THE STRUCTURE AND ACTIVITIES OF MAMMALS 413 



paws. Then it bounded across the creek, covering about five 

 feet at each leap. Again it landed facing the track it had made. 

 Being satisfied that it had escaped observation, it entered the 

 swamp at a leisurely pace. 



Fig. 



- Tracks in the snow showing where a musk rat has come from and 

 returned to the water. (From Dugmore.) 



Hibernation. — The problem of maintaining life during the 

 winter is solved by most birds by migrating. Mammals, on the 

 other hand, usually remain active, like the rabbit, or hibernate. 

 During hibernation the temperature of the body decreases and 

 the animal falls into a profound torpor. A cold-blooded animal, 

 like the frog, can be almost entirely frozen without being injured, 

 but warm-blooded animals must protect themselves from the 



