718 



THE MONTHLY BULLETIN. 



food residue, passes into that very remarkable and profound deathlike 

 condition of aestivation and hibernation. 



The securing of data upon hibernation is one of the most arduous 

 tasks facing the investigator. Frequently the hibernating den 

 (Fig. 40) is remote from any surface indication of its presence, and 

 although it is in most cases associated with a summer den, is so 

 completely and thoroughly plugged off from it as to be very difficult to 

 find. It appears to be made in this way: A burrow is run out from 

 a summer den and a perfectly circular cell is excavated for the reception 

 of the hibernating nest. Often a shaft is started towards the surface 

 of the ground for exit in the spring. This shaft is not completed until 

 winter is past and the squirrel ready to come from hibernation 



Den (Z) 



March IZ,l9llt Pollman,Wask. 



(C. co/omhionus) 



Fig. 41. Sectional view of a hibernation den. When the den is opened for 

 spring the earth taken in making the exit shaft is spread along the bottom of 

 the shaft and dumped into the drain. 



(Fig. 41). Having the cell completed the nest is placed in position, 

 coarse grass on the outside and very finely shredded material in the 

 center, completely filling the cavity. As a last resort the squirrel 

 plugs himself in by digging a drain under the nest and using the dirt 

 taken from this drain to close the entrance to his hibernating den 

 (Fig. 40, D). Thus all connection with the exterior is closed by this 

 earth being firmly tamped into place by the nose of the animal. 



126 



