338 THE BIOLOGY OF THE FROG chap. 



remain in this attitude, with its muscles in a state of tonic 

 contraction often for half an hour. A normal frog of the 

 species experimented with {R. esculenta) does not show 

 this response except in a slight degree, and then only after 

 it has become fatigued. Frogs devoid of otoliths were 

 found to utter the so-called " pain cry " upon slight provo- 

 cation, whereas this response is rarely elicited from normal 

 specimens. Locomotion may take place in a normal man- 

 ner, but the muscular tonus seems to be low, and the animal 

 quickly becomes fatigued. The compensatory motions are 

 unaffected. 



Frogs have a well-developed sense of hearing, although 

 they do not usually manifest it by any clearly evident sign. 

 In croaking, one frog frequently responds to the croak of 

 another individual, so that one often hears a pair answering 

 each other by regularly alternating notes. If, after a concert 

 of frogs has been silenced by some one who intrudes too 

 near their haunts, one individual ventures to resume its 

 croaking, it is speedily followed by one after another of its 

 comrades. Landois tells of a tree frog which he kept in 

 captivity which would give an answering cry every time that 

 he would imitate its note. Yerkes observed that when he 

 caused a frog to croak by rubbing its sides, the other frogs 

 under observation occasionally gave signs of attention by 

 straightening up and raising the head as if listening. The 

 same observer noticed, when carrying on experiments with 

 frogs in a labyrinth, that the animals often gave signs of 

 hearing the sound made when other individuals jumped into 

 the water. They would " straighten up and hold the listen- 

 ing or attentive attitude for some seconds. As the animals 

 could not see one another, there is good evidence of their 

 ability to hear the splash made by a frog when it strikes the 

 water." This, according to Yerkes, explains the fact that 



