Life History of the Kangaroo Rat Part 5

[Footnote 3: Changing from poor summer season of 1918 to excellent spring growth of 1919.]

[Footnote 4: From near the Sandia Mountains, N. Mex.; others from United States Range Reserve, near the Santa Rita Mountains, Ariz.]

In presenting Table 2, showing quant.i.ty of storage per burrow correlated with the time of year and the character of the preceding growing season, the fact may be emphasized that the growing seasons in southern Arizona are two in number--early spring and midsummer. The spring season is the less important, the plants consisting chiefly of a variety of small annuals, while the important range gra.s.ses make their chief growth and head out almost exclusively in the July-August rainy season. It may be noted also that the actual increases in storage appear somewhat after the growth period proper, since storing does not get well under way until the seed crop is mature. The banner-tailed kangaroo rat shows a marked adaptability to different foods available in the neighborhood of its burrows. It must, perforce, adapt itself and its storage program to the food that it can get, and this varies enormously with the climatic conditions of successive seasons. The large numbers present in suitable localities clearly indicate that the animal is successful in meeting the changing and sometimes extremely adverse conditions of its environment.

[Ill.u.s.tration: PLATE VIII. FIG. 1.--CONTENT OF DEN EXCAVATED IN NEW MEXICO.

Storage content of Den No. 24, of Table 1, from Sandia Mountains, N.

Mex. This is the largest lot of storage taken in the course of the investigations. The larger pile consists wholly of a valuable gra.s.s, _Sporobolus cryptandrus strictus_: the smaller of Russian thistle (_Salsola pestifer._)]

[Ill.u.s.tration: PLATE VIII. FIG. 2.--GROWTH FOLLOWING ELIMINATION OF KANGAROO RATS.

The same mound as shown in Plate III, Figure 1, after three years of protection, the rodents having been killed out. Nearly as good gra.s.s recovery following poisoning operations occurred in the single excellent season of 1921.]

At times, more especially in the seasons of active growth, some of the green and succulent portions of plants are eaten. This was very noticeable in the spring of 1919, when a most luxuriant growth of Mexican poppy (_Eschscholtzia mexicana_) occurred. Stomachs at this time were filled with the yellow and green mixture undoubtedly produced by the grinding up of the buds and flowers of this plant. Small caches of about a tablespoonful of these buds were also found in the burrows at this time. Occasionally in spring one may find a few green leaves of various plants, _Gaertneria_ very commonly, tucked away in small pockets along the underground tunnels, indicating that such materials are used to some extent. As has been shown in detail, however (Table 1), the chief storage, and undoubtedly the chief food, consists of air-dry seeds.

The character of the storage, the absence of rain for months at a time in some years, and the consequent failure of green succulents show that without doubt _spectabilis_ possesses remarkable power, as to its water requirements, of existing largely if not wholly upon the water derived from air-dry starchy foods, i.e., metabolic water serves it in lieu of drink (Nelson, 1918, 400), this being formed in considerable quant.i.ties by oxidation of carbohydrates and fats (Babc.o.c.k, 1912, 159, 170). During the long dry periods characteristic of southern Arizona, no evidence that the animal seeks a supply of succulent food, as cactus, is found; and if it may go for two, three, or six months without water or succulent food, it is reasonable to suppose that it may do so indefinitely. In the laboratory _spectabilis_ ordinarily does not drink, but rather shows a dislike for getting its nose wet. During the periods of drought the attacks upon the cactuses by other rodents of the same region, as _Lepus_, _Sylvilagus_, _Neotoma_, and _Ammospermophilus_, become increasingly evident. The list of plant species thus far found represented in the storage materials of _spectabilis_ on the Range Reserve is shown in Table 3.

TABLE 3.--_List of all plant species found in 22 dens of _Dipodomys spectabilis_ on the United States Range Reserve, near the Santa Rita Mountains, Ariz., with approximate total weights._

GRa.s.sES.

Grams.

Aristida bromoides (six-weeks needlegra.s.s) 536 Aristida divaricata (Humboldt needlegra.s.s) 9,412 Aristida scabra (rough needlegra.s.s) 344 Bouteloua aristidoides (six-weeks grama) 3,093 Bouteloua radicosa (grama) 1,269 Bouteloua eriopoda (black grama) Tr.

Bouteloua rothrockii (seeds, 8,495; crowns, 3,517 grams) (crowfoot grama) 12,012 Festuca octoflora (fescue gra.s.s) 70 Panic.u.m arizonic.u.m (Arizona panic-gra.s.s) 11 Panic.u.m hallii (Hall panic-gra.s.s) Tr.

Pappaphorum wrightii Tr.

Tridens pulch.e.l.la Tr.

Valota saccharata Tr.

OTHER PLANTS.

Alternanthera repens Tr.

