Dolphin self-talk: unusual acoustic behaviour of a solitary bottlenose dolphin

Solitary dolphins occurring outside their normal range provide a unique opportunity to study acoustic signals in the complete absence of conspecifics, drawing insights into their communicative functions. In this study, we analysed the acoustic behaviour of a solitary bottlenose dolphin residing in the Baltic Sea. We expected that he would produce few, if any, communicative sounds in the absence of potential recipients. Contrary to this expectation, we found the dolphin to be highly vocal, emitting burst-pulse and tonal sounds in rhythmic bouts. The sounds did not co-occur with echolocation trains, indicating that they were not produced during feeding activities, despite the presence of bray calls previously identified as feeding-associated in this population. The dolphin produced three different types of stereotyped whistles, deviating from the typical pattern of one individually distinctive signature whistle per dolphin. Additionally, three different biphonic sound categories were identified: combinations of two whistle components, a whistle and a low-frequency tonal component, and two burst-pulse components. In conclusion, the production of vocalisations in the absence of conspecifics suggests that these sounds may be emitted unintentionally as emotional signals, serve functions other than direct communication, or simply be a byproduct of dolphins’ intrinsic need for social interaction.