32 THE BIOLOGY OF THE FROG chap. 



is croaking, it may be seen that the air is forced back and 

 forth between the mouth and the lungs, while only a little, if 

 any, is allowed to escape through the nares. 



Under conditions which are particularly agreeable, frogs 

 often give out a low grunting sound as if of contentment. 

 On the other hand, when frogs are severely injured, they 

 sometimes utter a sort of cry which is called the pain 

 scream. When seized by a snake or other enemy, many 

 species of frogs may respond by making this piteous cry. 



Instincts for Protection. — When a frog is seized in the 

 hands, it usually makes violent efforts to escape. If it is 

 held by the anterior part of the body, the hind legs are used 

 to push against one's hand with considerable force. At the 

 same time the body is generally inflated with air, which 

 enables it to slip away more readily from one's grasp. The 

 sudden ejection of fluid from the bladder, which takes place 

 when the frog is caught, may also be of occasional service in 

 its attempts to get free. 



Frogs sometimes swell the body before being seized as if 

 in anticipation of their capture, and they are especially apt 

 to do this after being lightly touched. Touch a frog that is 

 resting quietly, and if the creature does not hop away, one 

 may see the body puff up ; and if the body is touched two 

 or three times, the swelling will continue until the lungs con- 

 tain their maximum amount of air. An animal such as a snake 

 which was attempting to swallow a frog would find the 

 operation somewhat more difficult if the body of its victim 

 were strongly inflated. Frogs often avoid capture better by 

 remaining perfectly quiet than by attemi)ting to get away 

 by jumping. Fear prompts the creatures now to the one 

 and now to the other method of escape. Safety is also 

 sought occasionally by crouching close to the ground, and 

 more often by crawling under some object that promises to 

 afford shelter. 



