Prey detection by bottlenose dolphins, Tursiops truncatus: an experimental test of the passive listening hypothesis

Abstract

Bottlenose dolphins possess a sophisticated echolocation system, but evidence suggests that they use this sensory modality sparingly in the wild. Several authors have noted that soniferous fish are prevalent in the diet of bottlenose dolphins, leading to the hypothesis that these predators detect their prey by passive listening. We tested this hypothesis by performing controlled acoustic playback experiments with free-ranging dolphins in Sarasota Bay, Florida. We used recorded calls of prey fish and sounds of snapping shrimp as experimental and control treatments, respectively, and measured the dolphins' direction of travel and rate of echolocation as response variables. Dolphins changed their direction of travel significantly, turning towards the sound source when fish sounds were played. In addition, dolphins significantly increased their rate of echolocation immediately following playbacks of fish sounds. The sounds of snapping shrimp elicited neither directional nor echolocation responses. The occurrence of echolocation sounds was low, except following playback of fish sounds. We conclude that bottlenose dolphins use passive listening extensively during the search phase of the foraging process. By listening passively, dolphins may obtain useful information on the identity, number, size and location of soniferous prey. Once dolphins discover prey by passive means, they then appear to use echolocation to track the prey during the pursuit and capture phases. Such judicious use of echolocation suggests that this sensory modality incurs significant energetic or ecological costs. These findings have implications for coevolution of dolphins and their prey with regard to sound production and detection.