Chimpanzees detect strange body parts: an eye-tracking study

Abstract

The knowledge about how body parts are located and what they look like is important for species and individual discrimination in animals. In this study, we used eye-tracking to investigate whether chimpanzees have this body knowledge or not. We tested six chimpanzees and recorded their gaze behavior towards chimpanzee body stimuli. We focused on manipulations of arms or legs. For either arms or legs, there were four conditions. The first was normal bodies as a control. In the second condition, we misplaced one arm or a leg to a wrong position. In the third condition, we replaced an arm with a leg, or vice versa. In the fourth condition, we replaced an arm or a leg with a human arm or leg. The AOIs (areas of interest) were the strange body parts in the manipulated conditions or the corresponding ones in the control condition. We examined the number of trials in which chimpanzees had looked at the AOIs, the time to first fixation to AOIs, and the fixation duration of AOIs. Chimpanzees had more trials looking at the AOIs in the second and fourth conditions than control. Although there was no difference in the time to first fixation to AOIs across conditions, they had longer fixation durations in all manipulated conditions than control. The results showed that chimpanzees paid more attention to the strange body parts than control in general. This suggests that chimpanzees have the knowledge of the locations and appearances of body parts, as humans do, indicating that our common ancestor may have the knowledge about their body parts, too.