Vocal response of dugongs (Dugong dugon) to playbacks of conspecific calls suggest ranging function of chirps

Abstract

Dugongs vocalize bird-like calls, such as chirps and trills but the functional definitions of the calls are yet to be clarified. A series of playback experiments was conducted in Thai waters to investigate their call-back behavior. The population was exposed to 4 different playback stimuli; a recorded wild dugong’s chirp, a synthesized down-sweep sound having similar frequencies to the dugong chirp, a synthesized constant-frequency sound, and no sound as a control. Vocalizing dugongs were localized using an array of stereo-underwater-recording systems (AUSOMS-Ds). Total of 4068 calls were observed in reaction to the stimuli. Wild dugongs vocalized more frequently after the playback of dugong chirps (2.8 calls/min) than those of constant-frequency (0.55 calls/min) and control (0.2 calls/min). Ratio of the dugong chirps to all of the call type increased during the playback period. Dugongs were localized on 52 occasions within 25 m range from the playback source. Source level and duration of the chirps from wild dugongs responding to the playback showed positive correlation with distances between the caller and the playback speaker. These results suggest that dugongs can advertise their relative locations by exchanging chirps. Frequency-modulated chirps may have facilitated ranging between individuals.