[Ill.u.s.tration: FIG. 3.--Diagram of the system of surface runways and subsidiary dens of _Dipodomys spectabilis spectabilis_. The underground tunnels of the main den were too complicated to ill.u.s.trate on this scale, being very similar to those of Figure 2. The underground tunnels of the subsidiaries are shown in solid black. Some runways fade out in the gra.s.s in a manner that can not be indicated in a line drawing.]
Bailey gives the dimensions of nest chambers observed in New Mexico as about 6 by 8 inches to 8 by 10 inches. The nest is composed of finer, softer, and more chaffy material than the regular storage. The chaff refuse from the food probably contributes largely to it, though some leaves of gra.s.ses not stored for food may also be found, and a nest, especially the one in use, may be distinguished, if excavating is carefully done, by the distinct cavity about the size of a fist in its interior (Pl. IX, Fig. 1). One may sometimes find this cavity distinctly warm from the recent presence of the inhabitant.
The walls or part.i.tions between the chambers and tunnels are in places surprisingly thin, and it is no wonder that one is almost certain to break through in stepping on a mound, since the whole is a honeycomblike structure of from two to four stories in vertical plan, as shown by the transect of a mound in Plate VII, Figure 1. As Bailey writes, these part.i.tion walls are a mixture of earth and old food and nest material discarded years ago, resembling the adobe walls of the Mexican houses built of chopped earth and straw. This is the result of the continual ejection of refuse and earth as before mentioned, combined with the caving action of rains and disturbances from larger animals.
Apparently there are no special pockets for deposit of feces in _Dipodomys_ burrows; such matter may be found throughout the den, and is more or less mixed with the food refuse which carpets practically the entire tunnel system. The nest and food stores are, however, clean and neat, the droppings being dry and, though present on the floor of a storage chamber, not actually mingled with the food. Evidently the animal does not clean up the floor litter before storing food material.
The entire system for any one den seems to consist not only of the burrows within the mound itself, as described, but of those small outlying ones which we have referred to as subsidiary burrows. These are two to four in number, and are connected with the main mound by the runways already mentioned. They often seem to be way stations on the runways connecting main mounds, and there is seldom any mound of earth whatever in connection with them. One entire den system, the home mound and three subsidiaries, was mapped after being excavated (Fig. 3), all having been carefully ga.s.sed with carbon bisulphide. The subsidiaries were simple and contained no storage. Two of them were shallow, while in the third a depth of 48 centimeters was reached. They appear to be merely places of refuge, though the well-worn trails connecting them with the main mound indicate regular use. These runways are conspicuous on the Range Reserve, and are apparently characteristic of mounds throughout the range of the animal. Dwellers in different mounds must have rather extensive social contacts, notwithstanding the enmity of individuals toward each other in captivity. The main mound, in this instance very complicated, was in one place three stories high, and we have found as many as four utilized stories; but as a rule there are two or three only.
Since collapses are rather frequent during rainy seasons, aside from the trampling previously referred to, the kangaroo rats, where abundant, as on the Range Reserve, may well be a factor in increasing soil porosity and fertility; for in the course of time they probably have succeeded in plowing and cultivating the whole surface layer of the soil. They may thus be a factor in ecologic succession, tending to improve the character of the soil and adapt it to another stage.
Doubtless their own workings afford the only shelter the animals know.
In the course of our digging in one mound, the occupant, an adult male, did not forsake the den until the excavation was three-fourths completed; and even then it did not leave by a burrow leading away from our operations, but came toward us, escaped the active efforts of four individuals bent on its capture, and ran speedily along a used runway toward another burrow several meters distant. A sack had been stuffed in the mouth of this, however, and, baffled, the rat then returned to the original burrow and was captured. Observations on other rats thus driven from the home mound indicate that they are very familiar with the runways of the vicinity of the mound and the various subsidiary burrows, and it is a question whether they need to see clearly to follow these runs. Apparently they never attempt to escape by forsaking their well-traveled runways. Tests of the maze-running ability of these animals by animal-behavior experts would be of extraordinary interest, in view of the character of the homes which they always inhabit and the network of runs on the outside.
[Ill.u.s.tration: PLATE IX. FIG. 1.--KANGAROO RAT NEST AND YOUNG.
Nest and the two young, the ordinary number in the litter, of _Dipodomys s. spectabilis_, taken from den on January 31, 1920.]
[Ill.u.s.tration: PLATE IX. FIG. 2.--YOUNG OF THE KANGAROO RAT.
The same young as shown in Figure 1, above. They were probably about two weeks old, the pelage being short but with the white markings of the adult; the tails are relatively short and with scarcely any hair.]
COMMENSALS AND ENEMIES.
COMMENSALS.
