Chapter 3

Five breeding females were taken in January, all during the last three days of the month. One of these was a suckling female, the young of which were secured alive and were probably at least a week old when taken. This must have been exceptionally early for young, since of a number of adult kangaroo rats taken during the first week of January none have been found to be breeding. Two records from Vernon Bailey are as follows: May 19-June 8, 1903, young specimen in nest (Santa Rosa, N. Mex.); June 12, 1889, one female, two embryos (Oracle, Ariz.).

The considerable proportion (which we believe to be more than 50 per cent) of nonbreeding females taken during all those months in which breeding has been found to occur may also indicate an extended period of breeding, with a small percentage breeding at any one time. This period also furnishes ample time for the rearing of two litters a year by some females, but we have no evidence as to the occurrence of two litters. Young of the year, practically grown, are taken during and after the month of April.

The mammae are arranged in three pairs, pectoral, 1/1; inguinal, 2/2.

Kangaroo rats are among those rodents in which the vagina becomes plugged with a rather solid material, translucent, and of the consistency of a stiff gelatine, after copulation. This must occur very soon after coitus, since in those individuals taken in this condition no definite evidence of the beginning of development of embryos could be detected by examination.

The length of the gestation period of _spectabilis_ is unknown. The young are born naked, a fact inferred by failure to find any fetus showing noticeable hair development, and from the conditions observed in such young as have been seen. A suckling female was taken by Vorhies, January 31, 1920, and her den immediately excavated in the hope of securing the young. Two juveniles were found in a special nest chamber (see p. 30). These were estimated to be perhaps two weeks old. A serious effort was made to raise the little animals by feeding milk with a pipette and keeping them warm with a hot water bottle, but they survived only 10 days, without the eyes having opened. The uneven temperature as well as the character of the food was probably responsible for their deaths. On February 3 they were measured and weighed, with the following results:

| | Measurements (in millimetres). | Weight |------------------------------- | (in | Total | Tail | Hind | grams). | length. | vertebrae. | foot. ---------|---------|---------|------------|-------- No. 1 | 13.3 | 90 | 38 | 24 No. 2 | 12.6 | 93 | 38 | 24 ---------------------------------------------------

At this stage the young were partially clothed with a coat of fine velvety fur, more especially on the bodies, the tails being still nearly naked. The body color was dark plumbeous, just the color of the dark underfur of the adult, or a shade darker, while the characteristic white markings of the adult stood out sharply as pinkish-white areas against the dark background (see Pl. IX, Fig. 2, at p. 32). The proportions were much as in the adult, except that the tails were relatively much shorter and the feet relatively longer.

Only one other record of young is at hand, that by Bailey, who secured the young after capture of a suckling female at Santa Rosa, N. Mex. In this case the litter contained only one. This was squeaking when found, but was not large enough to crawl away. Its eyes and ears were closed, and its soft, naked skin was distinctly marked with the pattern of the adult, the colors being as given for the other two. This juvenile lived only a week. Young less than half grown were not trapped or noted in our poisoning operations outside the dens.

Kangaroo rats, if _spectabilis_ be representative, reproduce at a slow rate as compared with many other small rodents. We have records of 67 females with embryos or scars showing the number produced, and of the two litters of young described above. Of the 69 females thus recorded, 15, or 21.7 per cent, had but one offspring each; 52, or 75.3 per cent, but two each; while only 2 individuals, or 2.9 per cent, had three. Three young is the maximum litter recorded. This, taken in connection with the protracted breeding season and lack of sure evidence of the production of two broods a year, gives a surprisingly low rate of reproduction, indicating relative freedom from inimical factors.

Our breeding records for _merriami_ are fewer than for _spectabilis_, but are very similar in every way so far as they go, both as to the time of year and number of young.

FOOD AND STORAGE.

