THE GROUND SQUIRRELS OP CALIFORNIA. 



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period of dormancy extends from late summer well through midwinter, 

 and thus "aestivation" may be said to go over directly into true hiber- 

 nation. 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 gestivate until its second year. Then and during each 

 succeeding year of its life it activated 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 Escon- 

 dido, 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 rein- 

 festation 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. 









Total 



Number 



Number 



Ratio of 





Date received (in 1918) 





number of 



of old 



of young 



old adults 









skulls 



adults 



adults 



to total 



January 8 







18 



1 



17 



1 to 18 



January 14 







34 



4 



30 



1 to 8.5 



January 22_ 







86 



18 



68 



1 to 4.7 



January 24 







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 



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