A HISTORY OF VERTEBRATES: BIRDS AND MAMMALS 



493 



little use upon the ground, but they, too, are effective grasping organs, 

 and are used for cUnging to a perch from wiiich the bat liangs upside 

 down when at rest. 



Most bats are crepuscular in habits, flying about at dusk in search 

 of insect prey. As Galambos and Griffin have shown, most of them rely 

 upon a biologic sonar system for finding their ^\ay and avoiding objects, 

 rather than upon their eyes, which are small and weak. As they fly, they 

 emit ultrasonic clicks that bounce off objects and are reflected back to 

 their sensitive ears. A bat which has been blinded can successfully navi- 

 gate in a room full of obstacles, but bumps into objects if its ears are 

 plugged or its mouth covered. 



Bats are the only mammals to have evolved true flight, but some 

 other mammals can stretch a loose skin fold between their front and 

 hind legs and glide from tree to tree. The flying squirrel (Fig. 24.20 A) 

 of the order Rodentia is one. Another is the "flying lemur" of the East 

 Indies. This animal is not a lemur, which is a primitive primate, but 

 belongs to an order of its own— the Dermoptera. 



Toothless Mammals. Since primitive mammals were insectivorous, 

 it is not surprising that certain ones became specialized to feed upon 

 ants and termites, which are very abundant in certain regions. The 



Figure 24.15. A, A mole in its burrow; B, a bat with its baby; C, the giant ant- 

 eater. (Courtesy of the American Museum of Natural History.) 