_Dipodomys s. spectabilis_ does not hibernate, so must prepare for unfavorable seasons by extensive storage of food materials. There are two seasons of the year, in southeastern Arizona at least, when storage of food takes place, namely, in spring, during April or May, and in fall, from September to November, the latter being the more important. For the periods between, the animal must rely largely on stored materials. Not infrequently a season of severe drought precludes the possibility of any storage. The summer and fall of 1918 was such a season on the Range Reserve (Pl. II, Fig. 2). If food stores are inadequate at such a time the kangaroo rats must perish in considerable numbers. Fisher found many deserted mounds in the vicinity of Dos Cabezos, Ariz., in June, 1894, which may be accounted for in this way. In 1921 Vorhies found all mounds within 4 or 5 miles of Albuquerque, N. Mex., deserted by _spectabilis_, resulting probably from overgrazing by sheep and goats during a succession of dry years. In the arid Southwest natural selection probably favors the animals with the largest food stores, and it is not surprising that the storing habit has been developed to a remarkable degree.

Some stored material is likely to be found at any time of year in any mound examined, the largest quantity usually in fall and winter, the smallest in July or August (Table 1, dens 1, 2, 14, and 24). Amounts found by different observers vary from a few ounces to several quarts or pecks, and stored materials taken from 22 mounds on the Range Reserve vary in weight from 5 to 4,127 grams (more than 9 pounds). This is exceeded by one lot from New Mexico, which totaled 5,750 grams (12.67 pounds). It is fairly evident that in seasons of scanty forage for stock the appropriation of such quantities of grass seeds and crowns and other grazing materials by numerous kangaroo rats may appreciably reduce the carrying capacity of the range. Studies of cheek-pouch contents and food stores taken from dens show that the natural food of _spectabilis_ consists principally of various seeds and fruits, particularly the seeds of certain grasses. The study of burrow contents has been especially illuminating and valuable.

All of the stored material from 22 dens on the Range Reserve and from 2 near Albuquerque, N. Mex., has been saved and analyzed as to species as carefully as the conditions of storage would permit. Within the mound the food stored is usually more or less segregated by plant species, though the stores of material of any one kind may be found in several places through the mound, and often the material is mixed. In the latter case the quantities of the various species can only be estimated, but in the former the species may be kept separate by the use of several bags for collecting the seeds, and a fairly accurate laboratory weighing can be made later. Very frequently, the explanation of this separation of species lies in the different seasons of ripening, but sometimes where two species are ripe at the same time near the mound, one is worked upon for a time to the exclusion of the other. The one kind is often packed in tightly against the other, but with a very abrupt change in the character of the material.

A number of the more interesting and representative results of the weighing and analyses of burrow contents are presented herewith in tabular form. The data for each den, or lot, shows in grams the quantity of stored material removed and the best estimate it was possible to make of the percentages or weights of the various species. When the weight was less than 5 grams, the mere trace of the species frequently is indicated in the following tables by the abbreviation "Tr."

TABLE 1.--_Analyses of plants stored by _Dipodomys spectabilis spectabilis_ Merriam, obtained from examination of representative dens (all except Den 24 from U. S. Range Reserve, near the Santa Rita Mountains, Ariz.)._

DEN 1.

February 7, 1918. Burrow typical, located on bank of wash in partially denuded grass-land, _Bouteloua rothrockii_ and weed type; soil sandy; burrow photographed in section (Pl. VII, Fig. 1).

Species stored. Grams.

Bouteloua rothrockii 2,205 Bouteloua aristidoides (B. eriopoda and B. rothrockii, Tr.) 1,445 Plantago ignota 442 Eriogonum polycladon 35 ----- Total 4,127

Four species of plants represented in burrow contents (Pl. VII, Fig. 2). Maximum quantity for single burrow in series of 22 from Range Reserve.

DEN 2.

March 9, 1918. Surroundings overgrazed and partially restored by complete protection. Red soil, with much coarse rough gravel and stone.

Species stored. Grams.

Bouteloua rothrockii (nearly pure) 1,460 Bouteloua rothrockii (mixed with Aristida spp.) 945 Boerhaavia wrightii 660 Bouteloua rothrockii } Bouteloua aristidoides } 525 Aristida divaricata } Aristida bromoides } Kallstroemia laetevirens Tr. Heterotheca subaxillaris Tr. Plantago ignota 15 Fleshy fungi 10 ----- Total 3,615

Eight species of plants represented by seeds. One species of fleshy fungus in addition.

