaged 170 millimeters or 6¾ inches in length; a typical one weighed 61 grams, or a little over 2 ounces. They were well covered with hair, which already showed on the back the characteristic dappled pattern of the adult squirrel. The tail, however, was nearly round and showed little sign of the fringe of hairs along the sides. Their stomach contents showed no sign of their having eaten green vegetation or anything else than milk. Data from other sources indicate that the young are not completely weaned until they are at least half grown. The other female secured in an adjoining burrow not over ten feet distant was found to contain seven small embryos each of which measured three-eighths of an inch in length. These embryos could not well have reached full development short of two or three weeks, so we have a variation of nearly a month in time of birth at one locality.
Cases such as those just given are thought to be exceptional and may serve in part to explain the occurrence of late litters such as have been the basis of the claim that this animal has two litters a season. Litters of young squirrels which sometimes appear very late in the season are, too, likely to be merely the result of efforts to replace first litters of young which have met an untimely death. Thus two litters might be born in one season, though only one raised.
Shaw (1916, p. 4) gives 24 or 25 days as the period of gestation in the Columbian Ground Squirrel (Citellus columbianus) in the region about Pullman, Washington. The period of gestation of the California Ground Squirrel has not to our knowledge been determined; yet the facts at hand, such as the general rate of development of the embryos, and of the young after birth, lead us to believe that it is close to thirty days.
The bulk of the young ground squirrels in any one locality make their appearance with remarkable uniformity as to size and regularity as to date. Our data is incomplete, as to exact time of birth; but we have plenty of records of embryos in various stages of development, and we can observe the time of appearance of the young squirrels above ground. In the lowlands the majority are probably born the last of March, and by the last of April the first born are beginning to appear aboveground, playing about the mouths of the burrows. In the higher altitudes the young are born later. Females in the Transition Zone and lower part of the Canadian Zone had not yet given birth to their young in June. "Spring" in the lowlands comes in April, while the spring of the higher altitudes does not occur until late June or July. Hence the young do appear at the same season, considering the differences in temperature conditions at the different elevations. The accumulation of a certain quantity of heat from without seems to be necessary each year to start the squirrels breeding.
Young California Ground Squirrels may be considered fairly precocious. They ordinarily begin to venture outside their nest burrows when yet very small, in ascertained cases only one-fourth or even one-fifth the weight of the adults. They are then probably not over four weeks old. At Snelling, Merced County, on May 28, 1915, C. L. Camp (MS) observed that young "evidently just emerging for the first time in their lives, seemed confused when they saw a horse and buggy and often ran almost directly under the wheels." Two months later, in the high mountains, the young squirrels behaved the same way. A probably abnormal occurrence was that of a very young squirrel found on April 29 wandering aimlessly about in the grass near a burrow entrance. This squirrel weighed only 61.5 grams, or less than one-tenth the weight of adults. It was practically helpless and would have fallen easy prey to any sort of predaceous animal.
The first litter of young ground squirrels seen aboveground in the season of 1918 by the junior author was noted on April 28 at 1,500 feet altitude on San Emigdio Creek, Kern County. In this litter there were six young at least one-third grown. Judging from the "sign" about the burrow, these youngsters had been foraging above ground for a week or ten days. The season at this altitude was at least ten days later than it was down on the lower parts of the San Joaquin Valley. Several burrows of small diameter and amateurish construction were found at the edge of a thick patch of alfilaria that grew near the nest burrow. These young squirrels in spite of their small size were busily harvesting the heads of the ripening alfilaria and when alarmed ran down the small burrows which each had dug for himself. While the observer was standing over one of these burrows a youngster came up halfway out of a hole six feet away, but catching sight of him gave a hasty alarm note and scurried back down the hole. Twenty-five minutes elapsed after this before any of the young squirrels reappeared above ground.
As far as is to be observed, the male takes no active interest in the welfare of the young. Indeed, he dwells altogether separately from the family and does not see his offspring until they begin foraging out of doors. His only function at all, as regards the upbringing of the young, is that of sounding general alarm throughout the colony when danger threatens. As for the mother, even she is notably indifferent to her young after they appear above ground. When suddenly alarmed, she flees to safety on her own account, leaving the youngsters to shift each for himself as best he may.
The rate of growth of the young is such that they reach mature size by September, when they are from four to six months old (McCoy, 1912, p. 1069). But before this time, by the first of August, the young of the year begin to emigrate locally, so as to establish each for himself a new home. It is likely that this process of emigration is hastened by the development on the part of the parents of an attitude of incompatibility. According to this idea the initial solicitude of the mother for her young at the helpless age is later reversed, so that she becomes antagonistic to them and finally speeds their departure. The young, at the same time, begin to give evidence of an instinct to wander. At any rate, the month of August sees the important phenomenon of emigration or dispersion well under way. Young of the year then put in their appearance in unexpected places; new ground is invaded, and the total territory occupied by the squirrels increased in extent insofar as the increase in population makes necessary and the favorable nature of the country permits. Undue congestion of population tends thereby to be prevented.
