In the present study, a combined video-acoustic method was used to determine the identity of the whistling dolphin in an aquarium in Mexico City, where two adult bottlenose dolphins were housed. The hyperbola method was used to localize the sound source, finding that a dolphin was located along the hyperbola for 72.9% of the 960 recorded whistles. However, due to dolphin behavior, the identity of the whistling dolphin could only be determined for 60.3% of the whistles. Sometimes, it was possible to use other cues to identify the whistling dolphin: It could be the animal located closest to the hydrophone that captured the whistle or that which performed a behavior named observation when whistling occurred. Therefore, 15.4% of the whistles were further ascribed to either dolphin to obtain an overall identification efficiency of 75.7%. This is the first study to report such a high rate of identified whistles to the free swimming dolphin that produced them. The very simple and inexpensive video-acoustic method developed can be applied in other aquaria. Nevertheless, its actual efficiency in other aquaria will depend on how often dolphins spend time next to each other and on the reverberation conditions of the pool.
The whistle repertoire of bottlenose dolphins has been poorly described in the literature, thus little information is known to support the hypothesis that whistles are used by dolphins to communicate. In this study, whistles emitted by four captive bottlenose dolphins housed in two aquaria in Mexico City were recorded to analyze the whistle repertoire and whistle emission rate for each aquarium. The whistle repertoire was described using human and automated categorizations. Results show that whistle emission rate and the whistle repertoire are both larger for the two dolphins that are more active and which are housed in the same aquarium. As for whistles shared in the repertoire of both aquaria, these are very few, probably because dolphins of the two aquaria were captured in different oceans and are of different ages, besides of the different activities that dolphins can perform in each aquarium. Additionally, all four dolphins could use similar whistles for a specific activity, like "swimming", while different whistles were used for another specific activity, like "observing". These results show that dolphins use whistles selectively, supporting the hypothesis that whistles may be used to communicate information for particular situations. [Work supported by PAPIIT, UNAM & CoNaCyT]
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