POCKET GOPHERS. Genus Thomomys¹¹

Field characters.—Size near that of House Rat, but form stout, tail short, ears and eyes very small (pls. 27, 29); length of body six inches or less; tail less than half length of body, scantily haired, bare at tip; a fur-lined cheek pouch opening outside of mouth on each side; cutting teeth (incisors) project conspicuously beyond lips which never cover them; forefoot not spade-like but provided with long slender claws (fig. 5b), longer than those on hind foot; skin loose-fitting; fur short, smooth but not plush-like; coloration uniform, light to dark brown, varying according to age as well as to species. Habits: Fossorial; live in self-constructed burrows in ground, appearing above surface but rarely. Workings: Low mounds of loose earth with crescentic or moraine-like topography (pl. 28c), mouth of burrow being near one side and left plugged.

Occurrence.—Common practically without interruption throughout the Yosemite section up to timber line.¹¹ Most plentiful about margins of meadows and on semi-open, timbered slopes; absent only in densest forests and on bare rock formations. Individuals work independently, though often in close proximity to one another.

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¹¹The Pocket Gophers of the Yosemite section are representative of five distinct kinds, as enumerated below. Although two of these, pascalis and mewa, are indicated as subspecies of one species, bottae (upon the basis of conditions farther northward in California), actual intergradation between any two of them was not found by us to take place within the region studied; all the five forms in the Yosemite section behave toward one another as full species; no two were found living in exactly the same locality. The geographic habitat of each is distinct (fig. 24).

The characters upon which most emphasis is laid by systematists for separating the species of pocket gophers have to do with the skull and teeth, and determination of these requires preparation of materials and special technical knowledge. It does not seem desirable to deal with these internal characters here; for them the reader interested is referred to Bailey's Revision of the Pocket Gophers of the Genus Thomomys (N. Am. Fauna, no. 39, 1915, U. S. Dept. Agr., Bur. Biol. Surv.). As to external characters our pocket gophers, though much alike, do possess features of difference which are appreciable. These consist in tone of color, general size, and relative size of ear. These external features are here given. The sequence of species is from the west base of the Sierras across the mountains to Mono Lake.

FRESNO POCKET GOPHER, Thomomys bottae pascalis Merriam, a race occupying most of the floor of the San Joaquin Valley, was found to enter the Yosemite section only along the bottom lands of the Merced River around Snelling. There it was abundant and troublesome in gardens and alfalfa fields. This gopher is slightly the largest of the five kinds, and it is palest in color of those on the west side of the Sierra Nevada. In summer it is bright cinnamon-buff all over, save for whitish tail and feet and dull brown around mouth; in winter, darker, snuff brown, paler underneath. Head and body 5-1/8 to 6-5/8 inches (130-168 mm.), hind foot 1 to 1-1/4 inches (25-32 mm.), ear (from crown) about 1/4 inch (5-7 mm.), weight 3 to 6 ounces (82-172 grams). In this race and the next, maximum dimensions and weights are for males, minimum for females; but in the other three species, the sexes differ in size little or not at all.

DIGGER PINE POCKET GOPHER, Thomomys bottae mewa Merriam, a race inhabiting a long narrow north-to-south strip along the western flank of the Sierras, was found in the Yosemite section to occupy very closely the Upper Sonoran Zone. We found it from Lagrange and Pleasant Valley east to six miles east of Coulterville, El Portal, and Pinoche Peak (at 5500 feet). This gopher is smaller than pascalis; its color tone in summer is still brighter, more reddish, and in winter darker, almost blackish down the middle of the back. Head and body 4-3/4 to 6 inches (120-153 mm.), hind foot about 1 inch (24-28 mm.), ear (from crown) slightly less than 1/4 inch (6-7 mm.), weight 2-1/4 to 4-1/2 ounces (67-129 grams).

YOSEMITE POCKET GOPHER, Thomomys alpinus awahnee Merriam, occupying the Transition Zone along the west flank of the Sierras, chiefly south of the Yosemite section, was found to be the common gopher on the floor of Yosemite Valley, west to Cascades. Outside of the Valley it has been recorded only from Sequoia. This gopher is dark grayish brown both summer and winter, and nearly all the individuals examined have more or less blotching of pure white on the under surface, especially around the chin and on the chest. Head and body 5-1/4 to 6 inches (132-150 mm.), hind foot 1 to 1-1/8 inches (26-29 mm.), ear (from crown) about 1/5 inch (5-6 mm.).