The natural enemies of the California Ground Squirrel are of many kinds, and under original conditions so many as regards individuals as to provide a regular automatic check to any abnormal increase of the squirrel. The most important are golden eagles, red-tailed hawks, coyotes, badgers, wildcats, weasels, rattlesnakes and gopher snakes. Each of these various animals pursues the squirrels in its own particular way. Hawks and eagles swoop down on them from their vantage points in the air. Wildcats and coyotes lie in wait near the burrows until the squirrels venture forth in search of food, when they pounce upon them. Badgers, weasels and snakes capture the squirrels in their burrows. Some specific cases will be cited here. It must be remembered that, while casualties to squirrels may be inflicted by their customary enemies almost hourly in any general neighborhood where man has not exterminated these predators, the chances of a person's being in a position at the critical moment to witness a tragedy of this sort are rare. At the San Emigdio Ranch in Kern County on April 25, 1918, the junior author watched a Golden Eagle (Aquila chrysaetos) capture and devour an adult ground squirrel. The eagle was first observed flying quietly down a canyon. By weaving in and out in its course the bird was able to skirt the irregular hillside so as to keep within fifteen or twenty feet of the ground. At length the eagle skimmed abruptly around the shoulder of a hill, just clearing the tops of the wild oats, and dropped quickly down upon a luckless ground squirrel. The latter had evidently been on a foraging expedition and did not have time to reach his burrow, so complete was the surprise. The eagle seized the squirrel with both sets of talons, and the piercing grip by these effective instruments quickly dispatched it. The bird then proceeded to tear the animal to pieces with the stout beak and, perched on the ground, devoured it on the spot. The strategy and success of this method of attack was obviously dependent upon the eagle keeping close to the ground so as to remain out of the squirrel's range of vision until the last moment.
At Pleasant Valley, Mariposa County, on May 17, 1915, C. L. Camp (MS) fed a ground squirrel that had been shot, to a Golden Eagle kept captive by a storekeeper there. The eagle ate head, skin and bones, but discarded the stomach and large intestines. Other birds, such as the turkey vulture, have been observed by the junior author to similarly avoid the stomach and intestines of ground squirrels that have been killed by taking poisoned barley. Coyotes have also been known to show the same fine discrimination when eating ground squirrels which they themselves have not caught.
Some idea of the success with which Golden Eagles sometimes pursue ground squirrels may be had from the fact that at Lilac, San Diego County, on April 4, 1907, James B. Dixon (MS) found eleven freshly caught ground squirrels in and about an eagle's nest that contained two eaglets about a week old.
During the spring of 1904 W. L. Finley and H. T. Bohlman observed and photographed a pair of young Golden Eagles in various stages of development from the time the eaglets were nine days old until they left their birthplace nearly three months later. The aerie was a bulky affair placed in a horizontal fork of the upper limbs of a large sycamore tree that grew in a canyon back of Mission San Jose, Alameda County. In speaking of the food of the Golden Eagle, Finley (1906, pp. 9–10) says: "His food consists almost entirely of the ground squirrels that are so abundant through the California hills. On our second trip [on April 12], when we looked into the nest, we found the remains of the bodies of four squirrels lying on its rim. At each visit we examined the food remains and the pellets about the nest, and we are sure that a very large proportion of the eagles' food supply consisted of squirrels. . . . I am satisfied that this family of eagles regularly consumed an average of six ground squirrels a day during the period of nesting, and, very likely, more than that. . . . But even this low estimate would mean the destruction of 540 squirrels along the hillsides in about three months' time."
The nest of a Western Red-tailed Hawk (Buteo borealis calurus) examined by J. B. Dixon (J. Dixon, 1917, p. 12) on March 28, 1906, and containing one day-old chick, two pipped eggs and a rotten egg, was found to contain also the remains of two ground squirrels. This was near Vista, San Diego County. At Pala, in the same county, the same observer found the nest of a Red-bellied Hawk (Buteo lineatus elegans), April 3, 1916, containing three young, a week old, together with one ground squirrel and two pocket gophers. The dead squirrels counted in the nests represent, of course, merely the surplus which the old birds had just carried to the young. The squirrels that the old birds themselves or the young may have eaten on the day of observation are not taken into account.
At Dunlap, Fresno County, on September 30, 1916, H. S. Swarth (MS) found a large rattlesnake (Crotalus oreganus) which showed a bulge in the middle portion of its body. This proved to mark the location of a full-grown ground squirrel, which had been swallowed entire, head first.
Near the mouth of Tejon Creek, Kern County, on July 16, 1914, C. L. Camp (MS) watched a rattlesnake (Crotalus oreganus) about three feet and a half long, swallow a ground squirrel. He describes the incident essentially as follows: The snake had just bitten the squirrel on the side of the face below the eye. The squirrel flopped about for five or ten minutes and then dropped over a bank and died, out of sight of the snake. The snake then slowly crawled down over the bank after its prey, found it, touched it all over with the end of its tongue, and then seized the animal by the nose. The squirrel moved slightly. The snake drew back and waited motionless for some time. The snake then got a fresh hold on the squirrel's nose, pulled the body out straight, and started to work its jaws over the squirrel's head. Things went rapidly as far as the squirrel's ears, then operations proceeded more slowly. The snake writhed about and gradually worked its jaws over the shoulders of the squirrel, first moving the upper jaw forward with slight jerks and then pulling up on the lower jaw. Finally, after the rodent had been half swallowed, we approached closer to take a picture and the snake disgorged the squirrel as the result of a violent effort lasting a minute or so. We went away and in a little while the snake returned to its food and had swallowed it almost completely within 15 or 20 minutes more.
On Pine Flats, in the San Gabriel Mountains, a large, lazy rattler was secured which showed a bulge about halfway along its body. Dis- section disclosed a full-grown California Ground Squirrel which had been swallowed. (Grinnell, J. and H. W., 1907, p. 53.)
