Signature whistle shape conveys identity information to bottlenose dolphins

Abstract

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.

Fig. 1.

Spectrograms and waveforms of original whistles (Left) and their synthetic versions (Right) for three individuals. The first whistle is from a target animal, the second is its kin stimulus, and the third one is its nonkin stimulus whistle. This animal reacted more to its kin stimulus despite its own whistle being more similar to the nonkin stimulus. Synthetic whistles in this figure represent the input to the speaker system.

Fig. 2.

Turning responses toward the kin and nonkin synthetic whistle stimuli. Data for the same individual are connected by lines.