Anisolotus trispermus (bird's-foot trefoil) 186 Aplopappus gracilis 1,030 Apodanthera undulata (melon loco) 55 Astragalus nuttallia.n.u.s (milk vetch) 630 Ayenia microphylla Tr.

Boerhaavia wrightii 885 Chamaecrista leptadenia (partridge pea) 5 Echinocactus wislizeni (visnaga) 5 Eriogonum polycladon 35 Eschscholtzia mexicana (Mexican poppy) 250 Gaertneria tenuifolia (franseria) Tr.

Collomia gracilis (false gilia) Tr.

Heterotheca subaxillaris Tr.

Kallstroemia laetevirens Tr.

Lupinus sparsiflorus (lupine) Tr.

Martynia altheaefolia (small devil's-horns) 12 Mollugo verticillata (carpetweed) 324 Oenothera primiverus (evening primrose) 15 Opuntia discata (p.r.i.c.kly pear) 15 Loeflingia pusilla Tr.

Lepidium lasiocarpum (peppergra.s.s) Tr.

Plantago ignota (plantain) 818 Polygala p.u.b.erula (milkwort) Tr.

Portulaca suffrutescens (purslane) Tr.

Prosopis velutina (mesquite) 1,570 Sida diffusa (spreading sida) 30 Solanum elaeagnifolium (742 fruits) (trompillo, p.r.i.c.kly solanum) 156 Puffb.a.l.l.s and fleshy fungi (undetermined) 12

Total species, exclusive of fungi, 41.

It will be seen from Table 3 that while a large number of species of plants are represented in the totals from so many dens, a majority of them are of very minor importance, and that the seeds of gra.s.ses are the princ.i.p.al storage and probably therefore the princ.i.p.al food material.

Six of the most important species of gra.s.ses (disregarding species furnis.h.i.+ng less than 5 grams) comprise 85.6 per cent of the total weight of storage from 22 dens. Crowfoot grama (_Bouteloua rothrockii_) stands first in quant.i.ty in the total, forming 39.4 per cent of all stored material, 46 per cent of the six important gra.s.ses, and 45 per cent of all gra.s.ses. The largest amount of storage of any one species of gra.s.s in any one den on the Range Reserve also is of this species, 2,205 grams[5] (Table 1, den 1, p. 20, and Pl. VII, Fig. 2). This is exceeded by a dropseed gra.s.s, _Sporobolus cryptandrus strictus_, which amounted to 5,455 grams in a lot from Albuquerque, N. Mex. (Table 1, den 24, and Pl. VIII, Fig. 1).

[Footnote 5: This amount of dry grama gra.s.s seed (heads) amounts to approximately a bushel.]

Of the species other than gra.s.ses found stored in these dens, mesquite beans (_Prosopis velutina_) are most important both by weight and number of dens containing them. The total for the 22 Range Reserve dens is 1,570 grams, or 35.9 per cent of the seeds other than gra.s.ses, but only 5.1 per cent of the total storage. In bulk mesquite beans do not loom up large, as they are probably the heaviest material stored. Sections of pods which must have been dragged into the burrows are found, some of them certainly being much too long for carriage in the pouches. The species of plant other than gra.s.s found in the largest quant.i.ty in any one den, however, was _Aplopappus gracilis_, not recorded in quant.i.ty from any den until the excavation of the twenty-second, and then found in a very large bulk of soft, fluffy material, with most of the seeds separated from the heads, and weighing 1,030 grams (Table 1, den 22).

Any of the food materials above listed are likely to be found in the cheek pouches, while in addition such extraneous matter as stones and feces have also been found. All species of plants stored are accessible in the immediate vicinity of the mound, and when any particular plant is found seeding in abundance in the vicinity of the den it is likely to be represented in the storage. Usually the animals can be readily trapped with almost any kind of grain bait, as oats, rolled oats, rolled barley, and wheat; and nut meats also are attractive, though we have no record of the storing of any true nut in the dens, such not being available in the range of the animal on the Range Reserve.

The following plants not represented in the list stored by the kangaroo rat on the Range Reserve have been found in the cheek pouches or mounds of _spectabilis_ in other localities:

_Amaranthus palmeri_, _Sesuvium portulacastrum_, and _Atriplex wrightii_ (alluvial soil of Santa Cruz Valley, Continental, Ariz., Bailey).

Cut leaves and stems of a small sagebrush (Franklin Mountains, Tex., Gaut).

_Gutierrezia_ heads (San Juan Valley, N. Mex., Birdseye).

_Verbesina enceliodes_, _Portulaca oleracea_, _Bouteloua gracilis_, and _Munroa squarrosa_ (Rio Alamosa, N. Mex., Goldman).

Tops of buds of _Artemisia filifolia_ (Mesa Jumanes, N. Mex., Gaut).

Tumbleweed (_Amaranthus graecizans_), Russian thistle (_Salsola pestifer_), _Munroa squarrosa_, and _Sporobolus cryptandrus strictus_ (Sandia Mountains, Albuquerque, N. Mex., Vorhies).