SIERRA NEVADA POCKET GOPHER, Thomomys monticola monticola Allen. A species whose range covers the higher parts of the central and northern Sierras. It inhabits rather strictly the Canadian and Hudsonian zones of the Yosemite region. We found it from Aspen Valley and Chinquapin eastward across much of the intervening territory below timber line to Gem Lake. Highest point of actual capture, 10,350 feet, at Vogelsang Lake. This gopher differs from all the others in its finer, longer pelage and in its larger, pointed ears; the color tone on the back is dark brown tinged with russet, with lower surface dull buffy white; a slaty patch behind ear. Head and body 5 to 6 inches (126-150 mm.), hind foot 1 to 1-1/4 inches (26-31 mm.), ear (from crown) about 1/3 inch (8-9 mm.), weight 3-1/6 to 5-1/2 ounces (90-158 grams).

FISHER POCKET GOPHER, Thomomys quadratus fisheri Merriam. Occupies the western parts of the Great Basin, and inhabits the arid east slopes of the Sierras and the territory around Mono Lake, chiefly within the Transition Zone. We took specimens from Leevining Creek (at 9200 feet), Walker Lake, and Silver Lake, east to Mono Craters. This is the smallest and palest colored of the five kinds; its ear is very small. Color a dull pinkish cinnamon above, huffy white beneath; a dusky spot at base of ear. Head and body 4-5/8 to 5-3/4 inches (117-146 mm.), hind foot about 1 inch (25.5-27 mm.), ear (from crown) about 1/6 inch (3-5 mm.), weight 2-1/3 to 3-1/2 ounces (65-98 grams).

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The Pocket Gopher is a modest, retiring animal of subterranean habits, known chiefly by his works. Indeed so rarely, is one of the animals seen alive by the casual observer that the evidences of its presence are often ascribed to that totally unrelated but more widely known animal, the mole. There is close similarity in general appearance and habits between the several species of pocket gophers inhabiting the Yosemite region, and for this reason it has seemed better to combine in one account what we have learned about all of them. Brief descriptions of the five species and statements of their respective ranges are given in footnote 11.

Fig. 22. Illustrates method used by Mole in putting earth up from below-ground. Successive loads of earth are forced up one after another and topple out on the surface of the ground, volcano-like, without ever leaving the mouth of the tunnel exposed or open. Compare with figure 23 illustrating work of Pocket Gopher.

Gophers live in tunnel systems which they themselves excavate at a relatively uniform depth five inches or so below the surface of the ground. They appear on the surface from time to time only when necessary to push out earth loosened in extending their tunnels or to forage in the close vicinity of the open burrow. They seem to be most active about sundown and during the early hours of the morning; for it is then that the majority of new surface mounds appear, and that the animals themselves are most often seen at the mouths of open burrows.

The presence of gophers is indicated by small mounds of loose earth which the animals push out here and there on the surface of the ground. The typical mound is of a fan shape, the opening of the burrow from which the earth was pushed, although closed, being clearly indicated at the base of the fan. (See pl. 28b). The upraised surface of the fan is marked with more or less sharply indicated concentric 'moraines,' each registering the terminus of an operation from the mouth of the burrow. The rim of the mound is often irregular, the earth having been pushed farther out at some points on the periphery than at others. The mouth of the burrow is plainly outlined in a perfect circle of raised earth two or three inches in diameter, but this small circle is lower than the preponderance of the heap.

Gopher workings can easily be distinguished from those made by moles. Mole mounds never show an open tunnel at any time, even during construction; the animals themselves never come out on the surface when pushing out earth (fig. 22.) The earth is pushed straight up from the initial opening and new earth is placed only beneath the pile already started, with the result that the pile is raised still higher. In rising, the earth at the top separates and keeps toppling over, leaving a peculiarly porous or cleft surface (pl. 28b), with no indication of the location of the burrow from which the earth was extruded. And so it is that the concentric moraines which characterize the gopher workings are never to be seen on the mounds of moles.

Fig. 23. Illustrates method used by Pocket Gopher in removing earth from burrow. Note that each load of earth is brought up from below and shoved out on top of the ground opposite the mouth of the open "lateral." The pushing out of successive loads in different directions from the mouth of one lateral gives the surface mound a semi-circular outline as viewed from above. Compare with plate 28.

