Investigating temporal coordination in the duet contributions of a pair-living small ape

Social animals use complex communication to maintain social bonds. For pair-bonded animals in particular, acoustic communication is vital to reinforcing bonds and coordinating various behaviors within the bonded pair. Pair consistency in duets can be influenced by the time since pairing and may serve multiple functions; two of note are pair-bond reinforcement and advertisement of the relationship to conspecifics. Here, we assess the potential for pair consistency and plasticity in timing of vocalizations in 50 wild Northern gray gibbons (Hylobates funereus) from seven sites in Sabah, Malaysia. Specifically, we looked at the timing of the female gibbon’s great call and the subsequent male coda, the male duet contribution that follows the female great call (coda timing), and the duration of the male coda (coda duration). We found that for coda timing, pair-level variance was the most important source of variance, as opposed to intra-individual or inter-site variance. In contrast, for coda duration, individual-level variance was the most important source of variance. We also found that variability in the duration of female calls was not correlated with the timing or duration of male codas. Our results are consistent with previous work showing pair consistency in other paired, duetting species and contribute to the growing body of literature indicating that primate vocalizations—rather than being inflexible and innate—have a high degree of plasticity. Future work should aim to understand both how gibbons coordinate their duets and what impact this coordination may have on quality of the pair’s bond. SIGNIFICANCE STATEMENT: Duetting is seen across a diverse range of taxa, but in only a few nonhuman primates including indris, tarsiers, titi monkeys, and gibbons. Duetting in primates co-occurs with a suite of behavioral traits that include territoriality and pair bonding. Although the functions of primate duets remain a topic of debate, it is clear that duets provide information to neighboring conspecifics about the calling animal(s). Our results indicate that gibbon duets may contain information about pair identity. Investigating how duration of the pair bond influences either pair-level signatures or consistency of male timing will be an important next step in understanding the proximate influences that shape gibbon duet structure.