AISTMAL BEHAVIOR — WALKER 279 



after trip over as wide a radius as they can safely traverse or until 

 they encounter the opposition of the occupant of another range. 

 If the seed crop is good they may thus accumulate a supply of seeds 

 sufficient to last them for several years. In this respect nature 

 has taught them well; for there are times when the seed crop is a 

 total failure during periods of a year or more, and, in a dry region 

 where the food supply is scant, the little creatures would starve to 

 death if they did not have adequate supplies laid up. 



Pocket mice are very frugal. Apparently they never overeat. 

 If they did eat excessively they would unduly deplete their food sup- 

 ply so that when a food shortage arrived they would have nothing 

 to tide them over it. Competition for food in the region inhabited 

 by them is so keen that the smallest kinds apparently have to start 

 out first in the evening to get their share. They generally lead soli- 

 tary lives. If two meet they almost mvariably fight, until one is 

 killed or driven off. Evidently they have found through the ages 

 that a given area will produce only enough seeds to support a 

 limited population. Therefore, in order that the population may be 

 evenly distributed, they live alone, each within his own territory 

 in which he must maintain himself. If they adopted a colonial 

 habit it would be necessary for the individuals to range much farther 

 from their homes to obtain food supplies, thus greatly increasing 

 the hazards from enemies. If the burrows of pocket mice are dis- 

 turbed, the animals feel that their stores are threatened. There- 

 fore, as soon as it is dark and the disturbance has subsided, they 

 begin industriously to move their seed supply to another place that 

 they consider safer. 



The kangaroo rats, larger cousins of the pocket mice, lead similar 

 lives but dig larger burrows. Some of them make large mounds of 

 earth and elaborate storerooms. They range somewhat more widely 

 than their smaller relatives. They likewise store up seeds that they 

 carry in their furlined cheek pouches to their treasure rooms. How- 

 ever, there appears to be one exception to this rule. One of the 

 smallest species of kangaroo rat {Dipodomys mernami) is frequently 

 found entering the burrows of the larger kinds and is not known 

 to store up food supplies of its own. It is suspected of having 

 adopted more or less parasitic habits and of living chiefly on the 

 stores hoarded by its larger relatives. The pets of this species that 

 I have had have shown no disposition to store food. 



The grasshopper mouse {Onycho7nys) of the southwestern United 

 States is largely a carnivorous rodent, for, in addition to seeds, fruit, 

 and plant tissues, it eats insects (particularly grasshoppers), crickets, 

 beetles, scorpions, lizards, and mice. Its technique in obtaining prey 

 is singularly effective and well adapted to its type of life. It follows 



