Popular Science Monthly/Volume 34/November 1888/Habits of the Great Southern Tortoise
|HABITS OF THE GREAT SOUTHERN TORTOISE.|
By N. S. SHALER,
PROFESSOR OF PALEONTOLOGY IN HARVARD UNIVERSITY.
IN a recent paper published in "The Popular Science Monthly" for February, 1888, I called attention to the effect on the soil produced by various burrowing animals. At that time I had not seen the work done in the under-earth by the Gopherus Carolinus, the largest of our North American tortoises, a creature which, on account of its peculiar habits and the geological effects which it brings about, is worthy of an attention which it has not received. It is a well-known fact that land-tortoises are particularly abundant on the American continent. Though found elsewhere, and once extremely abundant in other lands, they are now most plentiful and of largest size in the Americas, and the Galapagos Islands, off the western coast of South America. The greater part of these creatures have the habit of spending the most of their lives on the surface of the ground, only resorting to the under-earth for occasional shelter or during the annual period of rest or hibernation. The gopher, on the other hand, has developed the habit of underground life to such a degree that it may fairly be reckoned as an essentially subterranean form. The greater of our Southern species, and the one to which I shall devote this paper, dwells for the greater part of its life below the surface, only coming occasionally from its burrows. It appears to be by far the largest species of our vertebrates which is normally subterranean in its mode of life. The region at present occupied by this species is narrowly limited; it includes, so far as I have been able to find, only the southernmost part of our Gulf States, the southern portion of South Carolina, and the seaboard region of Georgia, Alabama, and Florida. Within this considerable area it is limited to the regions of the sand-plains, for in those districts alone does it find the soil suited to its peculiar habits.
The gopher, it should be noted, is of considerable bulk, having in adult specimens a length of fifteen inches or more, a width of about twelve inches, and a thickness from the breast to back of from eight to twelve inches. At first sight of the creature, it seems as if its form was totally unfitted for underground movements. The front of the animal is very blunt, and calculated to oppose about the maximum of resistance to movements through the earth. The fore-limbs are not to any degree specialized for grappling with the earth which has to be moved to form the burrow, and the head does not differ from that of our ordinary turtles and tortoises, which is fitted for grasping prey and not at all specialized for use as a shovel or digging instrument. A very slight examination of the sandy woodlands which are principally occupied by this creature will, however, convince the observer that the gopher, though selective processes have not helped him in his arduous task, is by dint of sheer strength and admirable persistency capable of doing work of singular magnitude.
The traveler in Florida may notice even from the windows of the railway-train that all over the surface of the soil in the pinewoods lie little heaps of sand which contain about one half a cubic foot of material which has been thrown upon the surface since the last period of rains. Sometimes there are only a dozen or two of these heaps to the acre, but they often amount to as many as two hundred or more in that area. Where they are few in number, we may remark that they are distributed in a tortuous line. Where they are very plenty, these tortuous lines intersect each other in such varied directions that no order in the distribution of the hillocks is discernible. Closer observation will show that these heaps are thrown out upon the burrows formed by the gophers. At first it might be supposed that they represented the points of entrance or exit of the subterranean passages, for it seems possible that the soft sand has fallen down over the opening so as to conceal its original position, and thus give the accumulation the aspect of a mere heap; but if the student takes pains to dig down into these little mounds, he will find that they do not communicate directly with the burrows. It is generally impossible to trace in the loose sand the manner in which they have been thrust up to the surface.