On San Emigdio Creek, Kern County, on the morning of April 23, 1918, the attention of J. Dixon (MS) was attracted by the nervous barking and peculiar actions of a large male ground squirrel. With the aid of the binoculars, the actions of the squirrel, which was less than 75 yards distant, were easily followed. The squirrel was obviously much wrought up and his sharp, nervous notes were quite different in pitch and intensity from the ordinary metallic alarm note. The animal's attention was continually focused upon an opening just beneath a certain small white rock at the edge of a stone pile. While his attention thus remained fixed, the squirrel kept running back and forth in a semicircle about thirty inches distant from the object concerned. During this time the squirrel's tail, which was held arched over his back, was twitched violently sideways every time he barked. The alarm notes were uttered during a momentary pause at the end of each advance in the arc-shaped path of the squirrel. The squirrel's whole demeanor reminded the observer of that of a pup that has cornered some old pussy cat and still hesitates to make an attack. Having witnessed three similar performances by ground squirrels, in San Diego County, the observer proceeded to investigate and found, as in the three previous instances, that a coiled rattlesnake was the cause of the excitement. In the present case there were two, a male and female, tightly coiled together at the mouth of a squirrel burrow, and they were dispatched. An hour later this same squirrel, which was easily identified by a peculiarity in its pelage due to wear, was observed digging a new burrow some fifty feet distant from where the rattlesnakes had been. There was no way of determining whether the presence of the snakes had influenced this action, but it was evident that this squirrel made no effort to fill up the entrance to the burrow which had been preempted by the snakes. It is a popular notion that ground squirrels, when the opportunity offers, bury snakes alive.
As for mammals as enemies of ground squirrels, the evidence most readily obtainable is derived from examination of the excrement of the former. Coyotes have regular places for deposit of excrement, on hill tops or ridges. Bones and teeth of ground squirrels frequently have been found represented in these deposits (J. Grinnell, MS). The remains of two freshly eaten ground squirrels were found in the stomach of a wildcat killed in central San Diego County (J. Dixon, MS).
As to food, the California Ground Squirrel shows a wide range of taste, even though there are at the same time decided preferences. He cheerfully adopts substitutes when favorite foods are lacking; he is not averse to taking considerable barley with his wheat. A list of all the plants eaten by the ground squirrel would be a very long one, and if locality be taken into account great variation would doubtless be found from place to place. The above general statements will be borne out, in part at least, by the data presented in the paragraphs to follow.
On the University campus at Berkeley, on March 13, 1918, the majority of the California Ground Squirrels were feeding on the tender leaves of alfilaria (Erodium). A female squirrel was observed by the junior author at this time to eat the leaves of young plants of the star thistle (Centaurea). On San Emigdio Creek, Kern County, on April 28, 1918, a squirrel was seen to disappear down a hole carrying a sheaf of freshly cut heads of foxtail (Hordeum) held tightly in his mouth. A few minutes later this squirrel was gassed and when the burrow was dug out the fresh foxtail heads were found on the edge of the nest. Previously this squirrel was seen to gather heads of both foxtail and alfilaria, but preference was given to the latter (J. Dixon, MS). The young as well as the old squirrels seem to prefer alfilaria when obtainable to any other plant.
In the region about Walnut Creek, Contra Costa County, on July 26 and August 16, 1918, the authors found ground squirrels feeding extensively on seeds of bur clover (Medicago hispida). Dried burs of this plant were abundant on the hillsides in the near vicinity of the squirrel burrows, and although there was a plentiful supply of barley on the adjacent stubble fields this was in major part passed up in favor of the clover seeds. Hulled seeds of the bur clover were found to predominate in the cheek pouches and the stomachs of the score or more squirrels that were shot. This fondness on the part of the ground squirrels for bur-clover seed suggests a possibly better way of poisoning these rodents by using the entire bur of the clover than by the use of barley, wheat or other grains, which are now so badly needed for human consumption.
In southern California the seeds of the plant known as wild cucumber, manroot, or chilicothe (Echinocystis macrocarpa) is eagerly sought by ground squirrels. Gnawed hulls of the seeds of this plant are frequently found in large quantities near the summits of rock piles where the husking or lookout stations of the squirrels are located. In Yosemite Valley a ground squirrel was seen gathering green fruits from the top of a four-foot manzanita bush.
C. H. Merriam reports (1910, p. 5) that the seeds of the manroot (Echinocystis fabacea) are eaten in the vicinity of Modesto from the middle of May to the middle of December. The seeds are eaten from
Fig. 13. Drawings from dissections to show relative extent of cheek-pouches in (a) Cahfornia Ground Squirrel and (b) Belding Ground Squirrel. One cheek-pouch opens into the mouth cavity on each side; it is lined with membrane continuous with that lining the mouth and is used for carrying food materials such as seeds and bulbs from the forage ground to either the store house or the husking place. It is to be inferred that the California Ground Squirrel is much more of a seed-gatherer than the Belding. The latter, like the Oregon Ground Squirrel, is more of a grass-eater, and also does not garner food to the extent that the "digger" squirrels do.
the time they begin to form until they are fully ripe. "Other favorite seeds are those of elderberry (Sambucus), jimson weed (Datura), wild nightshade (Solanum), turkey mullein (Eremocarpus), tarweed (Madia), and numerous grasses. . . . In southern California the squirrels are fond of the fruit of the prickly pear (Opuntia)."
