A Review of the Individual-specific Vocal Behavior of Bottlenose Dolphins (Tursiops truncatus) and Killer Whales (Orcinus orca): The Role of the Signature Whistle Hypothesis

Defense Date


Document Type


Degree Name

M.S. Marine Biology

First Advisor

Edward Keith

Second Advisor

Denise L. Herzing


Bottlenose dolphins (Tursiops truncatus) and killer whales (Orcinus orca) each have unique social structures. However, they both depend on individual and group recognition to maintain their social relationships. Bottlenose dolphins live in a fission-fusion society where group composition changes regularly. They are able to maintain both long-term relationships, such as between foraging partners or mother-calf pairs, as well as temporary associations. In contrast, resident fish-eating killer whales remain in highly stable matrilineal groups with minimal dispersal by either males or females.

Bottlenose dolphins and killer whales share a common dependence on acoustic communication to maintain their social structure. Since the marine environment poses significant problems for visual communication, especially at depths and distance, bottlenose dolphins and killer whales have developed a communication strategy based on individual- and group-specific vocalizations. Bottlenose dolphins utilize clicks for echolocation and whistles for communication. Their vocal repertoire is comprised mostly of individually-specific signature whistles. These signature whistles are used for individual recognition and to maintain group cohesion. Killer whales also use echolocation clicks and whistles but discrete pulsed calls are their most common vocalization. The discrete pulsed calls are used to maintain group cohesion and whistles are used during socializing to serve as affiliative signals.

Evidence in favor of the Signature Whistle Hypothesis as it relates to bottlenose dolphins has been provided since the hypothesis was established by David and Melba Caldwell in 1965. Recent studies have tried to refute the hypothesis based on the discrepancies found using qualitative versus quantitative analysis techniques and the effect of studies conducted in captivity versus in the wild.

Since the group dynamic is more important in killer whale societies, it is believed that group-specific vocalizations are the basis of their vocal repertoire rather than individual vocalizations such as signature whistles. However, recent studies have identified some amount of individual signature information present within the shared stereotyped killer whale call. This provides the first evidence that the Signature Whistle Hypothesis may also apply to killer whales and that individual identity along with group level information is transmitted through their shared calls. In order to further support the Signature Whistle Hypothesis in bottlenose dolphins as well as apply the hypothesis to killer whales, additional studies are needed to identify how these species categorize the whistles of conspecifics.

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