Communicative complexity in primates: sociality and multimodal communication in two species of lemurs

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Animals exhibit stunning variation not only in the type, but also in the number of signals they produce. Recently, variation in signal diversity has been conceptually and empirically attributed to social complexity, with species living in more complex societies exhibiting more complex signals. However, how to best quantify social and communicative complexity is a topic of ongoing debated. Moreover, as compelling as studies highlighting a link between social and communicative complexity are, in most studies only one communicative modality has been considered, and alternative hypotheses, such as the role of a species ecology, morphology or phylogenetic position in shaping communicative diversity, remain largely unexplored. In this study, we will compare the communicative systems of two closely related lemur species, Eulemur rufifrons and E. mongoz, that are similar in morphology and ecology, but which exhibit different social systems. Eulemur rufifrons lives in multi-male multi-female groups characterized by high levels of social tolerance, whereas E. mongoz is pair-living, with females being dominant over males. Both species are conspicuously vocal, produce a variety of olfactory signals, but also use visual signals for communication. Specifically, we aim at comparing the communicative systems, including acoustic, olfactory and visual as well as the multimodal usage of signals, between the two species by taking different measures of social as well as communicative complexity into account. We will operationalize social complexity by using various proxies to characterize variation in social structure between the two species, but also among different groups of both species, and relate it to variation in the communicative system. We propose to extend the social-complexity hypothesis for communicative complexity by also considering variation in social contexts, i.e. the audience, as a potential parameter of social complexity promoting flexibility in signal production and usage. Hence, our study may contribute to a new theoretical framework for testing the audience effect and to study the main drivers of communicative flexibility.