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

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.