Whistle variability and social acoustic interactions in bottlenose dolphins

Bottlenose dolphins exhibit a sophisticated social structure, known as a fission-fusion society. To sustain this complex system, dolphins rely on a rich vocal repertoire: clicks exclusively used for echolocation, burst-pulse sounds associated with emotions during social interactions, and whistles, including signature whistles that serve as individual-specific identifiers ( names). How dolphins maintain their complex social structure based only on a limited repertoire of sounds remains elusive. Previous studies suggest that contextual information can be transferred by the modulation of the whistles. Here, we investigated the whistle variability using a comprehensive computational approach, and studied the structure of the interactions between the whistle variants. Using a unique large dataset, acquired in a natural environment, we observed that signature whistles exhibit variations in their frequency contours. Unsupervised clustering revealed that signature whistles could be classified into sub-categories (signature whistle variants). The existence of these categories, and their independence on the emitter dolphin, indicate that these variations are not random. Analysis of pairwise interactions between sub-categories revealed a clustered structure similar to that of their social hierarchy. Network analysis of this structure showed that whistle sub-categories had different functional roles: some acted as hubs, others as bridges, and certain were used for turn-taking between the main whistle categories. We also found that the dolphins emit signature whistles of their deceased mothers, a phenomenon only observed in human language. Overall, these findings suggest that dolphins modulate signature whistles to transmit additional information beyond individual identity, and that they engage in "dialogue-like" acoustic interactions.