CHARACTERISTICS OF MAMMALS 5 



In mammals that eat almost everything — plant food, small 

 mammals, insects, and eggs — the cheek-teeth also have rounded, 

 mound-like cusps. Such are present in the pigs, some rats, 

 bears, and most primates. The leaf monkeys have sharp-cusped 

 teeth which chop up leaves. In many rats the cusps form cross- 

 ridges or zig-zag patterns. 



Limbs. The limbs, next to the teeth, provide the best external 

 guides for recognizing and classifying mammals. The several 

 different kinds of limbs are all modifications of single basic pat- 

 terns of the fore and hind limbs. By reduction or enlargement 

 of various parts, involving changes in proportions, hands, flip- 

 pers, wings, paws, and hooves have evolved. Only whales and 

 sirenians, which have lost all but internal traces of the hind 

 pair, lack four limbs. The front limbs of these animals have 

 become steering paddles, while the tail has become flattened 

 and enlarged to provide propulsion. The seals and their allies 

 likewise have converted their front limbs into paddles. Their 

 broadened hind feet are used somewhat like the whale's tail, 

 moving up and down in the water. 



The hoofs of hoofed mammals, both those with odd and those 

 with even numbers of toes, are nothing but enlarged fingernails 

 and toenails. In most of them the central toe, or pair of toes, 

 is enlarged in order to bear most of the weight. Moreover, the 

 bones of the hand and foot are lengthened, a condition which 

 is an advantage in running. 



The slow-moving, massive elephant, which has hoof-like nails, 

 walks partly on the tips of the toes and partly on a pad of 

 fatty tissue. The foot-bones are short, the limb-bones long and 

 massive. 



Meat-eaters, rabbits, rodents, insect-eaters, flying lemurs, 

 scaly and spiny anteaters, and pouched mammals have claws. 

 Some meat-eaters, rabbits, and rodents walk on their toes ; in 

 these, pads are developed under the joint between the free finger 

 or toe and the hand or foot, and other pads are formed at the 

 tip of each toe. The other clawed mammals, including the bear 



