Abstract
Facial expressions are a main communication channel used by many different species of primate. Despite this, we know relatively little about how primates discriminate between different facial expressions, and most of what we do know comes from a restricted number of well-studied species. In this study, three crested macaques (Macaca nigra) took part in matching-to-sample tasks where they had to discriminate different facial expressions. In a first experiment, the macaques had to match a photograph of a facial expression to another exemplar of the same expression produced by a different individual, against examples of one of three other types of expressions and neutral faces. In a second experiment, they had to match a dynamic video recording of a facial expression to a still photograph of another exemplar of the same facial expression produced by another individual, also against one of four other expressions. The macaques performed above chance in both tasks, identifying expressions as belonging to the same category regardless of individual identity. Using matrix correlations and multidimensional scaling, we analysed the pattern of errors to see whether overall similarity between facial expressions and/or specific morphological features caused the macaques to confuse facial expressions. Overall similarity, measured with the macaque facial action coding system(maqFACS), did not correlate with performances. Instead, functional similarities between facial expressions could be responsible for the observed pattern of error. These results expand previous findings to a novel primate species and highlight the potential of using video stimuli to investigate the perception and categorisation of visual signals in primates.
Highlights
Facial expression is one of the most common communicative systems used by primates
Overall, our findings demonstrate that crested macaques can discriminate between different facial expressions
Most of our subjects performed above chance after only a few sessions, generalising their understanding of the MTS task to a novel context
Summary
Facial expression is one of the most common communicative systems used by primates. Facial expressions can convey important social information about the producer’s internal state and possibly its forthcoming behaviour (Leopold and Rhodes 2010; Waller and Micheletta 2013). Macaques had difficulties when the pictures depicted similar facial expressions produced by different individuals and made more errors when facial expressions shared highly conspicuous features such as the mouth opening and teeth exposure (Parr and Heintz 2009). In both species, the presence of distinctive features could not account for the overall performances of the subjects, which suggest than chimpanzees and rhesus macaques, as humans, use both featural and configural information to discriminate between facial expressions (Leopold and Rhodes 2010; Parr 2011)
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