Ground squirrels are provided with more or less extensive, membrane-lined cheek pouches opening inside the mouth, which are used in gathering and transporting food (see fig. 13). Often when the animals are scared out of weed patches or bushes, or away from some supply of roots or bulbs which they have discovered, their cheeks are seen to be bulging with the contents of these pouches. They are able to operate their teeth and lips even when these pouches are copiously distended. The cheek pouches of the California Ground Squirrel are especially well developed and this, we think, is correlated with the pronounced seed gathering and storing propensities of this species. The following records of cheek-pouch contents, as secured from specimens collected, contribute further to our knowledge of the kinds of food of this animal and also of the quantity in which these may be gathered.
A female taken in a stubble field near Walnut Creek, Contra Costa County, July 26, 1918, held in her cheek pouches 26 seeds of bur clover. A male taken August 15, 1918, at the same place had 78 seeds of bur clover and one seed of needle grass. Two other males had one and three bur-clover seeds, respectively. Another female taken at the same time and place contained 212 seeds of bur clover and 12 seeds of some kind of wild grass. Another male held 97 grains of barley and three bur-clover seeds. A ground squirrel taken at Cisco, Placer County, on October 9, 1913, was carrying 92 seeds of the green manzanita (Arctostaphylos patula), while a squirrel secured near Pleasant Valley, Mariposa County, on May 28, 1915, had dug up and was carrying in its pouches 12 bulbs of a species of wild hyacinth (Brodicea hyacinthina). At El Portal, Mariposa County, a squirrel was secured with three large acorns of the golden oak in its cheek pouches.
We will now consider those feeding habits which make the California Ground Squirrel come into conflict more directly with man's interests. "Of cultivated nuts, almonds and walnuts are preferred; of other crops, apples, prunes, peaches, apricots, figs, olives, . . . the seeds of cantaloupes, watermelons and citron melons, and all the grains are eaten whenever they are to be had, and green alfalfa and clover are sometimes taken" (Merriam, 1910, p. 5). Frank Stephens (1906, p. 66) has summed up the food taken by this animal as follows: "The food is principally of a vegetable nature, preferably grain and other seeds, fruit, potatoes, green plants, etc. Eggs of poultry and wild birds are relished." We have heard considerable testimony from ranchers to the effect that individual ground squirrels in different localities have learned to raid henneries, so that the above statement is not exceptional.
A great deal of damage is done by California Ground Squirrels each year in orchards and vineyards. The following instances, given by Merriam (1910, p. 6) are typical of such depredations. "Ground squirrels are particularly fond of green almonds and of the pits of green peaches and apricots, eating these from the time the kernels begin to form until the fruit is ripe, thus doing serious damage. They are very destructive to apples also, and in places in the foothills of the Colfax-Auburn region are said to take fully half the crop. . . . In the fall of 1907 E. A. Goldman reported that they were doing serious damage to young vineyards about Orosi, in Tulare County, by biting off the leaves and tender shoots of the vines. . . . In the orange groves between Porterville and Springville, in Tulare County, it is reported that they occasionally gnaw the bark of the orange trees and sometimes cut the fruit and carry it off. Besides destroying nuts and fresh fruits they attack drying prunes and carry off large quantities."
On May 14, 1918, near the mouth of Caliente Creek wash, Kern County, in one corner of a 640-acre field planted to wheat, four large bare spots were counted by the junior author in an area of not over ten acres. These denuded areas were circular in shape and averaged 75 yards in diameter. They were caused by the ground squirrels having eaten and destroyed the ripening wheat and even the stalks so that nothing but weeds remained. In a single one of these denuded areas twenty-three occupied squirrel burrows were counted. In this same field, within a six-foot circle the center of which was a lone squirrel burrow, 113 heads of wheat were picked up (see fig. 14). These heads
Fig. 14. These 113 heads of wheat were picked up within a six-foot circle, the center of which was a "digger" squirrel burrow situated in the edge of a wheat field. They were part of what had been gathered within three or four days, apparently by the one squirrel.
had all been cut and carried to the burrow within three or four days, as they were not yet dry. This was evidently the work of a single squirrel, since no other squirrel was seen to go near the burrow. These 113 heads of wheat probably constituted part of what was intended for storing, and did not include that required for current consumption.
In gathering food California Ground Squirrels slink along slowly close to the ground, often half hidden in the grass. In gathering ripe alfilaria only the clusters of seed cases are taken, with relatively little of the stem. However, when the plants are young, the stems and leaves are much relished by the squirrels. The usual method of feeding as revealed by the binoculars, is for the squirrel to sit up on his haunches within reach of the alfilaria heads, which are dexterously gathered into little bunches by the front paws of the animal and then quickly snipped off by the sharp incisor teeth. During this last operation the head of the rodent is often inclined to one side. In gathering bulky food materials such as the heads of foxtail the cheek-pouches are not always used, the material being carried crosswise in the squirrel's mouth directly to the burrow.
On the San Joaquin River near Mendota R. M. Hunt (MS) reports seeing squirrels go out into the tules of the sloughs seemingly to eat the green stalks. Several times on following up rustling sounds squirrels were discovered on thick mats of dry fallen tules among the standing green ones and just above the water. One, on being alarmed, jumped into the water with a splash and, although lost to sight, probably reached safety by swimming. Davis "Island," near Mendota, is part of the mainland at low water, but in May, with high water, becomes a true island and with the highest water the ground everywhere is completely submerged. On June 20, 1918, a squirrel was discovered on this island. It jumped from a piece of ground into the water and swam, in much the manner of a dog, to a tree up which it took refuge.
