Characterisation of the acoustic repertoire of the common bottlenose dolphin (Tursiops truncatus) in the Aegean Sea and effects of environmental, behavioural and anthropogenic factors

Tursiops truncatus, the common bottlenose dolphin, is known for its vast and elaborate acoustic repertoire, on which the animal relies for many tasks that it performs daily, including navigation, foraging and social interactions. Vocalisations emitted by dolphins are divided into three categories: whistles, communication signals used to vehiculate complex information, including the animal’s identity; burst pulses, stereotyped calls used for close-range communication of the dolphin’s emotional state; and clicks, signals that used for echolocation, typically emitted in trains. Their repertoire is also notoriously plastic, meaning that the characteristics of the vocalisations emitted by bottlenose dolphins can adapt and respond to changes that happen in their habitat. This is particularly relevant in increasingly anthropized marine environments, where, due to the effects of noise pollution, the levels of background noise have been constantly and starkly increasing over the last century, posing a challenge to all marine animals. In this study, we carried out passive acoustic monitoring to provide a first description of the vocal repertoire of the T. truncatus population that inhabits the East Aegean Sea by detailing the values for each of their main temporal, frequency and contour parameters of their vocalisations. We then investigated how bottlenose dolphins are able to modify their whistles, burst pulses and echolocation clicks in response to environmental, behavioural and anthropogenic drivers, focusing in particular on the effects of marine traffic and noise pollution. Our results indicate that T. truncatus employs specific strategies to mitigate possible limitations to the effectiveness of its vocalisations caused by other sources of noise in its proximity, as well as diversifying its emissions depending on the activity that it is involved in. Clicks are modulated to maintain biosonar efficiency, whereas whistles and burst pulses undergo structural modification to carry on social communication. This research contributes the broader understanding of the T. truncatus repertoire, allowing for comparison with bottlenose dolphins that reside in other basins, the characteristics of their vocalisations and their adaptations, that vary across different populations.