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.