Development and functions of signature whistles of free-ranging bottlenose dolphins, Tursiops truncatus

Abstract

This thesis presents data on the development and functions of individually distinctive signature whistles of free-ranging bottlenose dolphins, Tursiops truncatus. Recordings demonstrate that free-ranging dolphins produce individually distinctive signature whistles, as was previously documented for captive dolphins. Each dolphin produced a distinctive frequency contour, or pattern of frequency changes over time, and this whistle comprised a large portion of all whistles produced. Comparisons of whistles recorded from the same individuals over periods of more than a decade indicate that these signature whistle contours are markedly stable. This extensive database of recordings of signature whistles produced by known individuals formed the basis for much of the work described in this thesis. Playback experiments conducted during temporary capture-release projects indicated that free-ranging dolphins were able to discriminate among signature whistles of familiar individuals. When these results are taken in the context of what is known about dolphin societies, which are characterized by stable individual associations intermixed with fluid patterns of association among many individuals, it appears highly unlikely that dolphins use signature whistles to recognize one another as individuals. Comparisons of whistle contours of parents and offspring, both in the wild (Sarasota) and in captivity (Miami Seaquarium), do not indicate that signature whistle structure is strictly inherited. Instead, it appears that learning plays a role in determining whistle structure. This contrasts with other non-human mammalian species, where learning does not appear to be involved in vocal development. Focal observations and acoustic recordings of four free-ranging Sarasota mother-calf pairs were conducted in order to examine the effects of the early social and auditory environment on signature whistle development. Mothers may have actively taught their calves to produce a distinctive whistle contour by producing "model" contours while their calves were very young. Strength of the mother-calf association, number of associates other than the mother, overall number of whistles heard, and number of whistles produced by the mother all may affect the time course of whistle development and whether or not a calf develops a contour similar to that of its mother. (Copies available exclusively from MIT Libraries, Rm. 14-0551, Cambridge, MA 02139-4307. Ph. 617-253-5668; Fax 617-253-1690.) (Abstract shortened by UMI.)