Socially mediated overlap in vocal interactions between free-ranging black howler monkeys

"Conversation rules" such as overlap avoidance and coordinated overlap have been reported in nonhuman animals, and seem to be adaptive responses to the requirements of social life. Some species display both patterns in an apparently flexible way, but the social factors mediating their respective usage remain poorly documented. We investigated the potential social factors guiding the usage of these temporal rules during collective howling in six free-ranging groups of black howler monkeys (Alouatta pigra). First, we found that adult males' vocalizations rarely overlap those of other callers while most adult females' calls overlap other members' calls, especially that of other females. Second, whereas some call types (notably affiliative calls) are typically emitted without overlap, roars (agonistic loud calls) overlap more frequently. Third, coordinated overlap is more frequent during intergroup competition. Our findings support the hypothesis that overlap avoidance and coordinated overlap are two different (here sex-related) vocal alliance social behaviors, at least for some nonhuman primates. More comparative investigations are now needed to explore further their evolutionary trajectories in this lineage.