Fig. 15. Metropolis of "digger" squirrels under a small oak on a grain-sown hillside; photographed near Walnut Creek, Contra Costa County, August 15, 1918. Owing to the dryness of the season and to the depredations of the squirrels, the grain on the hill above the oak had been left uncut.
A current report was to the effect that each year at high water ground squirrels are marooned on this island and live for the time being in the big hollow-trunked willows there. This shows that flooding does not necessarily drive out or drown these squirrels in such localities as afford refuges on high ground or in trees.
On wild land, alfilaria, foxtail and bur clover are perhaps the three plants that are eaten to a greater extent than any other of our forage plants. Alfilaria is eaten from the time it appears above ground until it ripens, and even after that, when the seeds have scattered out, they are gathered and either eaten at once, or stored. The long, curled "propellers" are broken off and discarded. In Strawberry Canyon on the University campus, in April, the squirrels were harvesting foxtail and alfilaria on sunny southern exposures where the plants had matured early. Later in the season, during late June and early July, these same squirrels with their families of half-grown young were found to have moved down the hillsides, some 150 yards, to the moister, shady ground near the creek bed where the foxtail was still green, and here they were busily gathering the foxtail heads just ripening on July 6. There is an obvious rotation in the use of the different important plants for food, dependent upon the sequence in which they become available. Thus, alfilaria is eaten during winter and early spring; then the foxtail crop claims attention; and the bur clover, after its seeds ripen, is harvested all through midsummer and autumn. Of course the above statements are only of local application.
Examination of the food stores of ground squirrels would go far toward providing adequate knowledge of their food habits. Such investigations should be made preferably in the fall. Specific information now available is as follows:
In digging out a colonial burrow near Bakersfield, Kern County, on May 3, 1918, a storehouse was uncovered. This consisted of a cavity or pocket off the main run (see a, fig. 8), which measured five and a half by eight inches in two diameters and was eighteen inches beneath the surface of the ground. The stored food consisted of a double handful of nearly dry heads of foxtail grass carefully packed in dry sand. A few alfilaria seeds were also included with the foxtail, but alfilaria was scarce at this locality.
Upward of fifty of the button-like seeds or "cheeses" of the mallow (Malva) were observed at the entrance of another burrow at the same place, but the observer was unable to determine whether or not these seeds were being stored. The mallow seeds were found for the most part on the lookout station at the entrance to the burrow.
"At Modesto in May, 1909, Piper found stores of alfilaria seeds packed in cavities and well mixed with dry sand. In December of the same year he examined a number of stores of grain unearthed by a farmer while scraping and leveling his land. Each of these caches consisted of from a pint to a quart of oats stored in cavities and packed in dry sand. They varied from 8 to 18 inches in depth beneath the surface; some were in short blind holes; others at the ends of branches, of the main burrow" (Merriam, 1910, p. 5).
An idea of the quantity of food eaten by the California Ground Squirrel can be derived from the following data:
A female taken near Coulterville, Mariposa County, on June 3, 1915, weighed 553.5 grams, or about a pound and a quarter. The stomach and its contents alone weighed 77.5 grams, or about 2¾ ounces (C. L. Camp, MS). Figuring out the ascertained weight of the stomach in other individuals, 5 grams, the ratio of stomach contents to total weight in this squirrel proves to have been about 1 to 7. The material represented is presumed to have been fresh green stuff.
Some experiments have been carried on at the Museum with captive squirrels with the purpose of determining the amount of green forage consumed daily. Fifty grams, or nearly two ounces, of green alfilaria was found to be the average daily ration for an average-sized squirrel. In cases where all food had been withheld from the squirrels the previous day, the greatest amount of succulent alfilaria, the favorite food of the squirrel, consumed in one day was 80 grams, or somewhat less than three ounces.
Five immature ground squirrels taken July 26, 1918, near Walnut Creek, Contra Costa County, gave an average total weight of 504.3 (427.2–517.2) each in grains. The average weight of the stomach contents in these five squirrels was 13.2 (10.3–19.0), so that the avejrage ratio of the weight of the stomach contents to the total weight was 1 to 38 (1 to 50–1 to 22). The stomach contents in these cases consisted almost entirely of finely chewed seeds of barley and bur clover. The squirrels were shot between noon and 2 p.m. Seven full-grown males taken on August 15, 1918, near Walnut Creek, gave an average total weight of 659.4 (576.8–724.7) grams each; the average weight of the stomach contents was 17.9 (11.5-20.5); and the average ratio of contents of stomach to total weight was 1 to 37 (1 to 63–1 to 30). Seven full-grown females taken at the same time and place gave an average total weight in grams of 500 (370.7–681.4) each; the average weight of the stomach contents was 9.7 (5–15.8); and the average ratio of contents of stomach to total weight was 1 to 42 (1 to 101–1 to 37). All these fourteen squirrels were foraging in stubble fields, and the stomach contents consisted of barley and bur clover seeds finely chewed and of nearly the same degree of moistness as ordinary baker's dough. The squirrels were shot between 10 a.m. and noon.