It is doubtful whether any animals live in a truly commensal relationship with _spectabilis_, but of not unfriendly a.s.sociates there are a great number. It is the experience of Bailey, corroborated by observations of Vorhies on living animals, that these kangaroo rats are active in defending their caches of food, and will even fight individuals of the same species savagely and to the death. One moonlight night a strange individual was liberated on a mound. It deliberately entered one of the openings, but after about two minutes" time made an exceedingly rapid exit, running rapidly out of sight as if pursued, though the owner of the home did not appear outside of the burrow. There can be little doubt that the stranger was precipitately ejected by the owner. We suspect, though this is a point difficult to prove satisfactorily, that _merriami_ does not always store food supplies for itself, but visits the burrows of _spectabilis_ regularly to pilfer the seed stored therein. The observed facts thus far recorded which suggest this are that in no _merriami_ burrow examined has a store of food been found, and also that in trapping for _spectabilis_ on its own characteristic mounds one catches a large percentage of _merriami_.
On two separate occasions Vorhies has observed the smaller species running over the mounds of the larger, actually carrying away the grain which had been placed to entice the larger when it might appear. (In these cases the larger species did not put in an appearance until near morning.) Furthermore, the dens of _merriami_ are often connected by distinct runways with those of _spectabilis_, indicating much traveling or visiting. That this is probably not friendly visiting is suggested by the certainty with which an individual of the larger species will strike and kill one of the smaller when they are placed together in the same inclosure. The word "thief" expresses this suspected relationship better than would the term "parasite."
It is not to be expected that such obvious shelter retreats as the mounds of _spectabilis_ should fail to attract the attention of other animals. We have found a small gecko (_Coleonyx variegatus_), scorpions of two or three undetermined species, and certain insects (of the Order Orthoptera) to be very common inhabitants of the dens. With the exception of the parasitic insects the most common are wingless locustids (_Ceuthophilus_ spp.) and the peculiar wingless females of a species of c.o.c.kroach (_Arenivaga erratica_). These two are seldom absent when a burrow is excavated, the female c.o.c.kroaches being abundant, although the winged males have never been taken in the burrows.
Cary"s observations at Monahans, Tex., and those of others at numerous localities, combined with our own, show that at various times the dens furnish protection and shelter for various species of cottontail rabbits (_Sylvilagus_), ground squirrels (_Citellus_ and _Ammospermophilus_), wood rats (_Neotoma_), gra.s.shopper mice (_Onychomys_), rattlesnakes (_Crotalus_), and most of the common lizards. Of these the ground squirrels _Citellus tereticaudus_ and _Ammospermophilus harrisii_ are most often noted on the Range Reserve using the dens as a retreat, the _Ammospermophilus_ seldom being observed to enter any other kind of burrow. It should be added that the total observations include dens which have been deserted by their rightful owners.
NATURAL CHECKS.
The enemies of the kangaroo rat are not determined in detail, or as to relative importance, but the badger (_Taxidea taxus berlandieri_) and the kit fox, or swift (_Vulpes macrotis neomexicana_), may well be foremost. Dens which have been deeply excavated by badgers are frequently seen, and sometimes two or three badger tunnels penetrate one burrow system. Dens thus despoiled are probably soon reoccupied even if the original owner is captured, and in the course of a few months the reworking of the abode obliterates the signs of destruction.
Droppings of the kit fox show an abundance of bones of small mammals of kangaroo rat size, among them those of _spectabilis_.
Bobcats (_Lynx baileyi_) and coyotes (_Canis mearnsi_) probably are a prejudicial factor. Skunks may sometimes be able to surprise the kangaroo rats, but probably not often. The western horned owl (_Bubo virginia.n.u.s pallescens_), the barn owl (_Tyto alba pratincola_), and perhaps others may well be among the most feared enemies, but no special investigation of owl pellets on the reserve has been possible. In 592 barn-owl pellets from California were found remains of 230 kangaroo rats, only one other rodent being represented by a larger number (McAtee, 1921, 258).
Much more information on enemies is needed. The relatively low rate of reproduction (see p. 18) indicates comparative freedom from inimical factors.
PARASITES.
_Dipodomys s. spectabilis_ is regularly infested with a species of flea, _Ctenophthalmus_ sp. Seldom or never is a specimen taken in reasonably fresh condition without some of these parasites present on its body, though of course they desert the body of the host after it becomes cold, and hence dead specimens left too long may be free from them. The den conditions are ideal for the breeding of this parasite, because of the great quant.i.ties of fine, dusty, organic refuse littering the tunnels and furnishing food and refuge for the larvae. As demonstrated to us by F. C. Bishopp, of the Bureau of Entomology, a handful of this refuse taken from the floor of a burrow within arm"s length of the entrance is almost certain to contain these larvae.
Less regularly present, perhaps because of its different life history, is a small tick, _Trombicula_ sp. At times this parasite is very common, being present on nearly every individual rat, and at other times specimens are difficult to find; it appears to be more commonly present in summer and fall than at other seasons, and is found attached chiefly to the ears.