A little careful searching will show the way in which these curious mounds have been formed. Here and there, but rarely perhaps in one amid a hundred of these mounds, we find the place where the reptile entered the ground. This opening is at once seen to be quite separate in character from the mounds which first attract the eye. It consists of a clearly defined tunnel, the sides commonly somewhat smooth and compacted by the energy with which the body of the creature has been driven through it. The passage inclines steeply downward, descending at the outset at an angle of from 20° to 30°, then turning at the depth of two or three feet to a more horizontal position. On the surface, a little beyond this entrance, is a heap of débris, which consists of the sand taken from the passage. A few feet in from the opening, the passage appears to be closed by loose material which was not ejected from the mouth of the tunnel. Although I have been unable to catch these tortoises at work, I have succeeded by tolerably safe inferences in tracing their method of operation. When they begin the burrow, they endeavor at once to penetrate downward to the level in which they obtain their food. At the outset they manage, by frequently backing out of the passage, and thrusting the earth behind them in their retreat, to clear a considerable opening. When they have advanced a few feet in the excavation, they cease to discharge the material excavated in their advance, but thrust it behind them, and leave it lying in the chamber, which it entirely closes. With this storage-room provided, the gophers are able to advance through the earth for the distance of some yards; but as the earth compacted by its own weight, by the pressure exercised through the expansion of roots, and the action of the rain, occupies less space than the same material loosened in the progress of the burrow, they soon become hampered in their movements. They then turn toward the surface and continue the excavation upward until they have attained very nearly to the open air. They then use the great strength which they clearly possess to thrust a quantity of the burrowed material upward until it rises above the surface in the form of a cone, and by the space in the burrow thereby gained they are able to go a few feet further in their tortuous line of advance, when they must again seek to discharge a portion of the earth in the manner just described. So the creature proceeds in its devious underground way, coming near the surface and pushing out a portion of the sand at intervals of from two to five feet in its path. In this manner it appears to journey at times for a distance of hundreds of feet before it again has occasion to come to the open air.
For the greater portion of its journey, the path of this creature seems to lie within two or three feet of the surface, that being the level in which it finds the roots which afford it food. It appears, however, from the points at which they emerge in the railway-cuts, not unlikely that they occasionally penetrate to the depth of six feet below the top of the soil. Although they plentifully occur throughout a region having a superficial area of nearly one hundred thousand square miles, they appear to exercise a considerable choice as to the ground they inhabit. They demand, in the first place, that the water-level shall not be within a dozen feet of the surface, and that the material they traverse shall be a very open-textured sand. This is probably because in the rainy season any considerable rise in the level of the groundwater would be destructive to them; unless they could quickly escape from their burrows, they would be drowned. It is often possible, through this habit, to determine in an approximate way the depth of the Tertiary beds of clay and other indurated materials which at many points lie near the top of the sand which envelop the surface in the Southern States. Where these beds, too compact for the uses of this creature, come within a short distance of the surface, they avoid the ground.
The geological effects of this creature in the district which it inhabits are considerable. In a region extensively occupied by them, they turn over the earth, to the depth of some feet, with amazing rapidity. On selected areas chosen to represent the work done by these animals I found that the number of hillocks varied from fifty to two hundred to the acre, and that the heaps contained an average of rather more than one fourth of a cubic foot of sand when reduced to the measure of compactness which it occupied when in its original place. I came to the conclusion that it would be safe to estimate that in each year this soil matter thrown up by the gophers on the surface of an acre amounted to an average of fifty cubic feet, the greater portion of which was uplifted from a depth of a foot or more below the surface. At this rate they would completely overturn the soil, for the depth of a foot or more, in about eight hundred years. In addition to the effect produced by the process of throwing out the earth upon the surface, these creatures accomplish a vastly greater amount of subsoiling by continually ascending and descending in the earth, pushing the earth behind them as they go. I am inclined to think that they displace vertically, about the amount of a foot or more, all the sands to the depth of about three feet in the course of less than a century. The result is, that in the regions they occupy there is no distinct soil coating whatsoever; the thin layer of half decayed vegetable matter, rarely exceeding an inch or two in thickness, lies immediately upon the sands, which are scarcely commingled with the humus material. Although the rapid decay of vegetation in the warm climate of the South may in part account for this peculiarity, I think that it in the main is due to the action of these creatures. This view is supported by the observation as to the character of the soil in places where the gophers have not done their underground work. Thus, where the ground is too wet for their occupancy, we commonly find a thick coating of vegetable matter and a soil which is charged with humus to a considerable distance from the surface. The wet "hammock" or "hummock" lands, which exist as occasional patches in the sandy districts where these animals are plenty, apparently owe in good part their more normal character of soil to the exemption from the overturning of the superficial materials which these creatures effect.
