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 quant.i.ty 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 quant.i.ties of gra.s.s 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 princ.i.p.ally of various seeds and fruits, particularly the seeds of certain gra.s.ses. 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 a.n.a.lyzed 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 quant.i.ties 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 a.n.a.lyses of burrow contents are presented herewith in tabular form. The data for each den, or lot, shows in grams the quant.i.ty 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.--_a.n.a.lyses 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 gra.s.s-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 quant.i.ty 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 coa.r.s.e 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 gra.s.s.
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 p.u.b.erula Tr. } Ayenia microphylla 2 } Portulaca suffrutescens 1 } 400 Aplopappus gracilis Tr. } Alternanthera repens 1 } Tridens pulch.e.l.la 1 } Plantago ignota 33 } Panic.u.m hallii Tr. } Fleshy fungi (puffb.a.l.l.s) 2 --- Total 734
Fifteen species represented in addition to the fleshy fungi. No perceptible gra.s.s 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 gra.s.s 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 a.s.sumed (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 ma.s.s 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 quant.i.ty._ The fresh storage material was weighed after becoming air dry. This ill.u.s.trates a late spring condition, storage running low.
DEN 14.
August 8, 1919. Excellent summer growth all over range. This burrow in mixed growth, gra.s.ses 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.