It is believed by us that two ounces of green forage or one-half ounce of dry grain is an average stomach-full for an average-sized California Ground Squirrel and that two stomach-fulls represent a day's ration. It is evident that the proportion between the weight of the stomach contents and the total weight averages considerably less in this species than it does in the Oregon Ground Squirrel. The California is more of a seed eater and less of a grass eater than the Oregon Squirrel and therefore enjoys a more, condensed ration.
During late summer digger squirrels, particularly the old adults, become exceedingly fat. In this condition they become obviously lazy and may often be seen lounging at the entrances to their burrows simply enjoying the sunshine. As the season farther advances, a decided decrease in squirrel population is noted. The active young of the year are still foraging abroad, but even these restrict their activities to the brightest hours of sunshiny days. What becomes of the squirrels which have altogether disappeared underground?
One would naturally expect that the life history of such a notorious animal as the California Ground Squirrel would be known pretty thoroughly. However, such does not seem to be the case; there are several features of the underground life of this squirrel in regard to which our information is very inadequate and of which from the standpoint of rodent control it would be most useful to know. As has been previously mentioned, little appears to be definitely known regarding the period of gestation of this animal. The condition of the young at birth and their subsequent care and development is also not well known. Another moot point is that of æstivation or hibernation of this species of ground squirrel, Merriam (1910, p. 4) states that "this species does not hibernate, except in the mountains, although in the foothills and valleys the animals usually stay in their burrows during stormy and severe weather. At the upper limit of their range, where the ground in winter is covered with snow, they may remain underground long enough to be said to hibernate, but over the greater part of the state they are out in numbers every month of the year." However, we believe we have evidence to indicate that a period of æstivation or hibernation (or the two combined), in other words a state of torpidity initially induced by the heat and dryness of summer, obtains among some at least of the adult ground squirrels even in the lowlands. This period of dormancy extends from late summer well through midwinter, and thus "gestivation" may be said to go over directly into true hibernation. The old adults seem to be the only ones that "hole up," for the young adults somewhat less than a year old, that is, the young of the year, may be seen about the burrows during suitable weather throughout the winter.
In support of the above belief, that a period of torpidity overtakes the older individuals of the squirrel population regularly each year, the following evidence is submitted:
(1) Close watch, extending over a period of between four and five years, was kept on a female ground squirrel that lived in the dooryard at the home of Mrs. Elizabeth Grinnell in Pasadena. This particular squirrel did not æstivate until its second year. Then and during each succeeding year of its life it æstivated regularly, becoming very fat and retiring to its burrow during the last week in August. It emerged lean and hungry, with marked regularity, about the twenty-second of each following February. When removed from the burrow at intervals during this period, the squirrel was found to be in a torpid state, with respiration not perceptible.
(2) In a case in the junior author's personal experience, near Escondido, San Diego County, all the squirrels that were active in a certain field in the fall were poisoned or otherwise killed, and yet old breeding squirrels suddenly appeared in this same field the following February. This occurred when there was seemingly no possible chance for reinfestation from the surrounding fields, which had been cleaned up also. Similar testimony has reached us from a number of men identified with efforts to exterminate these rodents.
(3) It occurred to the present writers that it might be possible through the examination of specimens to learn the extent to which old adults are out in midwinter. The heads of 186 ground squirrels were, at our request, secured by the United States Public Health Service, shot and trapped near Martinez, Contra Costa County, during January, 1918, and sent to the Museum of Vertebrate Zoology, where the skulls were cleaned and carefully examined. Relatively advanced age was determined from the skulls upon the following criteria: general size, zygomatic breadth, breadth of jugals, stoutness of postorbital processes, degree of development of sagittal crest, degree of approach of parietal ridges, breadth and degree of concavity of frontal surface, advance in coalescence of the adjacent bones along certain sutures, and amount of wear on the crowns of the molariform teeth.
The results of our examination are given in Table II:
Table II. Proportions of adult to young ground squirrels abroad in midwinter.
| Date received (in 1918) | Total number of skulls | Number of old adults |
Number of young adults |
Ratio of old adults to total | ||
|
18 | 1 | 17 | 1 to 18 | ||
|
34 | 4 | 30 | 1 to 8.5 | ||
|
86 | 18 | 68 | 1 to 4.7 | ||
|
48 | 11 | 37 | 1 to 4.3 |
The foregoing data are not nearly as complete as could well be desired, but as far as they go they show that "old adult" squirrels are relatively scarce aboveground in January as compared with the younger animals, those probably less than one year old. Further, the proportions of old to young increases rapidly towards the last of that month; in other words, as the breeding season approaches. It is probable that the full old-adult population is not abroad aboveground until the last of February, when the ratio of old adults to young of the previous year would certainly not be nearly so little as 1 to 4, which is the minimum possible at the immediate close of the breeding season.
In spite of the above lines of evidence, the real extent of this habit of æstivation among our ground squirrels is not satisfactorily known. It is exceedingly difficult to follow any individual squirrel under perfectly normal conditions through all its various activities for any great length of time. However, an important factor concerned in the work of destroying these animals is suggested; that is, the desirability of placing emphasis upon the need of poisoning in the spring rather than in the fall, when part of the breeding stock may be stowed away out of the reach of poisoned grain. It is a question, too, whether or not a dormant animal, in which respiration is extremely slow, would be fatally injured by a fumigant before the latter would be dissipated.
