period of dormancy extends from late summer well through midwinter, and thus "gestivation" may be said to go over directly into true hibernation. The old adults seem to be the only ones that "hole up," for the young adults somewhat less than a year old, that is, the young of the year, may be seen about the burrows during suitable weather throughout the winter.
In support of the above belief, that a period of torpidity overtakes the older individuals of the squirrel population regularly each year, the following evidence is submitted:
(1) Close watch, extending over a period of between four and five years, was kept on a female ground squirrel that lived in the dooryard at the home of Mrs. Elizabeth Grinnell in Pasadena. This particular squirrel did not æstivate until its second year. Then and during each succeeding year of its life it æstivated regularly, becoming very fat and retiring to its burrow during the last week in August. It emerged lean and hungry, with marked regularity, about the twenty-second of each following February. When removed from the burrow at intervals during this period, the squirrel was found to be in a torpid state, with respiration not perceptible.
(2) In a case in the junior author's personal experience, near Escondido, San Diego County, all the squirrels that were active in a certain field in the fall were poisoned or otherwise killed, and yet old breeding squirrels suddenly appeared in this same field the following February. This occurred when there was seemingly no possible chance for reinfestation from the surrounding fields, which had been cleaned up also. Similar testimony has reached us from a number of men identified with efforts to exterminate these rodents.
(3) It occurred to the present writers that it might be possible through the examination of specimens to learn the extent to which old adults are out in midwinter. The heads of 186 ground squirrels were, at our request, secured by the United States Public Health Service, shot and trapped near Martinez, Contra Costa County, during January, 1918, and sent to the Museum of Vertebrate Zoology, where the skulls were cleaned and carefully examined. Relatively advanced age was determined from the skulls upon the following criteria: general size, zygomatic breadth, breadth of jugals, stoutness of postorbital processes, degree of development of sagittal crest, degree of approach of parietal ridges, breadth and degree of concavity of frontal surface, advance in coalescence of the adjacent bones along certain sutures, and amount of wear on the crowns of the molariform teeth.
The results of our examination are given in Table II:
Table II. Proportions of adult to young ground squirrels abroad in midwinter.
| Date received (in 1918) | Total number of skulls | Number of old adults |
Number of young adults |
Ratio of old adults to total | ||
|
18 | 1 | 17 | 1 to 18 | ||
|
34 | 4 | 30 | 1 to 8.5 | ||
|
86 | 18 | 68 | 1 to 4.7 | ||
|
48 | 11 | 37 | 1 to 4.3 |
The foregoing data are not nearly as complete as could well be desired, but as far as they go they show that "old adult" squirrels are relatively
39
