Class Mammalia 



569 



relatively small surface. The reptile must obtain most of its heat from 

 outside; the smaller animal has a relatively larger surface. If external 

 temperature is so low that the heat necessary for metabolism cannot 

 be obtained, the reptile must hibernate or die. Some mammals hiber- 

 nate, but it is perhaps not unfair to them to describe it as an easy and 

 lazy way of getting through the cold season. The more energetic mam- 

 mals keep their internal fires effectively burning and maintain full 

 activity through even an arctic winter. 



Mammals have become adapted to living at external pressures 

 ranging from that at sea-level to that at mountainous altitudes, and 

 some whales can withstand the enormous pressures met in diving to 

 submarine depths of perhaps a mile. 



In size (Fig. 440), mammals range from the mouse and the even 

 smaller insect-eating shrews, some only an inch long exclusive of tail, 

 to the rorqual, a baleen whale which "approaches a length of 100 feet" 

 (A. B. Howell: "Aquatic Mammals"). 



Their diets are most varied. Many are vegetarian, each addicted 

 to a particular kind or part of the vegetation — grasses, grains, herbs, 

 roots, bark, foliage, fruit. Some eat ants or other insects. Many eat 

 their fellow vertebrates — "fish, flesh, or fowl." In short (but in long 

 words), they are herbivorous, frugivorous (fruit-eating), insec- 

 tivorous, myrmecophagous (ant-eating), carnivorous, or omniv- 

 orous. The nature of the mammal's diet is accurately reflected in the 

 specialization of the teeth and the digestive organs. 



Mammals use their legs in most varied ways. They walk, run, hop, 

 leap, climb cliffs or trees, glide or parachute from tree to tree, swing 

 from branch to branch by the forelegs, go erect on the hindlegs, walk 

 flat-footed or on tiptoe. Some dig burrows or dig up roots with the 

 forelegs. A squirrel uses the forepaws to hold a nut against the teeth. 



Fig. 410. Relative size of mammals. Several mammals and, for comparison, 

 one of the largest reptiles are represented, all drawn to the same scale. A mouse on 

 this scale would be microscopic. (Courtesy, American Museum of Natural History, 

 New York.) 



