500 VERTEBRATE LIFE AND ORGANIZATION 



Figure 24.20. Rodcnts and lagomorphs. A, A flying squirrel; B, the pika; C, a chip- 

 munk shelling a nut; D, a group of beavers (Courtesy of American Museum of Natural 

 History.) 



population. During the Pleistocene, or Ice Age, mastodons, mammoths 

 and other proboscideans were abundant in North America. 



The conies of the Middle East (order Hyracoidea), though super- 

 ficially rabbit-like animals, show an affinity to the elephants in their foot 

 structure, and in certain features of their dentition. 



A final group of contemporary subungulates are the sea cows or 

 manatees (order Sirenia). These animals live in warm coastal waters and 

 feed upon seaweed, grinding it up with molars that are replaced from 

 behind in elephant-like fashion. Sea cows have a powerful, horizontally 

 flattened tail, and well developed pectoral flippers. These features, to- 

 gether with a very mobile and expressive snout and a single pair of 

 pectoral mammary glands, led mariners of long ago to regard them as 

 mermaids. 



Rodents and Lagomorphs. Other herbivorous mammals gnaw, and, 

 in addition to high-crowned, grinding molars, have an upper and lower 

 pair of enlarged, chisel-like incisor teeth that grow out from the base 

 as fast as they wear away at the tip. This has been a very successful 

 mode of life; in fact, there are more species, and possibly more individ- 

 uals, of gnawing mammals, or rodents (order Rodentia), than of all other 

 mammals combined. Rodents have undergone their own adaptive radia- 

 tion and have evolved specializations for a variety of ecologic niches. 

 Rats, mice and chipmunks live on the ground, gophers and woodchucks 

 burrow, squirrels and porcupines are adept at climbing trees, and musk- 

 rats and beavers are semiaquatic (Fig. 24.20). 



Rabbits and the related pika of our Western mountains are super- 

 ficially similar to rodents, and were at one time placed in this order. 

 True rodents, however, have only one pair of incisors in each jaw, 