There is another important way in which these gophers have influenced the geological conditions of the districts they inhabit. The sands through which they make their burrows have evidently been deposited by water-action. They were probably originally in the stratified form, but so thoroughly have they been overturned to the depth of several feet by the burrowing of these animals that no trace of this original structure is shown in the railway and other cuts except where by a rare chance the section is so deep that it penetrates below the level of their migrations. At first sight I was greatly puzzled to find that the superficial sands of Florida, which have evidently been deposited beneath the sea, exhibited no trace of stratification, which is invariably brought about in deposits formed in shallow waters subjected to a strong current action. It was only when I came to reckon on the influence of these creatures in destroying the original planes of stratification that the riddle became plain. So far as I am aware, this is the only case in which a burrowing animal has done so much work as practically to efface the stratification planes in a wide field of sedimentary deposits. It appears to me likely that the absence of fossil remains in these superficial sands of Florida may be explained by the endless stirring of the beds which these creatures have effected. It is evident that in their motion through their underground ways they have exercised a very considerable degree of violence. A fossil which might have remained well preserved in undisturbed beds would, from repeated contact with the strong claws of the gopher, have been broken into fine bits, and made ready to pass into the state of solution. As is well known, ordinary marine fossils deposited in porous sands soon become very frail, and would certainly not resist any such rude treatment as they must again and again have received at the hands of these animals. The railway and cliff sections of Florida, which afford us the only opportunities for a careful examination of the superficial sands, are in positions where they are accessible to the gophers. In an examination of a good many miles of these escarpments, I found only one or two points in which a trace of the original bedding appeared to be distinguishable. It is evidently difficult to account for the unstratified as well as the non-fossiliferous nature of this deposit in Florida without some such explanation as that to which our study of the gophers leads us.
To the student who wishes to ascertain the limits of evolution under the influence of natural selection, the gophers present certain facts of great interest. It is evident, from the foregoing statements, that the habits of this creature are eminently peculiar, and yet there are no manifest modifications of the body which fit its peculiar needs. In shape the animal does not differ in any important way from our ordinary terrestrial species of Testudinata, which at most burrow in the earth for a little distance to secure temporary concealment or protection from cold in the hibernating season. All the most evident external modifications of the tortoise are directed to the end of securing protection against assault. The gopher is singularly exempt from the dangers encountered by the species which normally dwell on the surface, and its needs are totally different from its purely terrestrial kindred. How is it, then, that the form remains unchanged? Clearly the selectionist has to assume either that advantageous variations do not occur, or that there is some controlling element limiting the process of variation which absolutely prevents the accumulation of these chance modifications to a profitable end. Variations do occur in the shape of individuals. They seem to be about as plastic as other vertebrates in this regard. Here we must throw out the idea that the failure to produce advantageous modifications is due to the lack of variety on which selection can work. We are therefore reduced to the question-begging of which many naturalists now avail themselves in considering this process, and are compelled to say that there is a certain rigidity in the organization of the animal which prevents the accumulation of beneficial variations. This explanation is substantially like that of the doctor in Molière's play, who explained that "opium put people to sleep because of its soporific virtue"; but this does not suffice in the present case. It is worth while to note in this connection that, although the habits of the gopher have varied little with their peculiar habit of life, they have invented, as before noted, the very sufficient and ingenious custom by which they discharge the surplus earth from their burrows at the least expenditure of force and time. This peculiar intellectual adaptation appears to me one of the most interesting features connected with the life of this interesting animal.
- The only peculiar modification of the gopher's shell which can be deemed the product of selection with reference to its peculiar habits is the share-like projection of the plastron or lower shell, which is directed forward, and possibly serves in a slight way to divide the earth at the bottom of the burrow over which it crawls. My friend Mr. S. W. Garman, who has kept one of these creatures in captivity, has observed that the animal, by tilting the body downward at the anterior end, can project this share under the edge of a stone or into the crevices between two boards, and exercise a considerable amount of disruptive power by this process. If these creatures made their burrows in stony ground, it might be possible to conceive the structure as advantageous, but as they work altogether in fine-grained soils, I can not conceive that this curious projection is of any functional value.