Techniques for studying vocal learning in bottlenose dolphins, Tursiops truncatus

Abstract

The objective of this thesis is to develop the methods necessary for evaluating the role of learning in the natural whistle development of bottlenose dolphins. Bottlenose dolphins provide a unique opportunity to study social influences on vocal learning in a highly social non-human mammal. Vocal learning is critical for the development of human language but plays a much smaller role in the vocal development of most non-human terrestrial mammals. Preliminary evidence has indicated that the signature whistles of dolphin calves are modeled on the whistles in the calves' early environments and that the calves' social interactions influence the choice of model. The methods currently used to study the acoustic and social behavior of dolphins are insufficient to evaluate the role of learning in whistle development and the social influences on that development. The techniques necessary to perform such a study have therefore been developed and tested in this thesis. The methods used to study vocal learning in various species were reviewed and a study of vocal learning appropriate to dolphins was designed. A strategy for sampling the dolphins' social and acoustic behavior was developed. To test the sampling strategy, and to provide data for the development of analysis techniques, a pilot study was performed on dolphin calves born in captivity. Focal samples of the social interactions of dolphin mothers and calves were taken over several months before and after the births of four calves, with simultaneous acoustic recordings during all focal sessions. A test of sampling times determined that five focal samples spaced throughout the day adequately represented the dolphins' behavior for the entire day. The interactions recorded during the focal samples were analyzed with loglinear analysis, multidimensional scaling, and hierarchical cluster analysis to determine the types of social relationships that occurred between the dolphins. For both calves and adults, three types of relationships were found. An analysis of a prolonged alloparenting incident demonstrated that the social relationship between mothers and calves was a care-giving relationship independent of their genetic relationship. Measures other than the total association were found to be necessary to the evaluation of the subtle relationships between the dolphins. Methods for the quantitative analysis of the whistles produced by the dolphins were needed. Therefore, programs were developed to automatically detect and extract the whistles from the recordings in an unbiased manner. Several methods for categorizing whistles were compared and hierarchical cluster analysis of dynamic time warping of extracted contours was shown to perform well for comparing both stereotyped and un-stereotyped whistles. These techniques were then used to compare the early acoustic environments of the calves born in the pilot study. The early environments of the four calves were found to be distinctive. In particular, the putative signature whistle of each calf s mother made up a substantial proportion of the whistles in that calf's early environment. The combination techniques developed in this thesis for the analysis of the social and acoustic behavior of dolphins will allow a study of vocal learning in dolphin whistle development to be performed in a quantitative, unbiased manner.