In addition to their characteristic mounds, pocket gophers often afford much less conspicuous evidences of their activity, especially during the dry season. At frequent intervals circular openings in the ground are to be seen, which have been filled with loose earth nearly or quite to the level of the surrounding surface. These burrows have been used as exits from short side branches of the main subterranean tunnels, for the purpose of exploring the immediately adjacent surface for food. Gophers are exceedingly loath to leave shelter and ordinarily do not venture so far even as the length of their bodies from the open mouths of their burrows (pl. 29a). As an evident result of this timidity, each feeding exit becomes the center of a small circle, shorn of vegetation, the radius of which is less than the body length of the gopher. The haunches of the animal remain in contact with the orifice of the burrow as a sort of anchor by means of which he can pull himself back into safety at an instant's warning. It is well known to gardeners that a gopher will burrow underground some distance to a plant rather than risk capture by venturing forth on the surface even a short distance. Many times gophers tunnel toward the surface beneath plants and cut off roots and even the main stems, without causing any disturbance above ground until the plant begins to wither and die.

When excavating, gophers loosen the earth with their strong incisor teeth and the long claws of the forefeet (pl. 29c). The earth thus loosened is swept back underneath the body until a considerable amount has accumulated. The animal then turns around (being able to do so within the diameter of its own body), and pushes the earth along the tunnel to a surface opening where it is shoved out on top of the ground. (See pl. 29b and fig. 23.) Only the forefeet, in conjunction with the broad furry face below the level of the nose are used in moving the earth; the outside-opening cheek pouches (pl. 27b) with which the animal is provided, and which open at either side of the mouth, are used for the sole purpose of carrying food material. After tunnel excavation has proceeded a few inches beyond one surface opening, the opening is closed and a new opening made at a more convenient position, nearer the spot where earth is being removed. Most of the surface openings are at the ends of side tunnels which are but a few inches in length. Sometimes a great quantity of earth is pushed out at one surface opening. One mound observed in the Ten Lakes basin was 25 inches (62-1/2 cm.) in diameter at the base and 6 inches (15 cm.) high; the total earth pushed out amounted to 7825 cubic inches, or about four and one-half cubic feet (123,705 cc.).

Gophers at the higher altitudes show most activity during the late afternoon and early evening hours. It is then that most new mounds are to be seen and that trapping is most successful. In high meadows where there is a heavy frost, the surfaces of mounds made during the night are usually frozen stiff by morning, showing that the mounds were piled up before the nightly drop in temperature. However, especially in lower altitudes, gophers work a good deal in the morning and do some work at almost any hour of the day.

With regard to breeding habits in the Yosemite, we have little to report save what is shown by the specimens captured. We did not in any instance try to dig out the home burrows. A quarter-grown juvenal (pascalis) taken at Snelling, January 5, 1915, indicates early breeding at that low altitude (250 feet). A female (mewa) taken at Pleasant Valley, May 21, 1915, contained four large embryos. Two young (monticola), one-quarter to one-third grown, were taken at Mono Meadow on June 18, 1915. Five pregnant gophers (monticola) were taken in 1915 at the higher altitudes: Porcupine Flat, June 28, 6 large embryos; same locality, July 1, 5 small and 7 small; same locality, July 2, 3 embryos; Tuolumne Meadows, July 11, 5 embryos.

The pocket gopher is one of several Sierran rodents which carry on active existence throughout the entire year. It does not hibernate, so far as we know, even at the highest altitudes. As described beyond, there is good evidence of their continued work beneath the snow, however deep this may become. Yet there is some variation in degree of activity with the change of seasons and at different elevations. In the foothills and lower valleys the rains seem to have much to do with the behavior of these animals. Soon after the first soaking rain of the autumn, and with the first appearance of the annual vegetation, new outpushings of moist earth become conspicuous. In the higher mountains greatest activity, save for that in winter, is shown in September and October. Least surface work is shown during the first few weeks of spring after the snow melts.

Fig. 24. Cross-section of Sierra Nevada through the Yosemite region showing zonal and altitudinal ranges of Pocket Gophers (genus Thomomys).

