Escape response of Guiana dolphins to bottlenose dolphin playback

Abstract

An individual reaction to natural threats, such as the presence of predators or aggressors, can shape the social behaviour of an entire population. In southern Brazil, two coastal Delphinid species, Guiana dolphin (Sotalia guianensis) and Lahille's bottlenose dolphin (Tursiops truncatus gephyreus) co-occur on a regional scale. However, within Baía Norte, there is a clear spatial segregation between these dolphins at a fine-scale that, combined with records of bottlenose dolphins displaying agonistic behaviours towards Guiana dolphins, raised the hypothesis that such aggressive behaviours may contribute to shape this segregated distribution. Here, to investigate this hypothesis, we carried out a controlled exposure experiment in which surface behavioural responses of Guiana dolphins were evaluated before, during and after acoustic playbacks of bottlenose dolphins. Through aerial footages, collected with an unmanned aerial vehicle, we accessed significant behavioural responses of Guiana dolphins in terms of group organization and behavioural states. During playbacks, we observed a decrease in the cohesion index between individual Guiana dolphins and an increase in group synchrony, indicating that individuals coordinate their movements to aggregate when exposed to playback. In addition, time spent in foraging behaviours decreased significantly during playbacks, suggesting that foraging activities are interrupted, and individuals start to travel. These results demonstrate that Guiana dolphins perform a collective escape behaviour when facing bottlenose dolphins, a potential aggressor, reinforcing the hypothesis that such defensive behaviour is mediating the spatial segregation between these dolphins' populations.