The Mental Life of Monkeys and Apes: A Study of Ideational Behavior

by Robert Mearns Yerkes

IV

RESULTS OF SUPPLEMENTARY TESTS OF IDEATIONAL BEHAVIOR

1. Julius, Pongo pygmaeus

Box Stacking Experiment

In addition to the multiple-choice experiments which have been described in detail in the previous section, it was possible to conduct certain less systematic tests of ideational behavior in the monkeys and the orang utan. From the technical standpoint these tests were relatively unsatisfactory because only inexactly describable. But their results are in many respects more interesting, if not also more important, in the light which they throw on ideation than are those previously presented. First, in order of time, comes a test which may be designated as the box stacking experiment. The method will now be described in connection with an account of the behavior of Julius as contrasted with that of a child of three years and four months of age.

In the large central cage labelled Z, figure 12, which was twenty-four feet long, ten feet wide, and ten to twelve feet deep, the following situation was arranged. From the center of the wire covering of the cage, a banana was suspended on a string so that it was approximately six feet from the floor, five feet from either side of the cage, and twelve feet from either end. From all approaches it was far beyond the reach of Julius, since it was impossible for him to climb along the wire roof and thus reach the string. Two boxes were placed on the floor of the cage several feet from the point directly under the banana. The one of these boxes was heavy and irregular in shape, as is shown in figures 21, 23 and 24 of plate V. Its greatest height was twenty-one inches; its least height, eighteen inches; its other dimensions, twelve and sixteen inches respectively. The smaller and lighter box measured twenty-two by twelve by ten inches. According to the experimenter's calculations, the only way in which Julius could obtain the banana was by placing the smaller box upon the larger and then climbing upon them.

At 10 a.m. on March 5, Julius was admitted to the large cage, and the banana was pointed out to him by the experimenter. He immediately set about trying to get it, and worked diligently during the whole of the period of observation, which, because of the unfinished condition of some of the cages, was limited to slightly over ten minutes. Within this period he made upward of a dozen fairly well directed attempts to obtain the food. Chief among them were three attempts to reach the banana from different positions on the left wall of the cage (as the experimenter faced the laboratory); two attempts to reach it from different positions on the right wall; two from the large box in positions nearly under the banana; two from the large box with the aid of the experimenter's hand; and one from the distant end of the cage(?). There occurred, also, less definite and easily describable efforts to get at the reward.

On account of the unfinished condition of the cages, the experimenter had to remain in the large cage with Julius during the test. This interfered with the experiment because the animal tended both to try to escape and to get the experimenter to help him with his task. Particularly interesting is the latter sort of behavior. After the orang utan had made two or three futile attempts to obtain the food he came to the experimenter, who was standing in one corner of the cage, took him by the hand, and led him to a point directly under the banana. He then looked up toward the banana, grasped the experimenter's arm, raised it, and then tried to pull himself up. He was not allowed to get the food by climbing up on the experimenter. A few minutes later, he again led the experimenter toward the banana, but receiving discouragement in this activity, he proceeded to devote himself to other methods.

Apart from the distractions which have been mentioned above, Julius's attention to the food was surprisingly constant. Whatever his position with respect to it, he seemed not for an instant to lose his motive, and to whatever part of the cage he went and whatever he did during the interval of observation was evidently guided by the strong desire to obtain the banana. Frequently he would look directly at it for a few seconds and then try some new method of reaching it. His gaze was deliberate and in the handling of the boxes he accurately gauged distances. Several times he succeeded in placing the larger box almost directly under the banana, and repeatedly he located that portion of the side wall from which he could most nearly reach the coveted prize.