Although there does not seem to be a structural basis for a delicate sense of hearing, one must examine the physiological facts at hand before concluding that frogs do not possess a sense of hearing similar to our own.  First, the fact that frogs make vocal sounds is evidence in favor of the hearing of such sounds at least, since it is difficult to explain the origin of the ability to make a sound except through its utility to the species.  Granting, however, that a frog is able to hear the croaks or pain-screams of its own species, the range of the sense still remains very small, for although the race of frogs makes a great variety of sounds, any one species croaks within a narrow range.

Having satisfied myself that motor reactions for reaction-time measurements could not be gotten to any ordinary sounds in the laboratory, I tried the effect of the reflex croaking of another frog of the same species.  In attempting to get frogs to croak regularly, I tested the effect of removing the brain.  The animals are said to croak reflexly after this operation whenever the back is stroked; but for some reason I have never been successful in getting the reaction uniformly.  In many cases I was able to make normal animals croak by rubbing the back or flanks, and to this sound the animals under observation occasionally responded by taking what looked like an attitude of attention.  They straightened up and raised the head as if listening.  In no case have other motor responses been noticed; and the above response was so rare that no reaction-time measurements could be made.

Again, while working with the green frog on habit formation, I one day placed two animals in a labyrinth from which they could escape by jumping into a tank of water.  Several times when one frog jumped into the water I noticed the other one straighten up and hold the ‘listening’ or ‘attentive’ attitude for some seconds.  As the animals could not see one another this is good evidence of their ability to hear the splash made by a frog when it strikes the water.

B. Influence of Sounds in Nature.

In order to learn how far fear and artificial conditions were causes of the inhibition of response to sounds in the laboratory, and how far the phenomenon was indicative of the animal’s inability to perceive sounds, I observed frogs in their native haunts.

By approaching a pond quietly, it is easy to get within a few yards of frogs sitting on the banks.  In most cases they will not jump until they have evidence 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 additional 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 conclude 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.’