MAMMALIA. 345 
their slipping on the ice, while their white fur renders them 
invisible at any great distance. 
Fic. 368.—Polar bear and walrus, showing how the bear walks with the heel 
flat on the ground, and the walrus also. 
Note.—Many animals that can not migrate and are deprived of 
food by cold weather are enabled to enter into a state of torpor called 
winter sleep or hibernation, and thus bridge over the foodless season. 
In the extreme south certain animals during the dry period enter into 
a summer sleep called estivation. In complete hibernation all the 
functions of life are almost at a stand-still; the respiration is reduced 
and irritability of muscular fibre increased. Hibernation is favored by 
cold but not produced directly by it, and the hibernator is not insen- 
sible to extremes. In entering the sleep the temperature of the body 
sinks to nearly that of the surrounding atmosphere. If, now, the cold 
is-intense, they are awakened and then are frozen. According to Sem- 
per, the zizel, or Spermophilus, attains the lowest temperature in this 
condition of any known animal, namely, 2° (centigrade), the exact tem- 
perature of the outside air in one experiment, so that the animal may be 
