Phonation behavior of cooperatively foraging spinner dolphins.

Groups of spinner dolphins have been shown to cooperatively herd small prey. It was hypothesized that the strong group coordination is maintained by acoustic communication, specifically by frequency-modulated whistles. Observations of groups of spinner dolphins foraging at night within a sound-scattering layer were made with a multibeam echosounder while the rates of dolphin sounds were measured using four hydrophones at 6 m depth intervals. Whistles were only detected when dolphins were not foraging and when animals were surfacing. Differences in click rates were found between depths and between different foraging stages but were relatively low when observations indicated that dolphins were actively feeding despite the consistency of these clicks with echolocation signals. Highest click rates occurred within the scattering layer, during transitions between foraging states. This suggests that clicks may be used directly or indirectly to cue group movement during foraging, potentially by detecting other individuals' positions in the group or serving a direct communicative role which would be contrary to the existing assumption that echolocation and communication are compartmentalized. Communicating via clicks would be beneficial as the signal's characteristics minimize the chance of eavesdropping by competing dolphins and large fish. Our results are unable to support the established paradigm for dolphin acoustic communication and suggest an alternate coordination mechanism in foraging spinner dolphins.