The Notions of Size and Shape in Old World Monkeys (Macaca mullata): A Comparative Analysis of the Formation Process

Abstract

In everyday life, humans and animals occupying different rungs of the “evolutionary ladder” often have to evaluate the notions of larger/smaller size (predator/prey, rival/ally, etc.) and flat/volumetric shape (fruit/leaf, 2D/3D images, etc.). The aim of this study was to find out experimentally which of the following two tasks related to the formation of the preverbal notions of larger/smaller size or flat/volumetric shape is easier to train rhesus monkeys. The first task was to form or actualize the notion of larger or smaller size upon co-presentation of 4 flat or volumetric figures. The second task was to form or actualize the notion of flat or volumetric object among 4 figures of the same size (small, medium or large) presented simultaneously. To be rewarded during the formation of both notions, the animal was supposed to choose a figure which was different from the other three in the trial. In both tasks, the number of trials required to reach or exceed the 70% level of correct task implementation per each type of training was counted. This allowed optimization of the notion formation algorithm to rule out strong skill consolidation (overtraining) that might inhibit the formation of a new skill in further training. When the number of trials was averaged over all types of training, the notion of shape formed faster than the notion of size. This discrepancy was only observed under the difficulty of solving one of the task types–discriminating between objects poorly distinguishable (1.5 times) by their size. However, in the situation of no difficulty in discriminating between figures during their perception (more than a 2-fold difference in size), the number of trials in forming the notions of size and shape was statistically indistinguishable. Since training rates in forming the notions of larger/smaller size or flat/volumetric shape were indistinguishable in simple choice situations, we suggest that these notions are equally important for rhesus monkeys.