Influence of environmental, social and behavioural variables on the whistling of the common bottlenose dolphin (Tursiops truncatus)
Bottlenose dolphin communication has been studied extensively. Nevertheless, the combined effect of several variables on whistle structure has yet to be clearly understood. Here, we investigated the influence of underwater noise, boat presence, behaviour, group size and calf presence on the whistles of a small population of common bottlenose dolphins (Tursiops truncatus) Surface behaviour and acoustic data were collected in Sardinia (Italy Western Mediterranean Sea) between 2015 and 2017. Whistle frequency parameters (minimum, maximum, start, end frequencies and frequency range) duration, number of inflection points and number of harmonics were correlated with the ambient noise levels recorded prior to the whistle display and to behavioural and social factors by means of GAM models. Whistle frequencies varied as a function of the interaction between noise and boat presence, mainly when the noise increased at lower frequencies. Dolphin whistles also varied according to group size, which was further affected by the presence of calves, and behaviours characterised by high levels of arousal such as feeding. The study provides sufficient evidence to support that multiple factors need to be considered when investigating whistle structure as the species is capable of adopting different strategies depending on environmental, social and behavioural context. Finally, this data indicates that communication within a small population of bottlenose dolphins is vulnerable to acoustic pollution, thereby highlighting the need for further investigation into the effects of different noises on whistle structure, as well as the consistency of patterns across different populations. Exploring a dolphin's ability to adapt its communication to environmental context is crucial to understanding the consequences of ocean noise increase, especially in highly anthroposised areas such as the Mediterranean Sea. This is one of the few studies that investigate how noise and boat traffic affect bottlenose dolphin whistles whilst considering the concomitant effect of other variables such as behaviour, group size and composition. We demonstrated that dolphins vary their whistles as a function of increasing noise and boat presence, group size and calf presence, and during specific behaviour which requires intense contact between individuals. Finally, the results showed that other variables should be considered to fully understand those which influence whistle structure and adaptability.
Behavioral Ecology and Sociobiology