Natural History of the Ground Squirrels of California/Introduction
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 so successful when applied to the Oregon Ground Squirrel, which animal apparently does not habitually gather and store seeds and grain to a large extent, but is active chiefly during the spring and early summer when green vegetation, upon which it depends for food, is to be had. Some sort of green baits might be expected to be more effective with this species.
It is very desirable at this time that we know more about the extent of æstivation and hibernation among our ground squirrels. We know little or nothing of the effect of gases upon squirrels in this condition; and it is obvious that such part of a squirrel population as is dormant at the time of a poison campaign will escape destruction and become a nucleus of reinfestation.
During the past spring and summer our work of excavating squirrel burrows after the occupants had been gassed, has shown that one frequent cause of failure in the ordinary waste-ball method of applying carbon bisulphid is due to the fact that sudden elevation in the course of the burrow, of as much as two feet in some cases, prevents the heavy gas from reaching the animal. This condition was found to occur much more frequently than is generally supposed.
The instances just cited all suggest that the present knowledge of our ground squirrels is far from complete, and they serve to emphasize the above contention that a more thorough knowledge of this subject is essential to securing the most intelligent and efficient methods of control of these our chief rodent pests. It is the purpose of the present paper to supply the information available from all sources in as much detail as it has proven feasible for the authors to secure it at this time. The facts and inferences are given just as they came, irrespective of whether or not they show obvious and immediate economic bearing. They are here available to everyone who is concerned with methods of control.