Coordination challenges shape signal complexity in chimpanzees ( <i>Pan troglodytes verus</i> ), but not in mangabeys ( <i>Cercocebus atys atys</i> )

Abstract Increased coordination challenges, such as in the negotiation of group movement and activity, has been suggested to promote communicative complexity. This hypothesis, however, has rarely been tested within and between species, specifically in relation to signal combinations. We compared two primates, chimpanzees and mangabeys, living in the same dense forest habitat, but with varying degrees of fission–fusion dynamics. In both species, we examined whether spatial coordination challenges across daily contexts and social relationship strength impacted signalling complexity, considering combined signals as being more complex than single signals. As high-level fission–fusion dynamics increase coordination challenges, we additionally evaluated whether combined signalling predicted increased coordination (association) duration in chimpanzees. We found that greater coordination demands, i.e. during travelling, compared with feeding and resting, increased combined signalling production in chimpanzees only. In chimpanzees, coordination challenges were more influential in predicting combined signalling than relationship strength, with the latter being influential only during resting in both species. Finally, combined and socio-positive signal production predicted chimpanzee coordination duration. Our findings offer support that coordination challenges drive signal combination production, adding strength to the broader hypothesis that social complexity is a principal driver of the evolution of complex signalling.