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Complexity of facial communication in crested macaques (Macaca nigra)

Peter Clark, PhD student, Department of Psychology, University of Portsmouth, UK

In order to investigate why humans communicate in such complex ways, it is informative to make comparisons with, and between, other primates. The social complexity hypothesis proposes that humans’ advanced cognitive abilities, including our unparalleled communicative ability, have arisen due to the challenges, and selective pressure, of living in large, complex social groups. This hypothesis predicts that in primates with more complex social groups, communication systems, such as the use of facial expressions, are also more complex. Measures of facial communicative complexity used to test this prediction have generally relied upon simply counting the number of distinct signals present in a species’ repertoire; this is arguably an over-simplification, since a single facial expression may differ in form and function between or within species, and this approach ignores blending and grading of signals. In applying the Facial Action Coding System to crested macaques, I will quantify objectively and at high resolution the form of facial expressions used in this species, and subsequently, through detailed observation of social behaviour, compare the expressions’ form in different behavioural contexts. I will also compare the use of facial expressions between crested and rhesus macaques: these two species have different social styles, with the social system of crested macaques being more complex. Therefore, comparing the communicative complexity of crested and rhesus macaques will serve as a direct test of the social complexity hypothesis.