BURROW SYSTEMS, OR DENS.

The burrow system, or den, in which _spectabilis_ stores its caches of food materials, has its nest, and remains throughout the hours of daylight is a complicated labyrinth of tunnels. Ejection of refuse and soil from this retreat builds up the mound frequently referred to. These mounds are, as Bailey says, characteristic of the species, and are as unmistakable as muskrat houses or beaver dams, and as carefully planned and built for as definite a purpose--home and shelter. They are, furthermore, the most notable of all kangaroo rat dwelling places (Nelson, 1918, 400). They range in height from 6 inches to approximately 4 feet and from 5 to 15 feet in diameter.

The mound is built up not only through the cleaning out of chaff and other food refuse, but through extension and modification of the tunnels; old tunnels, entrances, and caches of musty food material are from time to time closed up and others excavated, repair and rebuilding being especially necessary after the collapse of portions of the den as a result of heavy rains or trampling by cattle. Ejected material is most commonly simply thrown out fan-wise from the openings, without much apparent effort to add to the height of the mound.

There are usually from 6 to 12 entrance holes in each mound opening into the subterranean burrow system, each hole from 4 to 5-1/2 inches in diameter. These holes are nearly all situated a little above the surface of the surrounding soil, and as Price has suggested (in Allen, 1895, 213), this is doubtless a wise provision against flooding, as torrential rains sometimes occur in the kangaroo rat country.

Both Bailey and Nelson state that as a rule several of the holes are closed with sand or miscellaneous earth and old storage material during the daytime, but our observations on the Range Reserve are that such closing is only occasional. Many occupied dens have not a single opening closed. Further, night observations disclose that the inhabitant of the mound will appear from some one of the two or three most-used openings when night falls, and not necessarily from one which has been closed by day. Recently an opening closed one day was observed in use during the night, but was left open all the next day.

In attempting to determine whether there exist similarities of plan or system in the dens, it was considered advisable to map them with some degree of accuracy. This we were enabled to do by laying off a square about a given mound, 2-1/2 or 3 meters each way, and subdividing it into a series of small squares of half a meter on each side by drawing cross-lines on the surface of the ground over the top of the mound. One person then did the digging and exploring of the tunnels, as to direction and depth, while the other noted the results on coordinate paper (Figs. 2 and 3); the proper excavation and mapping of one of these workings occupied from four to eight hours for the two.

[Ill.u.s.tration: FIG. 2.--Diagram of a typical den of _Dipodomys spectabilis spectabilis_. Double shading indicates where one portion of tunnel lies above another and solid black a three-story arrangement; _A_, _B_, _C_, etc., active openings to surface; figures without arrows, depths in centimeters to tunnel roofs; figures with arrows, tunnel widths in centimeters; _N._ nest chamber; _S_, storage; _OS_, old storage; _Y_, probably an old nest chamber; _Z_, old, unused, or partially plugged openings.]

While there is greater complexity in the larger, and probably older, mounds than in the smaller, all are extremely complicated and can only be described as labyrinthine in character. The tunnels wind about through the mound, rising and falling in vertical depth, intercommunicating frequently, but with occasional cul-de-sacs, and in places expanding into chambers, of which there may be three or four large ones. The stored materials are found in some, but not necessarily all, of these chambers, and may also occupy considerable lengths of ordinary tunnel, especially when the quant.i.ty present is large. Small ev.a.g.i.n.ations of the tunnels frequently contain lesser caches, and it is in such pockets that bits of fresh material are placed during a growing season, or that grain supplied the previous night for bait is usually found.

The main ma.s.ses of storage are most often found centrally located at depths of from 15 to 57 centimeters, although at times one may find a cache near the periphery of the system and as near the surface as 2 or 3 centimeters. In the latter case the materials are subject to wetting from rains, and consequent spoilage.

The major portion of the whole tunnel system is within about 50 centimeters of the surface of the mound, but usually some one branch tunnel goes to somewhat greater depth, and this is likely to be the one containing the nest; this is also likely to extend toward or beyond the periphery of the main system, and always ends blindly. Such a one, from which two young were taken on January 31, 1920, was at a depth of about 65 centimeters, and about 1-1/2 meters beyond the periphery of the mound itself.

The individual tunnels average about 8 centimeters in height, and about 11 centimeters in width, though the variation, especially in width, is considerable. The expansions mentioned as being the chief places of storage are from 15 to 25 centimeters in diameter, and may or may not involve a considerable increase in height. They are frequently located at junction points of two or more branches of the tunnel system.

The nest cavity is a chamber of approximately spherical shape and from 17 to 23 centimeters in diameter. Chambers of this character were observed and noted as "old storage" in a number of cases. They were sometimes cut off from the rest of the habitation, and at first were supposed to contain abandoned musty storage. As experience in excavating and interpreting results has been gained we have concluded that these chambers in fact represent abandoned nests.