Living together in a group involves constant interactions between individuals. Individuals need to constantly assess the behavior of others and react flexibly to it. Primates and other animals regulate and coordinate their interactions primarily through vocal, visual, tactile, and olfactory cues. However, it is not known what social or ecological factors influence the number of signals and the evolution of different signaling modalities.
One hypothesis holds that more complex signals have evolved in species living in pairs or groups to regulate their more diverse social interactions. To investigate these relationships, Claudia Fichtel and Peter Kentendre, researchers from the Behavioral Ecology and Sociobiology Unit of the German Primate Center – Leibniz Institute for Primate Research, investigated factors that may explain the diversity of signaling repertoires vocal, visual and olfactory in different species of lemurs. They were able to show that lemurs living in larger groups with more complex social systems also have more complex systems of interaction in all three modalities. The size of the signaling repertoire could not be attributed to specific environmental factors, nor associated with body size or brain size (Philosophical Transactions B).
For regulate and coordinate interactions, animals must communicate. The lemurs of Madagascar communicate according to different methods and present the main forms of social organization: they live either alone, or in pairs, or in groups. Additionally, the activity patterns of the more than 120 known species vary. There are diurnal and nocturnal species, as well as day and night active species. “Because lemurs have evolved in isolation from other primates for more than 50 millions of years, they provide an excellent opportunity to identify the fundamentals of coevolution of traits. social and communicative”, explains Peter Kentendre.
The vocal repertoire of lemurs is about as extensive as that of other species of monkeys. Lemurs also use vocalizations to signal their dominance status, resolve conflicts, signal their emotional state to others, maintain group cohesion, coordinate group movements, or defend territories. In addition, olfactory conversation is important in lemurs. They have specialized glands on their genitals, chest, hands, or head, and their secretions are applied to trees, but also to conspecifics. Lemurs also use gestures or facial expressions to regulate social relationships. As, for example, submission in rank conflict is indicated by visual cues in some species but vocal cues in others, it is crucial to investigate the extent of the signaling repertoire across all modalities to understand whether increasing social complexity facilitated the evolution of communicative complexity..
The study showed that lemurs living in larger groups also developed more vocal, visual and olfactory. From this, the researchers concluded that communicative abilities have diversified alongside increasing social complexity. Additionally, Fichtel and Kentendre were able to show that social complexity likely evolved first in evolution, followed by communicative complexity. Variation in other factors, such as habitat characteristics, activity patterns, or the number of lemur species existing in the same habitat, did not explain the evolution of more extensive communicative repertoires. Similarly, there was neither a relationship between the size of the vocal and visual repertoire and brain size, nor between the number of olfactory glands or olfactory signals and the body size of lemur species.
“Our study shows that the complexity of vocal, olfactory and visual interaction in lemurs co-evolved with the complexity of the social system, but not with socio-ecological factors, such as the variety of habitat or number of other species in the same environment. range”, concludes Claudia Fichtel.