NATURAL HISTORY OF THE GROUND 

 SQUIRRELS OF CALIFORNIA. 



By JOSEPH GRINNELL and JOSEPH DIXON. 



Contribution from the Museum of Vertebrate Zoology, of the University of 



California. 



INTRODUCTION. 



Human occupancy of a new country always tends to upset the primi- 

 tive balance of things. Man either purposefully or incidentally begins 

 at once to modify the original complement of animal and plant life both 

 through destruction of native species and by bringing in with him alien 

 kinds. Some native species become more and more restricted in range, 

 even disappearing altogether; others tend to increase and spread, 

 finding conditions for their existence to be improved through man's 

 activities. 



In the case of the ground squirrels of California, we have a group of 

 mammals which seems to have in many places benefited by human 

 invasion. This is probably due to the destruction by man of the many 

 predatory animals, such as hawks, eagles, coyotes and badgers, which 

 under original conditions kept the small herbivorous mammals in check, 

 and in part to the improved food supply made available to the ground 

 squirrels through his cultivation of crops. Because of the destructive- 

 ness of these rodents to the planted crops and native forage upon which 

 man is dependent to a large extent directly or indirectly for his own 

 food supply, the problem of ground squirrel control has become one 

 of very immediate agricultural and pastoral importance. 



It would seem that knowledge, as full as possible, of the ground 

 squirrels of California is necessary to determining the most successful 

 means of controlling them and to applying these means properly to the 

 varying conditions throughout our state. This knowledge should in- 

 clude the main distinctions by which each may be known from its 

 relatives, the distribution of each of the species, the extent of the bur 

 rows, the breeding rate, the food habits, and, indeed, every other class 

 of facts obtainable relative to their natural history. It is not often 

 apparent, in advance, which facts will and which will not prove of 

 critical importance in economic work. 



To illustrate the value of a thorough knowledge of the food habits 

 of the animal in question, when the most efiicient method of controlling 

 destructive rodents is sought, we need only to point to the present 

 method used in poisoning the California Ground Squirrel by the use 

 of barley coated with strychnine, rather than barley soaked in a strych- 

 nine solution. By applying a knowledge of the food habits of this 

 animal it was possible greatly to increase the effectiveness of poisoned 

 grain because of the discovery by Stanley E. Piper, of the United States 

 Biological Survey, that this squirrel is more readily poisoned through 

 the membranous walls of its cheek-pouches when merely carrying the 

 poisoned grain than through the stomach after the poisoned grain 

 has been eaten. Strychnine-coated barley has not, however, been found 