No internal parasites have been detected. The nocturnal and fossorial habits of the animal seem to give complete protection against a form of parasite which is very common among some other rodents of the Range Reserve, notably _Lepus_ and _Sylvilagus_. Nearly all rabbits are infested with "warbles," the larvae of a species of bot-fly, _Cuterebra_ (family Oestridae). Other small mammals also are occasionally parasitized by the _Cuterebra_, but in the handling and examination of perhaps 200 or more individuals of _spectabilis_ and _merriami_, we have yet to find a single case of infestation by an oestrid fly.
ABUNDANCE.
One"s first impression of a well-occupied _spectabilis_ area is that a large family must inhabit each den, but, as previously mentioned, we have gradually been compelled to shift from this conception to the idea of but a single animal to a mound, except when the young are present.
Therefore a census of the adult kangaroo rat population can readily be made, simply by counting the mounds. Such a census affords at least a conservative estimate of the number of adult individuals occupying a given area.
The first estimates of abundance on the Range Reserve were from actual counts of dens on areas measured off for experimental fencing, and gave the figure of about two mounds to the acre. From time to time rough estimates were made on different portions of the pastures, and these checked well with the above. Later still, a careful count showed 300 mounds on approximately 160 acres (see p. 8), or 1.87 mounds per acre.
Nine areas of 2 acres each, representing different environmental conditions, were later selected in different portions of the Range Reserve, and the dens accurately counted. The number of dens per 2 acres varied from none to a maximum infestation of 12, neither extreme occurring over large areas. The total number of dens was found to be 43 on the 18 acres, or an average of 2.38 dens per acre.
From all these estimates it may fairly be concluded that two mounds, or two animals, per acre is a conservative estimate for the infestation of the entire Range Reserve, with the possible exception of small areas at its upper edges, where the alt.i.tude limit of _spectabilis_ is pa.s.sed. It is, however, impossible to estimate the area of the State infested with kangaroo rats, for some large stretches of fine gra.s.sland show no kangaroo rats whatever, while others have more than are present on the reserve; and we have no estimates of the extent of either type.
ECONOMIC CONSIDERATIONS.
In May, 1894, Fisher found a ranchman at Willc.o.x, Ariz., who complained more bitterly of the depredations of _spectabilis_ than of those of any other mammal.
On the United States Range Reserve the food material appropriated by the kangaroo rat during good years is inappreciable. There is such an excess of forage gra.s.s produced that all the rodents together make very little difference. But with the periodic recurrence of lean years, when drought conditions are such that little or no gra.s.s grows, the effects of rodent damage not only become apparent, but may be a critical factor determining whether a given number of domestic animals can be grazed on the area (Pl. VIII, Fig. 2).
With two kangaroo rats to the acre (1,280 per square mile), there would be 64,000 animals on the 50 square miles of the Range Reserve. If each rat stores 4 pounds of gra.s.s seeds and crowns and other edible forage during the season (and in severe seasons we find that more crowns are stored than under ordinary conditions), a total of 256,000 pounds, or 128 tons, of edible forage are rendered unavailable to stock. In dry years it is probable that this amount of forage would be of critical importance. Allowing 50 pounds of food a day for each steer, the forage destroyed would be sufficient to provide for the needs of one steer for 5,120 days, or for the needs of 14 steers for one year. On a stock ranch the size of the Range Reserve this might mean the difference between success and failure.
It seems not unlikely, therefore, that during seasons of drought the banner-tailed kangaroo rat, where it is abundant on the grazing ranges of the Southwest, may be a factor of critical importance in relation to forage production and carrying capacity. It must be remembered, moreover, that the stored material consists largely of seeds, so that this loss is of greater importance than would be the case were it ordinary forage. Some of the range gra.s.ses of this region found in greatest quant.i.ty in the stored material depend in large part, under certain conditions, upon seed reproduction. Rehabilitation of a depleted range after severe drought and consequent close grazing and trampling is r.e.t.a.r.ded by the heavy toll of seed taken by the kangaroo rats.
CONTROL.
Kangaroo rats may be easily eradicated by the use of the poisoned grain used for prairie-dog control by the Biological Survey and the University of Arizona Extension Service. This can be obtained by application to the State representative of the Biological Survey or to the local county agricultural agent, or may be mixed as follows:
_Formula for poisoned bait._--Dissolve 1 ounce of strychnine sulphate in 1-1/2 pints of boiling water. Add 1 heaping tablespoonful of gloss starch, previously mixed with a little cold water, and boil until a clear paste is formed. Add 1 ounce of baking soda and stir to a creamy ma.s.s. Add 1/2 ounce of glycerine and 1/4 pint of corn sirup and stir thoroughly. Pour over 16 quarts of rolled barley and mix well until every grain is evenly coated.
Allow to dry before using.
In bushel quant.i.ties use as above directed, 2 ounces of strychnine, 2 ounces of soda, 1 ounce of glycerin, 1-1/4 ounces of starch, 1-1/2 quarts of boiling water, and 5/8 pint of corn sirup.
Scatter poison, when the natural food of the kangaroo rat is scarce, on clean hard places near the holes, 1 quart to 50 holes.