Vocal intervention : group leaders terminate social conflicts via vocalisations
Vocal intervention is a triadic social interaction, where a third party vocally responds to a conflict of group members, which ends most conflicts. It is a suitable behavioural strategy to intervene by minimizing costs of aggression. Proboscis monkeys live in harem and bachelor groups. It was hypothesised that vocalisations of the harem male are uttered in response to vocalisation-associated agonistic conflicts between group members, calming down the conflicts. To test this hypothesis, we analysed (1) vocal responses of the harem holder to agonistic vocal exchanges within the harem group and (2) context and conflict partners related to these vocal events. In the first study, audio recordings of 17 free-ranging groups living in the Kinabatangan Wildlife Sanctuary, Borneo, were scanned for Bray vocalisations of the harem holder and for Shriek vocalisation associated with agonistic conflicts. We found that Bray vocalisations of the harem holder occurred significantly more often after Shriek vocalisations than expected by chance. Thereby, Bray vocalisations terminated vocalisation-associated agonistic conflicts in 54% of the cases. In the second study, we video- and audio-recorded 5 groups of habituated proboscis monkeys at feeding platforms in the Labuk Bay Rescue Center. Vocalisations of the harem male occurred more often after vocalisation-associated female-female conflicts than after immature-immature conflicts. Thus, our study provided first empirical evidence that vocalisations of the harem holder function as vocal intervention signal to calm down agonistic conflicts between adult females of his group. Our findings point to the previously neglected role of vocalizations for governing conflict resolution in complex societies and suggest that vocal intervention in human societies is rooted phylogenetically deep in primate ancestry.