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 tnmcatus. Research was conducted at a study site near Sarasota, Florida, where a resident community of bottlenose dolphins have been the focus of a long-term, ongoing study. Through observations and censuses, researchers have gained information on home ranges and association patterns among individuals. A temporary capture and release program has provided opportunities to collect basic information regarding age, sex, genetic relationships, and life history of individuals, as well as to record vocalizations of known individuals. During the periods 1975-1976 and 1984-1992, 134 different individuals were recorded during temporary capture. More than half of these were recorded on two or more (up to 10) different occasions. These 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 likely that dolphins use signature whistles to recognize one another as individuals. Sex differences in whistle production were documented through analysis of whistles recorded during temporary capture. Naive judges rated the similarity of signature whistle contours of 42 Sarasota calves and their mothers, and found that males were more likely than females to produce signature whistle contours highly similar to those of their mothers. Conversely, females were more likely than males to produce contours highly distinct from those of their mothers. In addition, preliminary results indicated that male calves were more likely than female calves to produce whistles other than the signature whistle (called "variant" whistles). It was hypothesized that these sex differences may relate to the different roles males and females play in the social structure of the community. 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. Although there was considerable individual variability among these four calves, this study provided some preliminary insights into factors affecting the time course and outcome of signature whistle development in the wild. Two calves which exhibited relatively rapid whistle development and produced contours that resembled those of their mothers also heard proportionately more of their mothers' signature contours than did the other two calves. The other two calves exhibited more prolonged whistle development and produced contours that did not resemble those of their mothers. Preliminary data indicated that these two 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.