Whistles of Sympatric Species of Bottlenose Dolphins (Tursiops truncatus) and Spotted Dolphins (Stenella frontalis) in the Bahamas: Acoustic Characteristics and Contextual Use
Bottlenose and spotted dolphins in the Bahamas frequently interact in social, socio-sexual, and aggressive encounters, and whistles are thought to play a key role in their communication. Concurrent vocal and behavioral recordings of wild sympatric species of Atlantic spotted dolphins (Stenella frontalis) and Atlantic bottlenose dolphins (Tursiops truncatus) were collected from three Bahamas populations, and the acoustic parameters, structure, and contextual use of their whistles were analyzed. The mean acoustic parameters of spotted dolphins in the Bimini and White Sand Ridge Bahamas populations were higher in frequency than those of bottlenose dolphins, but bottlenose dolphins produced whistles that had higher delta and maximum frequencies than those of spotted dolphins. Bottlenose dolphins produced proportionately more rise-type calls and convex calls than spotted dolphins did, and spotted dolphins displayed greater use of amplitude-modulated whistles. Differences in acoustic parameters between these two sympatric species may enable them to differentiate between conspecifics and non-conspecifics. As with all odontocete species examined so far, the two whistle parameters with the highest intraspecific variability in these populations were duration and number of inflection points, which may aid in individual differentiation or identification. Whistle acoustic parameters were also found to vary with behavioral context and group composition in spotted dolphins. Specifically, significantly more whistles produced by dolphin groups comprised mainly or entirely of calves and younger juveniles were amplitude modulated, and had significantly higher frequency parameters, especially during people-oriented behavioral states. Whistles with amplitude modulation and higher frequencies may provide cues about the age and emotive state of the animals producing them. Biphonation, the simultaneous production of two sounds, is a commonly occurring phenomena in the Bahamas Atlantic spotted dolphins. Bitonal whistles have very rarely been reported in any species of dolphin, but both burst-pulse whistles and bitonal whistles have been recorded in this population. Bitonal whistles are produced far more frequently by adults than by sexually immature dolphins, while burst-pulse whistles are produced more often in younger rather than older animals. Biphonal components of whistles may provide cues as to identity, age, and social role in spotted dolphin whistles.
Psychology, bottlenose dolphin; communication; spotted dolphin; stenella frontalis; tursiops truncatus; whistle
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