Human interest in the California Ground Squirrel naturally concerns itself most especially with the questions of total population, rate of increase, and rate of re-invasion of territory previously cleaned of squirrels. As to the first question, we have found it difficult to find an accurate basis for determining the squirrel population living on any given unit of area, such as an acre or a square mile. Counts may be taken of living squirrels that happen to be aboveground at any one time, or of burrows which give evidence of current use. In the first case the count is never likely to cover all of the squirrels in the area, because the chances are overwhelmingly against all of the squirrels being aboveground at one time. Season of the year, time of day, and state of weather will affect profoundly this proportion of squirrels below ground to those in sight. In the latter case some sort of estimate of ratio of squirrels to burrows must have been arrived at. Season of the year must again figure importantly in the estimate, because of the jump in population following the breeding season, and progressive decrease thereafter.
On July 26, 1918, the authors took two censuses of ground squirrels on a badly infested ranch about three miles northwest of Walnut Creek, Contra Costa County (see fig. 16). The first census was taken on a south-facing hillside on an area 100 feet square, approximately one-fourth of an acre. Three counts gave 19, 16 and 17 squirrels, respectively, in sight at once. Twenty-five open burrows were counted in this area. This, therefore, was at the rate of 76 squirrels and 100 burrows per acre, or 1⅓ burrows to each squirrel. The breeding season at that date was well passed. Allowing one adult to every four young gave fifteen adults to every 100 burrows or between six and seven open burrows to each adult squirrel after the breeding season.
The second census was taken in a mowed field from which a crop of barley hay had been recently harvested. The area taken was on a north slope and measured 250 feet square, covering about 1⅓ acre. Three counts were taken between noon and one o'clock. The number of squirrels seen out at once was 25, 26 and 25, respectively. Sixty burrows were counted in this area, so that the infestation was at the rate of 20 squirrels and 50 holes per acre. This is at the rate of 2½ burrows to each squirrel.
On May 28, 1918, a single isolated colony was investigated at a point twelve miles west of Fresno. This colony was in a plowed field which had been planted to grain for several years past. Here, in an area 100 feet square 16 squirrels, eight of which were less than half grown, and 17 burrows were counted. The ratio here was close to one burrow to each squirrel. This figure, again, applies to a period after the close
Fig. 16. Hillside near Walnut Creek, Contra Costa County, badly infested with "digger" squirrels. On August 15, 1918, squirrels were here present at the rate of fifty per acre. From this breeding ground they were at this time invading the grain fields on the opposite slope.
of the breeding season, when the squirrel population had reached its maximum.
Counts taken before the breeding season naturally give different results. At Berkeley on March 13, 1918, the junior author counted 47 squirrel burrows in a colony which occupied about one acre on a hillside. By counting the squirrels which appeared aboveground in this area on several successive days it was ascertained that there were about nine adult squirrels inhabiting this acre of ground. This gave an average of over five burrows to each squirrel.
The above-cited instances are based on maximum infestation. Local distribution is often very irregular, since squirrels may be abundant on the southern exposure of a hill and yet be entirely absent on brushy northern slopes only a hundred yards or so distant. Even on the plains and in the valleys, although the distribution is much more uniform, there is often marked unevenness in infestation irrespective of human interference. Surgeon John D. Long (1912, p. 1596), in comparing the cost of the various methods of destroying ground squirrels, based his estimates of cost on an infestation of ten holes per acre. Presumably, this was taken as representing an average infestation according to the experience of the United States Public Health Service in their eradication work. This appeals to us as a fair estimate on well-populated territory, such as that around San Francisco Bay.
It is the authors' belief that if the entire area in California occupied by this species be taken into consideration a population of one squirrel per acre, or 640 per square mile, at the conclusion of the breeding season, would be a fair average. At this rate there would be a total population of 32,000,000 California Ground Squirrels in the state in July, and one-fourth this, or 8,000,000, in March, before the young of the year are out. If the closely allied Fisher and Douglas ground squirrels be included, as from an economic standpoint might well be done, the "digger squirrel" population of the state in summer, when crops are maturing, may be put at between 40 and 50 millions, in this state.
As for rate of increase, we are dealing with a prolific animal. As already shown, the average size of the litter in the California Ground Squirrel numbers practically eight. Males and females are present in a general population in about equal numbers. Even though but one litter is reared by each female squirrel each year, this would mean that for each pair of squirrels at the beginning of the breeding season there will be ten individuals at the close of the breeding season. The evidence we have examined goes to show that all the squirrels breed the first year of their lives—that is, when each is not quite one year old—as well as subsequently, and that the life-time of a squirrel, if it dies of old age, is five years. If we do not count upon any fatalities, one pair of squirrels can be reckoned on to give origin to a population in five years of 6,250!
In recent efforts to eradicate squirrels a 90 per cent efficiency has been currently estimated. This means that, if no follow-up campaign be waged, ten squirrels out of each original 100 will be left, to form a nucleus of future increase. At the end of the second year the population would be back to normal. Supposing, further, that a follow-up campaign is waged at the end of a suitable interval before the next breeding season, also with a 90 per cent effectiveness; then only one squirrel per original hundred would be left. Even then, when only six squirrels are left on one square mile, these in the third breeding season will produce, barring normal fatalities, the original 640, with a good margin to spare.
The factors limiting the population of ground squirrels under natural conditions, that is, as not affected by human agency, include the following, in the order of probable importance: (1) Quantity of food available at the season of the year when food is scarcest; (2) natural enemies, including predatory mammals, birds and reptiles; (3) adverse weather conditions, recurring rather infrequently, as when territory is inundated during exceptionally heavy rains; (4) disease; (5) old age. The rate of increase, through long ages, has been adjusted to more than meet the expected death rate from all causes combined. This rate of increase, fourfold each year, is now inherent and we have no reason for expecting any abrupt and permanent change in it either way.
