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).
[Illustration: PLATE VII. FIG. 1.--DEN EXCAVATED ON RANGE RESERVE.
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.]
[Illustration: PLATE VII. FIG. 2.--CONTENT OF EXCAVATED DEN.
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.[3]
11 | Apr. 9 | 117 | Good.[3]
12 | May 7 | 298 | Do.[3]
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[4] | Nov. 7 | 1,685 | Good.
24[4] | Nov. 8 | 5,750 | Do.
| | |
------------+-----------+-----------+-----------
[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 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.
[Illustration: 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 grass,
_Sporobolus cryptandrus strictus_: the smaller of Russian thistle
(_Salsola pestifer._)]
[Illustration: 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 grass
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 quantities
by oxidation of carbohydrates and fats (Babcock, 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._
GRASSES.
Grams.
Aristida bromoides (six-weeks needlegrass) 536
Aristida divaricata (Humboldt needlegrass) 9,412
Aristida scabra (rough needlegrass) 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 grass) 70
Panicum arizonicum (Arizona panic-grass) 11
Panicum hallii (Hall panic-grass) Tr.
Pappaphorum wrightii Tr.
Tridens pulchella Tr.
Valota saccharata Tr.