Sound characteristics of captive Irrawaddy dolphin Orcaella brevirostris

Abstract

Even though there have been several studies on vocal repertoires and behaviours of Irrawaddy dolphins (Orcaella brevirostris) in recent decades, important pieces of information are still missing, especially the sounds produced in captivity where environmental differences can affect their physical and acoustic behaviours. Over 40 hours of observations in March and November 2014 with almost 7,000 vocalization samples, it was determined that the captive Irrawaddy dolphins produced various types of sound including two types of clicks (n = 3,898), five types of pulsed sounds (n = 2,807), and nine types of whistles (n = 44). This work also describes three newly discovered pulsed sounds and all nine whistles types in this species. Clicks were observed most frequently during feeding and swimming in navigational contexts. Pulsed sounds were mainly used in social or emotional contexts associated with various behaviours, even aggression, sexual arousal, and tourist-encounter. Whistles, on the other hand, were scarcely heard which might imply that the communication was not necessary. Whistling behaviour was contradictory to those reported for other wild delphinids which were potentially caused by habituation in captivity. Overall vocalizations, which were concurrent with activity levels, were recorded at a significantly higher rate (p = 0.000009) when tourists were present during “swim-with-dolphin” programs. This is the first study to correlate Irrawaddy dolphin’s sounds and behaviours in captivity and represents the first time that all three sound categories have been recorded for this species in captivity.