With the arrival of the white man and his accessories in California, the natural balance has been upset. Man has destroyed a large percentage of the natural enemies of the ground squirrel. Cultivation of the land has, on the other hand, in portions of the state improved the food supply. The general tendency is for the squirrels to breed up on uncultivated land where they are least molested through human agency, and from this they spread out and invade nearby cultivated fields. The process is most conspicuously in evidence during late summer consequent upon the emigration of the young of the year, this being in compensation for the tendency to congestion of population brought on during the breeding season.
Reduction in the food supply locally causes the squirrels to spread out in search of new pastures. Such movements are usually less than a mile in extent, and of course come particularly to notice in the vicinity of grain fields and orchards to which the squirrels drift at the time the crops begin to ripen. Some idea of the rate with which ground squirrels reinfest cultivated fields which are adjacent to wild land may be had from the following instance. Mr. O. N. Garrison of Earlimart, Tulare County, stated in an interview that during the spring of 1918 thirty-six ground squirrels were drowned out on a five-acre field of alfalfa at the first irrigation and this in spite of the fact that the field had been free from squirrels at the end of the previous irrigation season in the fall of 1917.
From the earliest times of which we have record to the present day the California Ground Squirrels have given the impression of abundance. Changes in the status of the species within history have only concerned local occurrence. There is nothing to show that there has been any extension of the general range of the species, or any retraction in it either. As already set forth, the arrival of the white man and the institution of agriculture has undoubtedly had the effect locally of increasing the ground squirrel population. On the other hand, where man has been aroused by the seriousness of their depredations to the point of adopting and putting into force effective means of control the numbers of the squirrels have been conspicuously reduced. Thus at Earlimart on May 16, 1918, ground squirrel burrows were found to be abundant over a large acreage of "hog wallow" land. Live squirrels, however, were exceedingly scarce, only five being found on one tract of forty acres which had been thoroughly poisoned the previous season. A count taken on this tract showed that there was an average of fifty empty burrows to each squirrel present.
A very few localities have been reported in which the squirrels are, for the time being at least, things of the past; but the possibility of re-invasion presents itself, and this, as already shown, may be a very rapid process. It would seem that ground squirrels, like weeds or scale bugs, will have to be watched continually, and proper measures taken whenever necessary to prevent the reinfestation of land which is thought to have been freed.
The difficulties in arriving at a fair estimate of the damage done by the California Ground Squirrel, which is by far the most injurious species in the state, are many and various. We have tried to get at a satisfactory estimate (not a guess) in terms of dollars per annum, but have not succeeded. It may be of some interest, however, to give some other figures, indicative in partial degree of the loss that may be occasioned by this ground squirrel.
In order to ascertain the bearing of squirrels upon grazing interests we have found some basis for estimating squirrels in terms of livestock. We have weighed and examined the stomach contents of a series of squirrels, with results already given. These, summarized, show that one ounce of dry grain or seeds, or four ounces of green vegetation, is consumed each day by an average California Ground Squirrel. If we take fifty pounds of green stuff as representing the amount of forage consumed daily by one steer on open range, then 200 squirrels would appropriate the forage which would keep one steer. Twenty squirrels would eat as much as one sheep, and this last estimate would be most significant, because sheep graze closer and hence the competition here would be sharpest.
Expressing this relationship in another way, taking the average population of ground squirrels on open range as one per acre or 640 per square mile, the squirrels on each square mile appropriate the forage of three steers or 32 sheep. If the entire range of the California Ground Squirrel be taken into account and be supposed to consist purely of grazing lands (and so of minimum land value) grazed to their fullest capacity, then the squirrels of this species take the place of 160,000 cattle or 1,600,000 sheep. Of course, it is not likely that the squirrels come into actual close competition with livestock in ordinary years; but in extra dry years, such as that of 1917–18, when all the living things which depend on vegetation for support are hard pressed to maintain existence, then the squirrels cannot help but crowd the cattle interests of the country, which are of such vital human importance.
- ↑ The measurements given throughout the present paper have been taken according to the following methods. The external dimensions are those recorded on the label attached to the skin and were taken from the freshly killed animal by the collector in each case. Total length is the distance from the tip of the nose to the tip of the last vertebra of the tail (which is also practically the tip of the tail without the hairs), the body and tail being straightened out but not stretched; tail vertebræ is the length of the tail alone (again without hairs), from a point on upper side at base where tail can be bent at right angles to back, to tip of last vertebra; hind foot is measured when extended flat at right angles to leg, from heel to tip of longest claw; ear from crown is the distance vertically from top of head at inner base of ear to extreme tip of ear, not including hairs. The cranial measurements were all taken by the senior author, with parallel calipers reading to tenths of millimeters, from cleaned skulls. Greatest length of skull is taken parallel to axis of skull from anterior tips of nasals to most posterior point or points on skull (this in some skulls falls on the condyles, in some on the lambdoidal ridge); zygomatic width is the greatest width of skull at right angles to axis, from the outer surface of one zygomatic arch to the outer surface of the other; interorbital width is the least distance between the eye-sockets, but not counting the little notch usually present in ground squirrels on each edge of the interorbital portion of the roof of the skull.