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 primitive 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 destructiveness 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 include the main distinctions by which each may be known from its relatives, the distribution of each of the species, the extent of the burrows, 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 efficient 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 strychnine 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
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