Monkeying around: Non-human primate behavioural responses to humans reproducing their facial expressions

People are often observed mimicking animals’ facial expressions in an attempt to communicate with them. However, to date, there is limited understanding of how animals respond to humans reproducing their facial displays, or if this type of human behaviour presents a risk for either human safety or animal welfare. In the present study, we investigated how Barbary macaques (Macaca sylvanus) responded to pictures of humans and conspecifics displaying their facial expressions. Macaques viewed pictures of neutral, mildly threatening or highly threatening human or macaque faces. We recorded aggressive, submissive, and self-directed behaviours exhibited by individuals while in front of each stimulus. Macaques displayed more self-scratching toward human neutral face compared to the corresponding macaque face, and vice versa for the mild threat. They also exhibited more aggressive behaviours toward macaque neutral and mildly threatening stimuli compared to the human stimuli. However, macaques did not display any more submissive behaviour between human and macaque for any facial stimuli. There was also no significant difference in behavioural responses for highly threatening faces between species. These findings suggest that while the reproduced facial expression by humans might carry meaning for macaques, their responses vary between species. Therefore, these results highlight some potential issues for facial signalling (mis)communication between species, which has implications for animal welfare and human safety.