Signature whistles are a form of dolphin vocalization emitted by bottlenose dolphins (Tursiops truncates). They are used in communication within the species and have specialized functions and properties. Researchers define it as a whistle with a unique frequency curve that dominates in the repertoire of a dolphin. Each dolphin has a distinct signature whistle that no other dolphin has developed. They are typically used for locational purposes, however they also provide dolphins with identity information and behavioral context. Signature whistles also play an important role in group cohesion and social interaction.
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Identification
Signature whistles can be recorded in the wild or in captivity. Hydrophones are used in both cases, but the number of devices may vary based on the researcher's preference and methodology. Using multiple hydrophones allows the researchers to better identify which dolphin emitted which whistle.
Signature whistles are difficult to identify because bottlenose dolphins emit non-signature whistles as well. However, there is a distinct amount of time in between each signature whistle that helps researchers distinguish them from the rest of dolphin noises. Signature whistles are emitted within 1 to 10 milliseconds of each other. Non-signature whistles occur with longer or shorter intervals between each whistle.
An early definition of signature whistles defined them as the most common whistle emitted when a dolphin is kept in isolation. This description restricted research designs to only studying dolphins in captivity. The current method for identifying signature whistles is known as signature identification, or SIGID. Developed by Janik and King, this method combines human observations with vocalization analysis. The vocalizations are recorded by a single hydrophone. This method allows researchers to better identify signature whistles when studying wild dolphins because it can be used in situations where many dolphins emit whistles at the same time.
Development
A dolphin’s signature whistle usually fully develops within the first year of life, and rarely changes throughout adulthood. A calf develops its own signature whistle based on the signature whistle of an adult in its pod. The calf does not copy the whistle, instead it uses it as a model. Calves tend to model their signature whistles after those of adult dolphins who they do not spend much time with.
Vocal learning is essential in a dolphin calf’s signature whistle development. It is an animal’s ability to imitate vocalizations from other animals of the same species, and eventually produce its own sounds. Social interaction plays a significant role in vocal learning. Although vocal learning is often associated with aggressive behavior in some animals, this is not the case in dolphins. Rather, vocal learning strengthens social bonds, such as those between mother-calf pairs and alliance partners.
Within groups
Signature whistles are primarily used to locate group members, however they can also provide dolphins with the behavioral situations their group members are in. Dolphins primarily emit signature whistles when one individual is separated from the rest of the group. Non-signature whistles are the main vocalization when all of the group members are together. Although signature whistles are used to locate other dolphins, captive dolphins, who can use their sight to locate other animals, also emit these whistles. It is believed that signature whistles contain information other than location or identity, and can explain why dolphins in captivity use them. Signature whistle emissions also increase during socialization and predation.
Similar to vocal learning, bottlenose dolphins demonstrate vocal copying. Bottlenose dolphins repeat another dolphin’s signature whistle back in order to address that particular dolphin individually. Unlike other animals, dolphins do not display this behavior in aggressive situations. Humans and dolphins are the only known species to use vocal copying in cooperative contexts. Although this behavior is rare, dolphins with close relationships are more likely to demonstrate vocal copying. Also, bottlenose dolphins are capable of producing almost perfect copies of another dolphin’s whistle. Any differences in the repeated whistle could be deliberate, reinforcing the idea that the dolphins individually address close relatives.
Between groups
Dolphins also use signature whistles when meeting new groups of dolphins. Signature whistle emission rates are much higher when two groups encounter each other than in general socialization within pods; and they repeat them multiple times during the exchange. Whistle matching may occur when two groups of dolphins encounter each other, as well. It is observed when a dolphin copies another individual's whistle and repeats it back. Matching occurs in order for one dolphin to acknowledge another individual's presence. This can occur when the animals are up to 600 meters apart. Signature whistles play an important role in interactions between groups because dolphins use them differently than within their group.