^nzont'} LIFE HISTOEY OF THE KANGAROO EAT. 19 



on stored materials. Xot infrequently a season of severe drought pre- 

 cludes the possibility of any storage. The summer and fall of 1918 

 was such a season on the Eange Reserve (PI. 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, X. Mex., deserted by spectahilis. result- 

 ing probably from overgrazing by sheep and goats during a succes- 

 sion of dry years. In the arid Southwest natural selection probably 

 favors the animals with the largest food stores, and it is not surpris- 

 ing 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 varj' from a few ounces to 

 several quarts or pecks, and stored materials taken from 22 mounds 

 on the Eange Reserve vary in weight from 5 to 4.127 grams 

 (more than 9 pounds). This is exceeded by one lot from Xew 

 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 ma- 

 terials by numerous kangaroo rats may apj)reciabh^ reduce the carry- 

 ing capacit}' of the range. Studies of cheek-pouch contents and food 

 stores taken from dens show that the natural food of S'pectahil/'i 

 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, X. Mex., has been saved and an- 

 alyzed 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. 



