Hunting, Fighting, or Playing with Bubbles: Possible Usage and Acoustic Characteristics of Bubble Burst Sounds Produced by the Amazon River Dolphin (Inia geoffrensis)

Abstract

Acoustic characteristics of bubble production by an odontocete were documented for the first time. Bubble sounds produced by the Amazon river dolphin (Inia geoffrensis) were recorded incidentally as part of a survey of fish sounds in the Pacaya–Samiria National Reserve of Peru on six dates between 4 and 24 July 2012. Dolphins were observed to periodically produce large clouds of bubbles underneath or near the survey boat (averaging 7/survey or 8/h) as it drifted through areas of actively foraging dolphins. The bubble production was classified as bubble bursts due to their similarity to bubble bursts produced by other cetaceans. Bubble burst sounds had a mean peak frequency of 402 Hz and duration of 8.9 s (n = 51). Bubble bursts were temporally clustered with an average interval of 169 s (0.1 to 1,187 s) between bursts. Bubble bursts were disproportionately more likely to be present when fish sounds were also present, but it is not known if the association was due to predation or other factors. A review of the literature finds similar bubble production has been reported in at least 14 other species of cetaceans (4 mysticetes and 10 odontocetes). Most are commonly associated with play, surprise, agonistic, and foraging behaviors. We discuss each of these possibilities and conclude that Amazon river dolphin bubble burst behavior is most likely related to foraging or aggressive behavior because the behavior occurred in feeding areas and appeared to be directed at the drifting boat. We further propose a novel hypothesis that the bubble bursts are a hunting strategy used to disperse prey associated with floating vegetation mats and other forms of drifting materials used by fishes for shelter. Future research is needed to better understand the behavior associated with bubble production by the Amazon river dolphin.