Time-frequency fine structure of dolphin whistles

Data indicating that whistles of bottlenosed dolphins Tursiops truncatus are not simple tonal signals are presented. Whistles are not simply sinsoidal signals with rather slowly varying frequency contours, but also include more complex attributes, which may be related to a short-duration phenomenon, perhaps akin to a rapid frequency modulation of the basic tonal components. These subtle fluctuations are not revealed by spectrogram analysis. It is shown that methods of time-frequency analysis such as the reduced interference distribution (RID) reveal both the tonal structure and the more subtle modulation structure