Part 7

DEN 19.

October 31, November 1, 1919. In good grass. Entire burrow system mapped (Fig. 2, p. 29).

Species stored. Per cent. Grams.

Aristida spp. (probably mostly divaricata) 98 } Eriogonum sp Tr. } Bouteloua rothrockii 1 } 1,813 Bouteloua aristidoides 1 } Panicum sp Tr. } Prosopis velutina 1,213 ----- Total 3,026

Five species represented, in addition to those of Aristida. Largest storage of Prosopis found. Mound was near a good-sized mesquite tree. No storage in subsidiary burrows.

DEN 21.

January 31, 1920. Male trapped here night of January 29, and suckling female trapped at same place and same opening of mound, night of January 30. Burrow excavated to secure young, which were found in special nest chamber.

Species stored. Grams.

Aristida spp. (intimate mixture of undetermined species) 1,115 Eschscholtzia mexicana (from spring of 1919) 48 Opuntia (prickly pear, seeds only, no fruits) 10 ----- Total 1,173

Three species represented. Prickly pear hitherto found as fruits only.

DEN 22.

January 1, 1921. Rather good grass growth here in summer of 1920. Burrow typical, sandy soil. Two skulls of former residents unearthed.

Species stored. Grams.

Aplopappus gracilis (some B. rothrockii) 1,030 Astragalus nuttallianus 630 Bouteloua rothrockii (some A. gracilis) 530 Sida diffusa 30 Solanum elaeagnifolium (282 fruits) 53 Loeflingia pusilla Tr. Bouteloua aristidoides Tr. Plantago ignota Tr. Lupinus sparsiflorus Tr. Old storage (mostly Bouteloua aristidoides with traces of B. rothrockii and Aristida divaricata) 60 ----- Total 2,333

Eleven species represented. First instance of quantity storage of Aplopappus gracilis. First occurrence of Loeflingia pusilla and Astragalus nuttallianus.

DEN 24.

November 8, 1921. On mesa northeast of Albuquerque, N. Mex., near base of Sandia Mountains. Fair grass growth here during preceding summer.

Species stored. Grams.

Sporobolus cryptandrus strictus 5,455 Salsola pestifer 295 ----- Total 5,750

Two species represented. The heads of Sporobolus cryptandrus strictus are retained to a great extent within the leaf sheaths. This necessitates the cutting of the stems into suitable lengths for carrying, and the stored material appears to be merely cut sections of the stems. Close examination, however, discloses the heads within, and shows that as in other instances seed storage is the end sought. These pieces are packed beautifully parallel like so many matches, and vary from a minimum length of 20 to a maximum of 37 millimeters, averaging about 30. Count of 2 grams of the above Sporobolus material shows that there are 125 separate cut sections per gram, or a total of approximately 680,000 pieces in this one lot of storage, indicating a remarkable activity on the part of the individual rat (Pl. VIII, Fig. 1).

Vertical section through Den No. 1, of Table 1 (p. 20), showing the complex system of burrows, some of them plugged with closely packed storage (outlined in white), the depth of the den, and the widened chambers centrally located.]

Storage content of Den No. 1 (Fig. 1, above), showing the separate species of plants listed in Table 1. The rod is 1 meter long. The large pile on the left is composed of seed-laden heads of crowfoot grama (Bouteloua rothrockii), the large pile on the right consists of heads of six-weeks grama (Bouteloua aristidoides), the pile of heads in the center is desert plantain (Plantago ignota), and the smallest heap is composed of buckwheat-bush seeds (Eriogonum polycladon).]

The number of lots of storage (24) studied in detail, extending as it does over a period of three years with seasons of varying growth conditions, is not sufficient to permit the construction of a curve showing increase and decrease in quantity of stored material with growing seasons and intervals between; but the results indicate a very decided increase during the autumn storing season, and continuing large well into the winter, since some outside material can still be obtained until midwinter. From about February to April a decrease may be noted, followed, if the spring growth of annuals be good, by a slight increase; and we can very nearly predict the general character of the increases and decreases by the precipitation and consequent growth conditions.

TABLE 2.--Quantity of storage per den correlated with time of year and growth conditions of preceding season (chiefly from United States Range Reserve near the Santa Rita Mountains, Ariz.).

------------+-----------+-----------+----------- | | | Den No. | Date. | Quantity. | Preceding | | | season. | | | ------------+-----------+-----------+----------- | 1918. | Grams. | | | | 1 | Feb. 7 | 4,127 | Good. 2 | Mar. 9 | 3,615 | Do. 3 | July 25 | 401 | Poor. 4 | Sept. 20 | 734 | Do. 5 | Sept. 21 | 2,520 | Do. 6 | Oct. 17 | 2,315 | Do. 7 | Dec. 20 | 1,247 | Do. | | | | 1919. | | | | | 8 | Feb. 7 | 1,600 | Do. 9 | Mar. 13 | 370 | Do. 10 | Apr. 7 | 180 | Do. 11 | Apr. 9 | 117 | Good. 12 | May 7 | 298 | Do. 13 | May 11 | 1,590 | Do. 14 | Aug. 8 | 5 | Good. 15 | Sept. 4 | 151 | Do. 16 | Oct. 17 | 313 | Do. 17 | Oct. 18 | 583 | Do. 18 | Oct. 25 | 3,410 | Do. 19 | Nov. 1 | 3,026 | Do. 20 | Dec. 13 | 2,816 | Do. | | | | 1920. | | | | | 21 | Jan. 31 | 1,173 | Do. | | | | 1921. | | | | | 22 | Jan. 1 | 2,333 | Fair. 23 | Nov. 7 | 1,685 | Good. 24 | Nov. 8 | 5,750 | Do. | | | ------------+-----------+-----------+-----------

In presenting Table 2, showing quantity 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 grasses 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.

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 grass, Sporobolus cryptandrus strictus: the smaller of Russian thistle (Salsola pestifer.)]

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