Tolerant and intolerant macaques differ in the context specificity of their calls and how they ‘comment’ on the interactions of others

Abstract: The ‘social complexity hypothesis for communicative complexity’ posits that living in a complex social system requires complex communication skills. Since the complexity of a system can be measured by the amount of uncertainty it produces, we tested this hypothesis by studying species of macaque that differ in social tolerance and uncertainty of social interactions. We studied vocal communication in groups of macaques belonging to four species: Japanese and rhesus macaques, which are characterized by low levels of social tolerance and low uncertainty in the outcome of social interactions, and Tonkean and crested macaques, which display high levels of tolerance and uncertainty in interactions. We recorded the vocalizations emitted by adult females in agonistic, affiliative and neutral contexts. We measured call duration, entropy and time and frequency energy quantiles and processed these variables using cluster analyses and permutational multivariate analyses of variance. We found that tolerant macaques had a weaker relationship between the acoustic structure of calls and their context of emission compared to intolerant macaques. The study of ‘commenting calls’, i.e. calls made by individuals attending interactions between groupmates, also showed that their acoustic structure was more differentiated from other calls in tolerant Tonkean and crested macaques than in intolerant rhesus macaques. The flexibility of vocal production therefore appears to be correlated with the level of uncertainty of social interactions. Species with more complex social interactions were also those with higher degree of freedom in the association between acoustic structure and social context, which supports the social complexity hypothesis. Significance statement: Is there a relationship between the complexity of social systems and the complexity of communication skills? Animals living in complex social environments are expected to use a wide variety of messages related to different goals and contexts. The complexity of a system can be assessed by the amount of uncertainty it can produce. We investigated the complexity of vocal communication by comparing two species of macaque displaying low uncertainty in their social interactions, with two other macaque species displaying high levels of uncertainty in their interactions. The comparison showed that call flexibility was related to uncertainty levels. Species with higher levels of uncertainty had weaker associations between acoustic structure and social context. These results support the hypothesis of a link between social system complexity and communication complexity, which has important implications for our understanding of the evolution of social and communication systems.