342 GENERAL ZOOLOGY 



While the frog depends very largely on the sense of sight 

 to warn it of approaching enemies or enable it to distinguish 

 food, the sense of hearing is also of considerable value. 

 Dr. R. M. Yerkes of Yale University, who has studied the 

 sense of hearing in frogs both in the laboratory and in the 

 field, says that he is convinced that sounds which are of 

 importance in the life of the animal, as the splash made by 

 a frog jumping into the water, are not only heard, but that 

 such sounds serve to put other frogs on their guard. The 

 croaking of male frogs in the spring is undoubtedly heard 

 by the female and serves to make mating more certain. 



The observer just cited does not give the green frog 

 credit for much intelligence, since his experiments seem to 

 show that nearly all the frog's actions are repeated with 

 machine-like accuracy, and new habits are learned very 

 slowly. He is inclined to think that even the perch learns 

 more rapidly than the frog. He also notes that the frog is 

 very timid, and that fright tends to lengthen the process 

 of learning. 



When suddenly touched, the frog may do one of several 

 things : it may jump, using the strong hind legs sometimes 

 with force enough to carry it several feet ; it may remain 

 perfectly quiet; or it may crouch with its head close to 

 the ground, at the same time puffing itself out. This last 

 action, Dr. Yerkes has noticed, more often takes place when 

 the animal is touched in front and is probably useful to 

 render seizure difficult, or to prevent it altogether. If the 

 frog leaps away, it is usually into the water with a loud 

 " plunk." A few swift strokes of the hind legs serve to carry 

 the animal to shelter beneath protecting debris in the water. 

 There it is able to remain for a considerable period without 

 the necessity of rising to the surface for oxygen, owing to 

 the low state of all its life processes. The frog has numer- 

 ous enemies, among which are owls, "hawks, and herons, 



