Whistle rates of wild bottlenose dolphins (Tursiops truncatus): Influences of group size and behavior.

Abstract

In large social groups acoustic communication signals are prone to signal masking by conspecific sounds. Bottlenose dolphins (Tursiops truncatus) use highly distinctive signature whistles that counter masking effects. However, they can be found in very large groups where masking by conspecific sounds may become unavoidable. In this study we used passive acoustic localization to investigate how whistle rates of wild bottlenose dolphins change in relation to group size and behavioral context. We found that individual whistle rates decreased when group sizes got larger. Dolphins displayed higher whistle rates in contexts when group members were more dispersed as in socializing and in nonpolarized movement than during coordinated surface travel. Using acoustic localization showed that many whistles were produced by groups nearby and not by our focal group. Thus, previous studies based on single hydrophone recordings may have been overestimating whistle rates. Our results show that although bottlenose dolphins whistle more in social situations they also decrease vocal output in large groups where the potential for signal masking by other dolphin whistles increases.

The acoustic repertoire of wild common bottlenose dolphins (Tursiops truncatus) in Walvis Bay, Namibia

“…LFN sounds were produced during 7 encounters or 12% of encounters with bottlenose dolphins, compared with the 48 (81%) encounters in which whistles were found and 45 (76%) encounters in which BP sounds were identified. Similar proportions have been reported from other sites, for example in the Gulf of Mexico (GOM), LFN sounds were produced during 12% of encounters with dolphins (Simard et al 2011) and in Scotland whistles were identified from 84% of encounters (Quick and Janik 2008).…”

“…However, the majority of research has focused on echolocation clicks and whistles (e.g. Au et al 1982;Jones and Sayigh 2002;Quick and Janik 2008;Díaz López 2011) without looking at the whole repertoire of vocal production. Bottlenose dolphin echolocation clicks are high intensity, short duration, broadband sounds with ultrasonic peak frequencies, usually produced in rapid succession (click trains) for the detection and discrimination of targets (Au 1993).…”

“…range between 0.8 and 28.5 kHz in frequency (Schultz and Corkeron 1994;May-Collado and Wartzok 2008). Whistles are principally used in social contexts (Jones and Sayigh 2002;Quick and Janik 2008). Approximately, 52% of those produced by freely interacting animals are signature whistles (Cook et al 2004), which are used to convey identity information (Janik et al 2006), facilitate group contact (Janik and Slater 1998;Quick and Janik 2012) and address conspecifics .…”

Changes in bottlenose dolphin whistle parameters related to vessel presence, surface behaviour and group composition

“…Whistles paired with a point sample in which information was missing from the field observations were excluded from any further analysis. To account for the secondary behavioural states involving milling, which may indicate a transition between states (Quick & Janik, 2008), a mixed behavioural state (MIX) was introduced at the analysis stage. It was assigned to point samples where milling behaviour was documented as the primary behaviour with either socializing or surface feeding as the secondary behaviour, or in cases where socializing or surface feeding was the primary behaviour with milling as the secondary behaviour.…”

“…In favourable conditions, these signals can propagate over tens of kilometres (Janik, 2000). Whistle production is usually higher during socializing contexts (Jones & Sayigh, 2002;Quick & Janik, 2008) and when animals experience stress (Esch, Sayigh, Blum, & Wells, 2009). Each bottlenose dolphin uses an individually distinctive signature whistle which is acquired through vocal production learning in the first year of life (Caldwell & Caldwell, 1979;Janik & Slater, 1997).…”