During the winter and spring in the high country, where snow lies on the ground for several months, gophers are, as just stated, continually active, but are led to adopt a somewhat different method in extending their tunnel systems than that used during the summer months. Tunnels are made in the snow some distance above and more or less parallel to the surface of the ground; these "snow tunnels" are usually greater in diameter than the subterranean ones and obviously serve the purpose of allowing the gophers to reach food plants which are imbedded in the snow. Certainly many of these snow tunnels are also used in extending the subterranean system; the earth from below ground is carried up and packed into the snow and thereby solid earth-cores are formed above the ground. When the snow melts these cores are lowered intact onto the surface of the ground, where they often remain distinguishable for several months despite the summer thunder showers. (See pl. 30.) The height to which the snow tunnels extend above the ground depends upon the depth of the snowfall; but there is reason to believe that their course is also modified by the position of the vegetation encountered. In early spring, after the snow has gone, we have found portions of earth-cores lying on top of flattened branches of snow-bushes, over fallen tree branches, over logs and rocks, these various locations indicating that the animals had pursued courses through the snow well above these objects, that is to say, at least 12 inches from the top of the ground. When active right after a light fall of snow, the gophers run their tunnels along directly upon the surface of the ground, appropriating to their uses the stems of grass and the other plants encountered as they go.

Very often the material composing the cores is quite different in character from that of the top of the ground immediately underneath them. This makes the cores very conspicuous, for they are, with reference to the ground on which they lie, in the relation of a geological unconformity. This kind of gopher work is carried on even after but light snow storms in the fall when snow may lie on the ground only a few days. We noted evidences of such work in 1915, about Tuolumne Meadows after a light snowfall on September 24 and 25, and after another in Yosemite Valley on November 12.

Rather than being a drawback to the interests of the pocket gopher, snow seems to be of real benefit to them. Two factors are here involved. We have referred to the timidity of the animals because doubtless of relentless pursuit by certain carnivorous birds and mammals, and to the resulting precautions evinced by the gophers in keeping out of sight. The snow provides cover which conceals them still more effectually from their enemies. At the same time, the vertical range of accessible food sources is greatly increased, for the gophers are able to reach plant stems and leaves enveloped in the snow mantle many inches and even feet above the ground surface. All this is subject to indubitable proof through study of winter workings uncovered at the time of the spring thaw.

Some estimates made by our field party while near Porcupine Flat during the first week of July, 1915, will serve to indicate the amount of work done by gophers. It was found that the average amount of earth pushed up in the form of winter cores was, on a selected area, 1.64 pounds per square yard (0.90 kilograms per square meter). Assuming that, on the average, gopher workings cover 0.1 per cent of the land surface of the Park there would be 3.675 tons of earth accumulated per square mile or 4132 tons over the whole Park. And this in a single winter! It will be recalled that there are many square miles of either solid rock or slide rock in the Park, where gophers cannot work. On the other hand, in favorable localities workings sometimes occur on every square yard of surface; the average of 0.1 per cent is therefore believed to be conservative for the Park as a whole. In summer the amount of material excavated is probably at least as great as it is in winter—exactly how much has not yet been determined. But for the year we feel safe in doubling the total figure just given, which, to put it in another unit of measure, would be close to 160 carloads of 50 tons each. We estimate further that this great quantity of earth is lifted by the gophers an average distance of at least 8 inches; 5500 foot tons of energy is expended in excavation alone by these animals in Yosemite Park during a single year!

The question then presents itself, what are the general effects of all this work upon the terrane at large, upon the vegetation, and even upon the other animal life of the region. Some of these relations borne by pocket gophers to their environment may be enumerated as follows:

(1) The weathering of the substratum is hastened by the burrow systems carrying the water and contained solvents as well as air to the sub-soil particles and rock masses below.

(2) The sub-soil is further comminuted and brought to the surface where it is exposed to increased rate of weathering.

(3) The loose earth brought up and piled on the surface of the ground thereby becomes available for transportation by water; rain and melted snow carry it from the slopes down to fill up glacial depressions and make meadows of them, and when these are full the sediment is carried on still farther by the gathering streams to contribute to the upbuilding of the great and fertile valleys beyond the foothills.

(4) Water is conserved for the reason that snow melts more slowly on porous ground than on hard-packed soil or bare rock so that the spring run-off is retarded and the supply to the streams below is distributed over a longer period of time; furthermore, the porous soil retains the water longer than packed ground and gives it up with corresponding slowness. Spring floods are less liable to occur, and a more regular water supply is insured to the lowlands.

(5) A porous, moist soil produces a fuller vegetational cover—forest, brushland, and meadow—and this again favors water conservation.

