Gaze following in monkeys is modulated by observed facial expressions

Abstract

Gaze following and the ability to understand that another individual sees something different from oneself are widely considered important components of animal social cognition. Recent studies suggest that gaze following is taxonomically widespread, yet for many species there is no evidence that gaze following is employed in a flexible manner and is more than a simple so-called ‘orienting reflex’. Here, we measured the effect of social facial expressions, mimicking responses to social events, on gaze following in longtailed macaques, Macaca fascicularis, using a human demonstrator. Gaze-shifts accompanied by a socially meaningful facial expression (the Bare Teeth display) elicited stronger gaze-following responses than neutral gaze-shifts. Subjects also ‘check-looked’, that is, looked back and forth between the experimenter's face and their gaze direction, which has been proposed to indicate that a subject understands that another individual is looking at a specific stimulus. Monkeys' gaze following is thus modulated by the facial emotional expressions of the demonstrator, providing evidence that their gaze following is more flexible than was previously thought. This modulation may be due to a specific arousal- or attention-based mechanism or may be based on the subject understanding that the demonstrator is attending to something the subject cannot see.