Abstract:
Odontocete phonations have traditionally been classified into three mutually exclusive categories: (1) click trains, (2) burst-pulse sounds, and (3) whistles. An analysis of phonations from two false killer whales revealed that these categories are not actually distinct and that gradations exist between them. In a sample of 152 whistles, 80% had a pulsed component with a pulse train occurring before, during, and/or after whistle production. Many whistles could be seen to fade into and out of pulsed signals that aurally and spectrographically could appear as click trains or burst-pulse signals. These associated pulsed signals matched the signal characteristics of the whistles, including peak frequencies. The mechanism that produces whistles also seems to produce pulsed signals. Based on waveform and spectral analysis, at least three states in the gradation from whistle to click train can be identified: (1) amplitude modulations of the whistle, (2) a secondary spike in the whistle waveform, with the second harmonic appearing as the fundamentals, and (3) rapidly pulsing oscillations appearing as harmonic bands. The observation of independent whistles and clicks suggests that there are at least two sound-producing mechanisms. The occurrence of graded signals is significant when considering the function and information content of animal signals.