The Dancing Mouse: A Study in Animal Behavior

by Robert Mearns Yerkes

My experiments with the dancer differ from those which have been made by most students of mammalian behavior in one important respect. I have used punishment instead of reward as the chief motive for the proper performance of the required act. Usually in experiments with mammals hunger has been the motive depended upon. The animals have been required to follow a certain devious path, to escape from a box by working a button, a bolt, a lever, or to gain entrance to a box by the use of teeth, claws, hands, or body weight and thus obtain food as a reward. There are two very serious objections to the use of the desire for food as a motive in animal behavior experiments—objections which in my opinion render it almost worthless in the case of many mammals. These are the discomfort of the animal and the impossibility of keeping the motive even fairly constant. However prevalent the experience of starvation may be in the life of an animal, it is not pleasant to think of subjecting it to extreme hunger in the laboratory for the sake of finding out what it can do to obtain food. Satisfactory results can be obtained in an experiment whose success depends chiefly upon hunger only when the animal is so hungry that it constantly does its best to obtain food, and when the desire for food is equally strong and equally effective as a spur to action in the repetitions of the experiment day after day. It is easy enough to get almost any mammal into a condition of utter hunger, but it is practically impossible to have the desire for food of the same strength day after day. In short, the desire for food is unsatisfactory as a motive in animal behavior work, first, because a condition of utter hunger, as has been demonstrated with certain mammals, is unfavorable for the performance of complex acts, second, because it is impossible to control the strength of the motive, and finally, because it is an inhumane method of experimentation.

In general, the method of punishment is more satisfactory than the method of reward, because it can be controlled to a greater extent. The experimenter cannot force his subject to desire food; he can, however, force it to discriminate between conditions to the best of its knowledge and ability by giving it a disagreeable stimulus every time it makes a mistake. In other words, the conditions upon which the avoidance of a disagreeable factor in the environment depends are far simpler and much more constant than those upon which the seeking of an agreeable factor depends. Situations which are potentially beneficial to the animal attract it in varying degrees according to its internal condition; situations which are potentially disagreeable or injurious repel it with a constancy which is remarkable. The favorable stimulus solicits a positive response; the unfavorable stimulus demands a negative response.

Finally, in connection with the discussion of motives, it is an important fact that forms of reward are far harder to find than forms of punishment. Many animals feed only at long intervals, are inactive, do not try to escape from confinement, cannot be induced to seek a particular spot, in a word, do not react positively to any of the situations or conditions which are employed usually in behavior experiments. It is, however, almost always possible to find some disagreeable stimulus which such an animal will attempt to avoid.

As it happens, the dancer is an animal which does not stand the lack of food well enough to make hunger a possible motive. I was driven to make use of the avoiding reaction, and it has proved so satisfactory that I am now using it widely in connection with experiments on other animals. The use of the induction shock, upon which I depended almost wholly in the discrimination experiments with the dancer, requires care; but I am confident that no reasonable objection to the conduct of the experiments could be made on the ground of cruelty, for the strength of the current was carefully regulated and the shocks were given only for an instant at intervals. The best proof of the humaneness of the method is the fact that the animals continued in perfect health during months of experimentation.

The brightness discrimination tests demanded, in addition to motives for choice, adequate precautions against discrimination by other than visual factors, and, for that matter, by other visual factors than that of brightness. The mouse might choose, for example, not the white or the black box, but the box which was to the right or to the left, in accordance with its experience in the previous test. This would be discrimination by position. As a matter of fact, the animals have a strong tendency at first to go uniformly either to the right or to the left entrance. This tendency will be exhibited in the results of the tests. Again, discrimination might depend upon the odors of the cardboards or upon slight differences in their shape, texture, or position. Before conclusive evidence of brightness discrimination could be obtained, all of these and other possibilities of discrimination had to be eliminated by check tests. I shall describe the various precautions taken in the experiments to guard against errors in interpretation, in order to show the lengths to which an experimenter may be driven in his search for safely interpretable results.

To exclude choice by position, the cardboards were moved from one electric-box to the other. When the change was made regularly, so that white was alternately on the right and the left, the mouse soon learned to go alternately to the right box and the left without stopping to notice the visual factor. This was prevented by changing the position of the cardboards irregularly.

Discrimination by the odor, texture, shape, and position of the cardboards was excluded by the use of different kinds of cardboards, by changing the form and position of them in check tests, and by coating them with shellac.

The brightness vision tests described in this chapter were made in a room which is lighted from the south only, with the experiment box directed away from the windows. The light from the windows shone upon the cardboards at the entrances to the electric-boxes, not into the eyes of the mouse as it approached them. Each mouse used in the experiments was given a series of ten tests in succession daily. The experiment was conducted as follows. A dancer was placed in A, where it usually ran about restlessly until it happened to find its way into B. Having discovered that the swing door at I could be pushed open, the animal seemed to take satisfaction in passing through into B as soon as it had been placed in or had returned to A. In B, choice of two entrances, one of which was brighter than the other, was forced by the animal's need of space for free movement. If the right box happened to be chosen, the mouse returned to A and was ready for another test; if it entered the wrong box, the electric shock was given, and it was compelled to retreat from the box and enter the other one instead. In the early tests with an individual, a series sometimes covered from twenty to thirty minutes; in later tests, provided the condition of discrimination was favorable, it did not occupy more than ten minutes.

To exhibit the methods of keeping the records of these experiments and certain features of the results, two sample record sheets are reproduced below. The first of these sheets, Table 6, represents the results given by No. 5, a female,[1] in her first series of white-black tests. Table 7 presents the results of the eleventh series of tests given to the same individual.

[Footnote 1: It is to be remembered that the even numbers always designate males; the odd numbers, females.]