Testing mechanisms underlying vocal overlap during vocal exchange of signature whistles in common bottlenose dolphins

In animal communication, including human conversation, vocal exchange is a key phenomenon that organizes interactions so that partners exchange rapidly, taking turns and avoiding overlapping vocalizations. By contrast, vocal exchanges in cetaceans have generally been reported to involve frequent overlapping vocalizations. However, it remains unclear whether animals overlap their vocalizations incidentally or actively. Here, we explored the behavioural mechanisms underlying vocal overlapping during vocal exchange in common bottlenose dolphins, Tursiops truncatus . We first identified the signature whistles of five captive dolphins and then conducted a playback experiment in which a separated individual's signature whistle was played to dolphins housed together in the focal pool. The observed rate of vocal overlap was significantly higher than expected by chance, indicating that overlap did not occur incidentally. Analysis of spontaneously produced signature whistle sequences showed that dolphins produced whistles with isochronous timing, and that this isochrony exceeded that of a random model. This indicates that dolphins naturally produce signature whistles isochronously. We then evaluated two possible explanations for vocal overlap: dolphins might overlap by predicting the timing of a partner's next whistle, or by responding rapidly after hearing the next whistle, even before it had ended. However, in a playback experiment comparing vocal responses to isochronous versus nonisochronous sequences, the frequency of overlapping did not differ between conditions. This suggests that isochronous temporal regularities in vocal sequences did not facilitate prediction of the timing of the partner's next whistle and that vocal overlap is better explained by rapid responses following detection of conspecific whistles. These findings suggest that bottlenose dolphins, like other taxa, exchange vocalizations to reduce response delay. However, unlike most other species, they do not avoid overlapping and instead incorporate it into the vocal exchange. This study provides new comparative insight into both the universality and the diversity of vocal exchange patterns across species.