THE GROUND SQUIRRELS OP CALIFORNIA. 719 



Fig. 42. A hibernating squirrel in position in a hibernation nest {Vertical 

 Section). They curl very tightly in a vertical position. 



Having retired from active life, he sits down on his sacrum, curls 

 up vertically, so tightly that the air is quite driven out of his lungs 

 and in the course of a few hours has passed into a profound comatose 

 condition accompanied by a very low temperature, a very weak heart 



Fig. 43. An exit. After the long period of aestivation the squirrel digs from 

 his secluded den through to the surface, making a small nest hold which is not 

 easily detected by the enemy and through which little storm can pass. 



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