Abstract
The social complexity hypothesis for communicative complexity posits that animal societies with more complex social systems require more complex communication systems. Here, we test the social complexity hypothesis on three macaque species that vary in their degree of social tolerance and complexity. We coded facial behavior in >3000 social interactions across three social contexts (aggressive, submissive, affiliative) in 389 animals, using the Facial Action Coding System for macaques (MaqFACS). MaqFACS allowed us to define facial behavior in detail by the combination of muscle movements observed in the face. We quantified communicative complexity using three measures of uncertainty: entropy, specificity, and prediction error. We found that the relative entropy of facial behavior was higher for the more socially tolerant crested macaques as compared to the less tolerant Barbary and rhesus macaques across all social contexts. This result indicates that crested macaques more frequently use a higher diversity of facial behavior. The context specificity of facial behavior was highest in rhesus as compared to Barbary and crested macaques, demonstrating that Barbary and crested macaques used facial behavior more flexibly across different social contexts. Finally, a random forest classifier was able to predict social context from facial behavior better than chance in all three species, suggesting that there is predictive value to facial behavior. Prediction accuracy was highest (nearly perfect) for rhesus and lowest for crested macaques, indicating there is higher uncertainty and thus complexity in the communication system of crested macaques. Overall, our results support predictions of the social complexity hypothesis.
Significance Statement Primates have some of the most complex societies in the animal kingdom and exhibit an extensive repertoire of facial behavior that allows them to achieve a variety of social goals. However, the full complexity of primate facial behavior has yet to be properly quantified, especially in relation to social complexity. Here we compare three macaque species that have homologous facial musculature but vary in their social styles. We show that the more socially complex macaques exhibit more complex facial behavior than the less socially complex ones. Our results support the predicted link between social and communicative complexity and therefore help us better understand the evolution of communication.
Competing Interest Statement
The authors have declared no competing interest.
Footnotes
Competing Interest Statement: The authors declare no competing interest.
