Unravelling communicative complexity: a multimodal comparative study of two lemur species with different social systems

The ‘social complexity hypothesis for communicative complexity’ (SCHCC) suggests that greater social complexity promotes greater communicative complexity. This is because there is increased uncertainty in larger social groups with differentiated social interactions, providing an advantage for more diverse and more flexible signals to transfer diverse messages and to manage the behaviour of others. In this study, we offer a comprehensive approach to contrast the multimodal communicative systems of two closely related true lemur species having similar morphology and ecology but different social systems. We collected behavioural and acoustic data on 13 wild adult red-fronted lemurs, Eulemur rufifrons, belonging to three different groups, and 10 wild adult mongoose lemurs, E. mongoz, belonging to four different groups, in the Kirindy and Ankatsabe forests, respectively. We describe a new analytical framework to assess the complexity of signalling systems across modalities. Applying a multimodal approach may help uncover the different selective pressures acting on the communicative system and better understand adaptive functions that might not be obvious from the isolated study of its components. The results support the prediction of a more complex communicative system in E. rufifrons, which has a more complex social system than E. mongoz. E. rufifrons exhibited larger signalling repertoires, greater signalling rates, a greater number of signal combinations and a significantly lower level of predictability in its signalling network compared to E. mongoz. We discuss the differences in the communicative systems of the two species and the potential functions associated with nonhomologous signals. We further explore potential evolutionary pathways at the communicative system level and discuss the proposed framework’s advantages and limitations at a cross-taxonomic scale.