Yerkes, The Sense of Heaj'ing in Frogs. 287 



of being noticed. Repeatedly I have noted that it is never possible to get near 

 to any frogs in the same region after one has jumped in. In this we have addi- 

 tional proof that they hear the splash-sound. To make sure that sight was not 

 responsible for this on-guard condition in which one finds the frogs after one of 

 their number has jumped into the water, I made observations on animals that 

 were hidden from one another. The results were the same. I therefore con- 

 clude that the splash of a frog jumping into the water is not only perceived by 

 other frogs in the vicinity, but that it is a peculiarly significant sound for them, 

 since it is indicative of danger, and serves to put them ' on watch.' 



A great variety of sounds, ranging in pitch from a low tone in imitation of 

 the bull frog's croak to a shrill whistle, and in loudness from the fall of a pebble 

 to the report of a pistol, were tried for the purpose of testing their eflfects upon 

 the animals in their natural environment. To no sound have I ever seen a motor 

 response given. One can approach to within a few feet of a green frog or bull 

 frog and make all sorts of noises without causing it to give any signs of uneasi' 

 ness. Just as soon, however, as a quick movement is made by the observer the 

 animal jumps. I have repeatedly crept up very close to frogs, keeping myself 

 screened from them by bushes or trees, and made various sounds, but have never 

 succeeded in scaring an animal into a motor response so long as I was invisible. 

 Apparently they depend almost entirely upon vision for the avoidance of dangers. 

 Sounds like the splash of a plunging frog or the croak or pain-scream of another 

 member of the species serve as warnings, but the animals do not jump into the 

 water until they see some sign of an unusual or dangerous object. On one 

 occasion I was able to walk to a spot where a large bull frog was sitting by the 

 edge of the water, after the frogs about it had plunged in. This individual, 

 although it seemed on the alert, let me approach close to it. I then saw that the 

 eye turned toward me was injured. The animal sat still, despite the noise I 

 made, simply because it was unable to see me ; as soon as I brought myself 

 within the field of vision of the functional eye the frog was off like a flash. 



Many observers have told me that frogs could hear the human voice and 

 that slight sounds made by a passer-by would cause them to stop croaking. In 

 no case, however, have such observers been able to assert that the animals were 

 unaffected by visual stimuli at the same time. I have myself many times noticed 

 the croaking stop as I approached a pond, but could never be certain that none 

 of the frogs had seen me. It is a noteworthy fact that when one frog in a pond 

 begins to croak the others soon join it. Likewise, when one member of such a 

 chorus is frightened and stops the others become silent. This indicates that the 

 cessation of croaking is a sign of danger and is imitated just as is the croaking. 

 There is in this fact conclusive evidence that the animals hear one another, and 

 the probability is very great that they hear a wide range of sounds to which they 

 give no motor reactions, since they do not depend upon sound for escaping their 

 enemies. 



The phenomenon of inhibition of movement in response to sounds which we 

 have good reason to think the frogs hear, and to which such an animal as a 

 turtle or bird would react by trying to escape, is thus shown to be common for 

 frogs in nature as well as in the laboratory. This inhibition is in itself not sur- 

 prising, since many animals habitually escape certain of their enemies by remain- 

 ing motionless, but it is an interesting phenomenon for the physiologist. We 



