Intragroup Vocalizations in Captive Northern White-cheeked Gibbons (Nomascus leucogenys) and Southern Yellow-cheeked Gibbons (Nomascus gabriellae)

Abstract The vocal repertoire of nonhuman primates contains intergroup vocalizations, which are often transmitted over long distances, and intragroup vocalizations, which are usually transmitted over short distances. Unlike the well-documented intergroup vocalizations of gibbons, which are emitted as loud calls and are specific to species and sex, intragroup vocalizations have received little attention. In this study, we describe the structure of three types of intragroup vocalizations (grunt call, hoo call, and long hoo call) uttered in five contexts (grunt calls during duet, feeding, stress, animals, and attention to humans) in nine northern white-cheeked gibbons ( Nomascus leucogenys ) and five southern yellow-cheeked gibbons ( Nomascus gabriellae ) in zoos. We obtained most data from spontaneous vocalizations across the five contexts, supplemented by recordings during predator model (stress) and preferred food (feeding) presentations. Based on 806 vocalizations, we found that grunt calls were emitted in all contexts, hoo calls were emitted in four contexts and long hoo calls were emitted only in the feeding context. The grunt call structure, which constituted the proportion of the dataset, differed significantly between species and contexts, with significant context-dependent differences in value of the minimum and maximum fundamental frequencies and harmonic structure. Together, these findings highlight the structural and contextual flexibility of intragroup vocalizations in Nomascus gibbons and underscore their potential role in short-range social communication.