In a variety of experimental paradigms, chimpanzees have shown remarkable adaptive and flexible responses to others’ direct line of gaze (i.e. We discuss our results in relation to the theory of mind hypothesis that chimpanzees understand seeing in others, and we evaluate possible alternative low-level explanations. Results suggest that chimpanzees, through a possible process of experience projection based on their own prior experience with mirrors, infer that an experimenter looking at the mirror can see them. Chimpanzees produced significantly more visual gestures when the experimenter could see them than when he/she could not, even when the experimenter did not have a direct line of gaze to them. We investigated the frequency of chimpanzees’ visual gestures towards a human experimenter who could see them (as a result of looking into a mirror) but who lacked a direct line of gaze to them (as a result of having his/her head turned away). Here, we sought to resolve this ambiguity by dissociating seeing from direct line of gaze using a mirror. However, previous research has never experimentally ruled out the alternative explanation that chimpanzees are just responding to the geometric cue of ‘direct line of gaze’, the observable correlate of seeing in others. There is much experimental evidence suggesting that chimpanzees understand that others see.

Chimpanzee face side view