A signature whistle is a learned, individually distinctive whistle type in a bottlenose dolphin's (Tursiops truncatus) acoustic repertoire that gives the identity of the whistle owner.^[1] The whistles are identified and studied in the wild or in captivity by researchers using hydrophones.^[2] Vocal learning strongly influences the development of signature whistles,^[3] which can remain stable for up to at least 12 years.^[4] More often than not, calves develop signature whistles by the sounds they hear while making sure that they differ from close associates.^[1] However, male calves tend to learn signature whistles that resemble their mother's.^[1] They are believed to be most frequently used in the communication of captive bottlenose dolphins,^[4] along with having specialized functions and properties.^[5] Signature whistles are in a higher frequency range than humans can hear.^[6] Researchers define a signature whistle as a whistle with a unique frequency curve that dominates in the repertoire of a dolphin.^[7] Each dolphin has a distinct signature whistle that other members of its social group use to individually identify the whistler.^[4] They are typically used for localisation purposes, however they also provide dolphins with behavioural context.^[8][9] Signature whistles have an important role in facilitating mother–calf contact,^[6] group cohesion and social interaction.^[3]

Identification of signature whistle calls from wild, unrestrained bottlenose dolphins is essential to understand how they are used in their natural environment.^[1] Signature whistles of bottlenose dolphins can be recorded in the wild or in captivity.^[7][3] 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.^[9] Although multiple hydrophones are used, signature identification uses a single hydrophone and allows better identification.^[2] Variation between signature whistles can be tested by examining the whistle contours frequency modulation pattern.^[1] A frequency modulation pattern differs between dolphins, which is usually identified by using standard acoustic parameter measurements, such as beginning, end, maximum or minimum frequency.^[1] Combining the measurements of the frequencies will show the modulation pattern of the individuals signature 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.^[2] Signature whistles are typically emitted within 1 to 10 seconds of each other, while non-signature whistles can occur with longer or shorter intervals between each whistle.^[2] This distinction can be used to identify signature whistles in hydrophone recordings.

An early definition of signature whistles defined them as the most common whistle emitted when a dolphin is kept in isolation.^[1] 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.^[2] 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.

A dolphin's signature whistle development is influenced by auditory experience,^[1] and usually fully develops within the first year of life, rarely changing throughout adulthood.^[10] Stable whistle call structure for at least a decade is necessary for long term call recognition between bottlenose dolphins.^[1] A calf develops its own signature whistle based on the sounds they hear and most calves develop signature whistles that differ from their close associates.^[1] Calves tend to model their signature whistles after those of adult dolphins who they do not spend much time with.^[10] Due to the fact that signature whistles are unique, a calf's signature whistle never matches a single whistle but instead has similarities to multiple whistles.^[10] However, male calves tend to develop signature whistles that resemble the signature whistles of their mother's.^[1] The male calf does not copy the whistle, instead it uses it as a model.^[10]

Vocal learning is essential in a dolphin calf's signature whistle development.^[11] It plays an important role in development because it helps to build an individualized whistle with a unique frequency modulation pattern.^[1] It is an animal's ability to imitate vocalizations from other animals of the same species, and eventually produces its own sounds.^[12] Social interaction plays a significant role in vocal learning.^[10] Although vocal learning is often associated with aggressive behavior in some animals, this is not the case in dolphins.^[11] Rather, vocal learning strengthens social bonds, such as those between mother-calf pairs and alliance partners.^[11]

Apart from signature whistles being developed socially there are other behavioural, ecological, genetic and modification factors that can also influence the development of a dolphin's whistle.^[1] For example, signature whistles may vary from a behavioural factor and the temporal production of whistles can be affected by stress or other psychological influences.^[1] Determining the contribution of different influences on signature whistle variation, aside from social influence, is difficult to achieve due to minimal populations. It is best examined in multiple populations using recordings collected from various behavioural contexts.^[1]