DEN 4.

September 20, 1918. In _Calliandra_ type. Stony or gravelly soil, red, nearly denuded of grass.

Species stored. Grams.

Prosopis velutina 190 Mollugo verticillata (pure) 90 Anisolotus trispermus (mixed, but mostly of this genus) 50 Solanum elaeagnifolium (12 fruits) 2 Per cent. Mollugo verticillata (inseparable) 50 } Bouteloua rothrockii 1 } Bouteloua aristidoides 10 } Lepidium lasiocarpum Tr. } Polygala puberula Tr. } Ayenia microphylla 2 } Portulaca suffrutescens 1 } 400 Aplopappus gracilis Tr. } Alternanthera repens 1 } Tridens pulchella 1 } Plantago ignota 33 } Panicum hallii Tr. } Fleshy fungi (puffballs) 2 --- Total 734

Fifteen species represented in addition to the fleshy fungi. No perceptible grass growth from the summer rains here, therefore dependent on a wide variety of scattering plants.

DEN 6.

October 17, 1918. Mixed type, partially denuded, no growth from summer rains. Sandy soil.

Species stored. Grams.

Bouteloua rothrockii (crowns) (heads 1 to 2 per cent) 1,435 Bouteloua rothrockii (heads and crowns, about 50 per cent of each) 325 Bouteloua rothrockii (with small percentage of crowns) 315 Boerhaavia wrightii (with a few grass crowns) 150 Prosopis velutina 90 Solanum elaeagnifolium (3 fruits) Tr. ----- Total 2,315

Four species represented. Count of 100 grams of stored _Bouteloua_ crowns gives 1,700, or 17 crowns per gram. At this rate there were at least 27,000 crowns stored in this burrow. If a density of 250 plants to the square yard be assumed (a high estimate) these crowns represent the total _B. rothrockii_ on 104 square yards of range surface. Further examination of the vicinity of this den showed that the surrounding area was not completely cleared, but was devoid of _B. rothrockii_, while still having _B. eriopoda_ with crowns undisturbed.

DEN 11.

April 9, 1919. In partially denuded land where good spring growth of _Eschscholtzia_ was in bloom at time of excavation. Stomach of _spectabilis_ killed in this burrow contained a mass of fresh but finely comminuted green material, probably poppy leaves, strongly colored with yellow from blossoms. No summer growth here in 1918.

Species stored. Grams.

Bouteloua rothrockii (crowns) (miscellaneous chaff, etc.) 107 Eschscholtzia mexicana (buds and flowers) } Anisolotus trispermus (leaves and pods) } Gaertneria tenuifolia (leaves) } 10 Lupinus sparsiflorus (flowers) } Solanum elaeagnifolium (2 fruits) Tr. --- Total 117

Six species represented, some only by leaves or flowers and not by seeds. _Such storage is never in large quantity._ The fresh storage material was weighed after becoming air dry. This illustrates a late spring condition, storage running low.

DEN 14.

August 8, 1919. Excellent summer growth all over range. This burrow in mixed growth, grasses and weeds.

Species stored. Grams.

Miscellaneous portions of _green plants_ of mixed species, _no seeds_ 5

Representing minimum for any one of the 22 burrows studied. Active storage does not begin until September.

DEN 16.

October 17, 1919. In good grass, but mound overrun by a large _Apodanthera_ vine.

Species stored. Per cent. Grams.

Aristida divaricata 90 to 95 } Chamaecrista leptadenia 10 to 5 } 58 Bouteloua rothrockii Tr. } Prosopis velutina 200 Apodanthera undulata 55 --- Total 313

Five species represented. Two species, _Apodanthera_, and _Chamaecrista leptadenia_, new to storage records. Several whole fruits of _Apodanthera_, about 2 inches in diameter, stored in addition to seeds alone; seeds of this form not previously noted in burrows, but very abundant in this one, indicating importance of the factor of accessibility in storage.

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).

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.)._