Acoustic tag recordings of social pulse sounds in bottlenose dolphins (Tursiops truncatus) in human care during intra-specific aggressive interactions

• The tag recorded 549 regular click trains, 1056 slow and irregular pulse trains and 255 pulse bursts in 70 and 120 kHz band-pass filter bands. Pulse bursts occurred in high intensity aggressive situations, with animals face-to-face at close distance. Plotting peak repetition rate (PRR) against sound duration, two clusters were found, with a mean PRR of 642 pulses per second (pps) and 76 pps, respectively, and a mean sound duration of 110 msec and 224 msec, respectively. In 1085 audio band pulse sounds, recorded with a stationary hydrophone during 25 aggressive interactions between two females, also two PRR vs. sound duration clusters were found, with mean values 148 pps/203 msec, and 550 pps/196 msec, respectively. Also > 1 sec long pulse trains with <200 pps repetition rate were found, including "Machine-gun" sounds, which often initiated an aggressive interaction, when one animal aimed its rostrum at another from a distance. If the targeted animal turned towards the aggressor, they approached each other face-to-face, displaying "Open mouth", "Head & pectoral fin jerk" and "Jaw clap" behaviours, concurrent with "Pulse burst" emissions. We propose that these directional pulse sounds are analogous to the facial expressions seen in e.g. aggressive primates and canids, offering the dolphin the possibility to express underlying emotions, and to address aggressive signals to selected con-specifics. The strong, omni-directional audio band component allows others to monitor the progress of the aggressive interaction.

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