Vocal exchanges of signature whistles in bottlenose dolphins (Tursiops truncatus)

Bottlenose dolphins (Tursiops truncatus) develop individually distinctive signature whistles that they use to maintain group cohesion. Unlike the development of identification signals in most other species, signature whistle development is strongly influenced by vocal learning. This learning ability is maintained throughout life, and dolphins frequently copy each other's whistles in the wild. It has been hypothesized that signature whistles can be used as referential signals among conspecifics, because captive bottlenose dolphins can be trained to use novel, learned signals to label objects. For this labeling to occur, signature whistles would have to convey identity information independent of the caller's voice features. However, experimental proof for this hypothesis has been lacking. This study demonstrates that bottlenose dolphins extract identity information from signature whistles even after all voice features have been removed from the signal. Thus, dolphins are the only animals other than humans that have been shown to transmit identity information independent of the caller's voice or location.

We conducted playback experiments with wild bottlenose dolphins, Tursiops truncatus, to determine whether there is sufficient information in their individually distinctive signature whistles for individual recognition. We conducted experiments with members of a resident community of dolphins in waters near Sarasota, Florida, during temporary capture-release projects. We used a paired playback design, wherein the same two whistle sequences were predicted to evoke opposite responses from two different target animals. This design controlled for any unknown cues that may have been present in the playback stimuli. We predicted that mothers would respond more strongly to the whistles of their own independent offspring than to the whistles of a familiar, similar-aged nonoffspring. Similarly, we predicted that independent offspring would respond more strongly to the whistles of their own mother than to the whistles of a familiar, similar-aged female. Target animals were significantly (P<0.02) more likely to respond to the predicted stimuli, with responses measured by the number of head turns towards the playback speaker. In bottlenose dolphin societies, stable, individual-specific relationships are intermixed with fluid patterns of association between individuals. In primate species that live in similar 'fission-fusion' type societies, individual recognition is commonplace. Thus, when taken in the context of what is known about the social structure and behaviour of bottlenose dolphins, these playback experiments suggest that signature whistles are used for individual recognition. Copyright 1999 The Association for the Study of Animal Behaviour.