| Summary: | Differences in whistle types between species and populations of dolphins may arise from differences in body size, environmental conditions, geographic separation, and vocal learning between animals. Assessing vocalization differences between populations of delphinids, as well as the mechanism of divergence, has become a subject of interest since acoustic differences may help to distinguish between populations at sea. In this study, bottlenose dolphin (Tursiops truncatus), Atlantic spotted dolphin (Stenella frontalis), and pilot whale (Globicephala sp.) populations in U.S. waters were quantitatively compared to determine if differences in whistle structure exist between both neighboring and geographically separated populations. Comparisons were made for nine whistle characteristics between northern Gulf of Mexico and western North Atlantic populations of bottlenose dolphins and pilot whales and between continental shelf and offshore populations of Atlantic spotted dolphins in the western North Atlantic. Whistle characteristics were measured for 3,836 pilot whale whistles, 1,703 Atlantic spotted dolphin whistles, and 2,715 bottlenose dolphin whistles recorded between 2002 and 2004. Differences between groups were evaluated using principal components analysis and discriminant analysis. Bottlenose dolphin whistles in the Atlantic were significantly different (Hotelling’s T-squared, p < 0.0001) from those in the Gulf of Mexico, differing chiefly in the whistle characteristics of end frequency, duration, and the number of inflection points. Offshore Atlantic spotted dolphin whistles were significantly different (Hotelling’s T-squared, p < 0.0003) from those of the continental shelf population, differing principally in high frequency, central frequency, and bandwidth. No significant difference was found between pilot whale whistles in the two ocean basins. The whistle differences demonstrated in this study indicate that acoustic divergence exists between distinct populations and may arise from geographic isolation or due to habitat separation between neighboring but genetically distinct groups. This study suggests that acoustic studies are an excellent and cost-efficient method to assess population structure.
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