Figure 4. Typical waveforms and spectrograms of leopard seal vocalizations. (A, B) High double trill, (C, D) Medium single trill. The raw waveform and the bandpass-filtered waveform are shown by cyan and red solid lines, respectively (48,000 fast Fourier transform points and 4,800-point Hanning window).
LDT, consisting of two trills in series, begins with a relatively short and narrowband component. The spectrum level of this initial narrow component is higher than that of the subsequent double-trill parts, and trill parts exhibit a broader bandwidth of several tens of Hz compared to that of the narrowband component. HST consisted of a relatively stronger single narrowband component followed by a single trill, which had a slightly higher center frequency than the narrowband component, with distinct time intervals separating them. DT, characterized by a center frequency that gradually decreases within approximately 290 and 570 Hz, shows a tendency for the amplitude modulation to slightly increase around 20 Hz. AT, previously reported as a single trill, was observed as three trills in series (hereafter referred to as triple ascending trill) in our measurements. A total of 362 triple ascending trills were recorded, each composed of three ascending trill parts in the frequency range of approximately 400 to 1,540 Hz (Figure 5D). Single ascending trills were also observed sporadically with a significantly low signal-to-noise ratio, but they were not addressed in this study due to high uncertainty in detection. These issues are discussed in Section 4. The first ascending trill part, starting at approximately 400 Hz, was characterized by a progressively accelerating pulse repetition rate within the range of 10 to 60 Hz, similar to MST (Figure S2). The second ascending trill part showed a rapid increase in center frequency, connecting the other two trill parts within a relatively short duration. The last ascending trill part, with a center frequency slowly increasing within the frequency range of 1,000 to 1,500 Hz, exhibited a slow increase in the pulse repetition rate at approximately 40 and 50 Hz and its harmonic components. The acoustic characteristics of the six call types extracted from sample calls are listed in Table 1. The reverberation effects and amplitude modulation patterns of the calls are discussed in Section 4.