Body Perception in Chimpanzees (Pan troglodytes): The Effect of Body Structure Changes

Chimpanzees have been found to show the inversion effect to visual stimuli of intact chimpanzee bodies, suggesting that they have a specific way of body processing. In this study, we examined how changes of body structures affect the inversion effect to reveal the properties of their body processing. We focused on two aspects of body structures: the first-order relations (i.e., body part arrangements) and body proportions. The experimental conditions had stimuli of chimpanzee bodies with scrambled first-order relations in Experiment I and chimpanzee bodies with distorted body proportions in Experiment 2. Both experiments had a control condition consisting of stimuli of intact chimpanzee bodies. A total of 7 chimpanzees participated in matching-to-sample tasks on touch screens. In each trial, the stimuli were chimpanzee bodies with the same kind of manipulation of body structures and were either all upright or all inverted. We compared their performances in the upright and inverted trials to examine the inversion effect. The chimpanzees did not show any inversion effect to scrambled bodies but showed the inversion effect to distorted bodies and to intact bodies. The results suggest that chimpanzees' configural body processing relies on correct first-order relations, and distorted body proportions do not interfere with configural processing. It also implies that chimpanzees may have knowledge of the arrangement of body parts, but they may be less sensitive to body proportions. This study could facilitate the understanding of the evolution of visual processing of bodies. faces, and other objects.