The spatial context of free-ranging Hawaiian spinner dolphins producing acoustic signals

Abstract

An important requirement for understanding the function of animal signals is the ability to determine the context in which they occur. To improve our understanding of how dolphins use acoustic signals in the wild a three-hydrophone towed array was used to investigate the spatial distribution of Hawaiian spinner dolphins (Stenella longirostris) relative to each other as they produced whistles, burst pulses and echolocation clicks. Groups of 10 to 60+ animals were recorded while they traveled and socialized in nearshore waters off Oahu, HI. Broadband recordings up to 62.5 kHz were obtained using a PC-based multi-channel system with simultaneous analog-to-digital converters. Signaling animals were localized using time of arrival difference cues on the three channels. Localized whistles produced closely in time typically originated from individuals spaced widely apart (10–100+ m), supporting the hypothesis that whistles play a role in maintaining contact between animals in a dispersed group. Multiple burst pulses, on the other hand, usually came from individuals spaced closer to one another (5–20 m), suggesting they function as a more intimate form of signaling between adjacent individuals. The spacing between echolocating animals was variable.