(6) The ground is rendered more fertile through the loosening of the soil as well as through its permeation by the tunnels themselves, as thereby both air and water are admitted to the roots of the plants; the mineral constituents of the soil become more readily available, and the rootlets are better able to penetrate the earth.

(7) The accumulated vegetational debris on the surface of the ground is eventually buried by the soil brought from below by the gophers, and becomes incorporated to form the humus content so necessary for the successful growth of most plants.

Our readers will have been reminded by a portion of the above considerations of Darwin's classical study on the relation of earthworms to soil formation. There is undoubtedly a parallel here, the more significant in that the earthworm is a relatively rare animal in the Sierra Nevada, and what there are of him, are of small size, and of relative inconsequence in cultivating the soil. The pocket gopher is wonderfully equipped to handle the refractory young soils of the semi-arid Sierran slopes, and his is the role here of Darwin's earthworm in England.

Now that the greatest of all agencies of erosion, the glaciers, so stressed by John Muir, have almost ceased to operate, the less obvious agencies stand ready to claim their due prominence, if we will but look for them. The element of time granted, we are able to conceive of vast accomplishments on the part of even so humble a contributor as the pocket gopher.

A real service, it seems to us, performed by burrowing animals, among which in the Sierras the pocket gopher stands foremost, is that of counteracting the packing effect of large mammals on uncultivated pasture lands. The impact of heavy feet on the soil, especially when wet, crowds the particles together and renders the earth less suitable for plant growth. Close tamping tends to exclude air and hence to suffocate the plant roots, to which oxygen is as essential as it is to animal life. One has but to observe the condition of mountain meadows outside the limits of National Parks, to appreciate the point here made. Often where the country has been overstocked with cattle or domestic sheep, the grasslands have become poor—the crop of grass is scrawny—except where gopher workings occur; the sites of these are marked by patches of vivid green. Indeed, on ordinary hill slopes within the Yosemite section we have repeatedly noted the rejuvenation of the plant cover here and there due directly and obviously to the activity of the gophers. Before the advent of the white man with his cattle and horses a similar service was rendered, though in lesser degree, perhaps, when the wild deer, mountain sheep, and bears frequented the same meadows.

The pocket gophers, then, are the chief natural cultivators of the soil, and upon their continued activity the maximum thrift of wild vegetation is dependent.

The question of damage by gophers to forests under natural conditions, for example, injury to young trees, has been raised by foresters. There is no doubt that gophers do girdle or cut off the stems of many seedlings and thus terminate the existence of numerous individual trees. But the great number of seedlings observable on parts of any forest floor, vastly more than could ever reach maturity, would seem to indicate that an adjustment in this direction had been reached long ages ago. Plants in general provide for a rate of replacement sufficient to meet the maximum probabilities of casualty, this involving all stages from the seed to the mature fruiting plant.

In the arable lowlands of California the pocket gopher is well-nigh universally condemned for pursuing his normal activities, while making his living, on lands that have been appropriated and cultivated by man. There, man has disturbed the original balance of natural relations between plants and animals; he aims to make the land produce crops of selected plants in the largest measure possible, and to that end he cultivates the ground himself by very effective 'artificial' means. He resents the levy upon the land and its products by any other animal. Most of the original quota of herbivorous mammals have gone before him; but the gopher and ground squirrel have been able to persist under the changed conditions and have availed themselves of man's crops. Yet it is clear that we have here, most surely, a reversal of the relationships obtaining in the wild. In the wild, there is no cultivation in the artificial sense. The crops of wild plants—grasses, herbs, shrubs, and even trees—depend upon whatever favorable agencies cooperate in natural ways. The happy relation found by our pioneers was the result of eons of adjustment among all of the elements concerned. Gophers have been at work as gophers of modern type since Miocene time. We grant that the farmer must combat the gopher in his fields; we sympathize with him for yearning for the total eradication of the rodents there. But we do not agree with the policy of wholesale extermination advocated by some persons for all areas alike. We hold that the native plant life on hill and mountainside, in cañon and mountain meadow, would at once begin to decline, were the gopher population completely destroyed. Not that such a thing is at all possible; but it should not be thought of, even, by any intelligent person who seeks to interpret nature correctly. On wild land the pocket gopher, with its fellow-rodents of burrowing habits, constitutes a necessary link in the system of natural well-being.

Fig. 25. Cartoon suggesting relation between work of Pocket Gophers on the Sierra Nevada and accumulation of fertile sediments on floor of San Joaquin Valley.