II HABITS AND NATURAL HISTORY OF THE FROG 31 



the left side, thus assisting the hind legs in the act of 

 rotation. 



The Voice. — The croaking of Rana pipiens may be rep- 

 resented, although rather inadequately, by the syllables " au- 

 au-au-au-auk." The voice of the male is louder and deeper 

 than that of the female and is more often heard. In large 

 frogs the notes are deeper than in small ones. The notes 

 of frogs are more often heard during the breeding season, 

 when they are supposed to serve the purpose of a sex call. 

 In the summer, however, it is not unusual to hear the 

 croaking of frogs, especially in the evening. A damp at- 

 mosphere is conducive to their song, and for this reason the 

 voices of these animals are often heard upon the approach of 

 a shower. The tree frogs seem to be especially sensitive 

 to atmospheric changes, and the popular reputation which 

 these creatures enjoy as prognosticators of the weather is 

 not entirely unmerited. 



The croaking of frogs is readily produced by rubbing the 

 back or side of the body. After each stroke the frog usu- 

 ally responds by a croak and then lapses into silence. 

 Croaking is often caused through accidental contact with 

 other individuals. Two frogs which were kept in a dish on 

 my table were in the habit of croaking at frequent intervals, 

 and I observed that each time the back or side of one frog 

 was touched by the other, the individual would respond by 

 a croak. If not disturbed, the frogs would remain silent 

 indefinitely. 



Frogs croak as well under water as on land. As the air 

 is forced out of the lungs, past the vocal cords, into the 

 mouth, the external nares are closed so as to prevent its 

 escape. Then the buccal cavity contracts, forcing the air 

 back into the lungs again ; and the same process is repeated. 

 If the head of the frog is held under water while the animal 



