Structure of Signature Whistles

Dolphins produce three types of sounds: broadband clicks used in echolocation; broadband burst pulses; and whistles. Whistles are frequency modulated narrow bands of sound, which are thought to be used as a nametag for each individual dolphin (Caldwell, Caldwell and Tyack 1990). Caldwell et al. (1990) described a hypothesis in which captive dolphins would produce individually distinctive and stereotyped whistles. These whistles were named signature whistles due to their individually distinctive features.

The constant components of each signature whistle are thought to indicate the identity of the whistler. Caldwell et al. (1990) suggested that the other more variable sections of the whistle might indicate other information related to the dolphin’s position or corresponding to environmental cues. Caldwell et al. (1990) attempted to determine the structure of signature whistles, as well as the portion of each that was constant, thus representing the individual’s identity.

Signature whistles, when recorded and analyzed acoustically, display variable rises and falls in frequency, which range from 4 – 18 kHz. Durations for whistles have been far from constant, some only lasting 0.1 seconds, while others last for up to 3.6 seconds (Caldwell et al. 1990).

The following sound file is an example of a signature whistle from Tursiops species.

Caldwell et al. (1990) also found supporting evidence for "loops" in the signature whistles of bottlenose dolphins. Loops repeated portions of the whistle. The repeated segment is simply a pattern of rising and falling frequency. Acoustical analysis of loops indicated that the dolphins tended to vary the structure of the first and last loops of the whistle. These first and last loops were deemed to be the introductory and terminal loops (Caldwell et al. 1990).

Interpretation of signature whistles has not been perfected, as many difficulties in field observations have arose. For example, in natural settings dolphins are constantly in contact with one another, and so it is difficult to pinpoint which dolphin may have emitted a whistle. In this case, it may be thought to be beneficial to the study of signature whistles if bottlenose dolphins are studied in captivity. However, problems arise in captive situations since the dolphins have been removed from their native and familiar settings. It is not known whether the placement of these animals in such foreign settings would alter their behaviour so much as to affect the production of signature whistles.

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