The Dancing Mouse: A Study in Animal Behavior

by Robert Mearns Yerkes

Were not the dancing so extremely variable in rapidity and duration, it might be used as an index of the influence of auditory stimuli. Cyon's statements would indicate that sounds interfere with the dancing, but as I obtained no evidence of this, I worked instead with the following indirect method, which may be called the method of auditory choice.

The apparatus which was used is described in detail in Chapter VII. Figures 14 and 15 will greatly aid the reader in understanding its essential features. Two small wooden boxes, identical in form and as closely similar as possible in general appearance, were placed in a larger box in such positions that a mouse was forced to enter and pass through one of them in order to get to the nest-box. On the bottom of each of these small boxes was a series of wires through which an electric current could be made to pass at the will of the experimenter. The boxes could readily be interchanged in position. At one side of the large wooden box and beyond the range of vision of the mouse was an electric bell which could be caused to ring whenever the mouse approached the entrance to one of the small boxes. The point of the experiment was to determine whether the dancer could learn to avoid the box-which-rang when it was approached. The method of conducting the tests was as follows. Each day at a certain hour the mouse was placed in that part of the large box whence it could escape to the nest-box only by passing through one of the small boxes. If it approached the wrong box (whether it happened to be the one on the right or the one on the left depended upon the experimenter's decision), the bell began to ring as a warning against entering; if it approached the other box, all was silent. As motives for the choice of the box-which-did- not-ring both reward and punishment were employed. The reward consisted of freedom to return to the nest-box via the passage which led from the box-which-did-not-ring; the punishment, which consisted of a disagreeable electric shock, was given whenever the mouse entered the wrong box, that is, the one which had the sound as a warning. Entering the wrong box resulted in a disagreeable stimulus and in the necessity of returning to the large box, for the exit to the nest-box by way of the passage from this box was closed. My assumption, on the basis of extended study of the ability of the dancer to profit by experience, was that if it could hear the sound of the bell it would soon learn to avoid the box-that-rang and enter instead the one which had no sound associated with it.

Systematic tests were made with No. 4 from the 3d to the 12th of February, inclusive, 1906. Each day the mouse was permitted to find his way to the nest-box through one of the small boxes ten times in succession. Usually the experimenter rang the bell alternately for the box on the left and the box on the right. The time required for such a series of experiments varied, according to the rapidity with which the mouse made his choice, from ten to thirty minutes. If in these experiments the animal approached and entered the right, or soundless box, directly, the choice indicated nothing so far as ability to hear is concerned; if it entered the wrong, or sounding box, despite the ringing of the bell, it indicated either the lack of the influence of experience or inability to hear the sound; but if it regularly avoided the box-which-sounded it thus gave evidence of ability to hear the sound of the bell. The purpose of the test was to determine, not whether the mouse could learn, but whether it could hear.

For ten successive days this experiment was carried on with No. 4 without the least indication of increasing ability to avoid the wrong box by the association of the sound of the bell with the disagreeable electric shock and failure to escape to the nest-box. In fact, the experiment was discontinued because it became evident that an impossible task had been set for the mouse. Day by day as the tests were in progress I noticed that the animal became increasingly afraid of the entrances to the small boxes; it seemed absolutely helpless in the face of the situation. Partly because of the definiteness of the negative results obtained with No. 4 and partly because of the cruelty of subjecting an animal to disagreeable conditions which it is unable to avoid, the experiment was not repeated with other individuals. I have never conducted an experiment which gave me as much discomfort as this; it was like being set to whip a deaf child because it did not learn to respond to stimuli which it could not feel.

By a very similar method No. 18 was tested for his sensitiveness to the noise and jar from the induction apparatus which was used in connection with many of my experiments on vision and the modifiability of behavior. In this experiment the wrong box was indicated by the buzzing sound of the apparatus and the slight vibrations which resulted from it. Although No. 18 was tested, as was No. 4, for ten successive days, ten trials each day, it gave no evidence of ability to avoid the box-which-buzzed.

Since both direct and indirect methods of testing the hearing of the dancer have uniformly given negative results, in the case of mice more than five weeks old, I feel justified in concluding that they are totally deaf and not merely irresponsive to sounds.

Rawitz's statements, and the fact that what may have been auditory reactions were obtained with a few individuals of five weeks of age, suggest that the mice may be able to hear at certain periods of life. To discover whether this is true I have tested the young of twenty different litters from the first day to the twenty-eighth, either daily or at intervals of two or three days. In these tests König forks, steel bars, and a Galton whistle were used. The results obtained are curiously interesting.

During the first two weeks of life none of the mice which I tested gave any visible motor response to the various sounds used. During the third week certain of the individuals responded vigorously to sudden high tones and loud noises. After the third week I have seen only doubtful signs of hearing. I shall now describe in detail the method of experimentation, the condition of the animals, and the nature of the auditory reactions.

Between the twelfth and the eighteenth day the auditory canal becomes open to the exterior. The time is very variable in different litters, for their rate of growth depends upon the amount of nourishment which the mother is able to supply. Without exception, in my experience, the opening to the ear appears before the eyes are open. Consequently visual stimuli usually are not disturbing factors in the auditory tests with mice less than sixteen days old. There is also a sudden and marked change in the behavior of the mice during the third week. Whereas, for the first fourteen or eighteen days they are rather quiet and deliberate in their movements when removed from the nest, some time in the third week their behavior suddenly changes and they act as if frightened when taken up by the experimenter. They jump out of his hand, squeak, and sometimes show fight. This is so pronounced that it has attracted my attention many times and I have studied it carefully to determine, if possible, whether it is due to some profound change in the nervous system which thus suddenly increases the sensitiveness of the animal or to the development of the sexual organs. I am inclined to think that it is a nervous phenomenon which is intimately connected with the sexual condition. Within a day or two after it appears the mice usually begin to show auditory reactions and continue to do so for three to five days.

I shall now describe the results obtained with a few typical litters. A litter born of Nos. 151 and 152 gave uniformly negative results in all auditory tests up to the fourteenth day. On that day the ears were open, and the following observations were recorded. The five individuals of the litter, four females and one male, were taken from the nest one at a time at 7 A.M. and placed on a piece of paper in the bright sunlight. The warmth of the sun soon quieted them so that auditory tests could be made to advantage. As soon as an individual had become perfectly still, the Galton whistle was held at a distance of about four inches from its head in such a position that it could not be seen nor the currents of air caused by it felt, and suddenly blown. Each of the five mice responded to the first few repetitions of this stimulus by movements of the ears, twitchings of the body, and jerky movements of the legs. The most violent reactions resulted when the individual was lying on its back with its legs extended free in the air. Under such circumstances the four legs were often drawn together suddenly when the whistle was sounded. Similar responses were obtained with the lip sound already mentioned. Two other observers saw these experiments, and they agreed that there can be no doubt that the mice responded to the sound. The sounds which were effective lay between 5000 and 10,000 complete vibrations.