Midshipman grunt trains. Midshipman grunt trains. The spectrogram (A) and waveform (B) of the series of grunts show their broadband characteristic and inter-grunt interval (IGI) of ~0.5 to 1 s. As the IGI between the first two grunts and the preceding four is at least 2-fold the IGI of the first two grunts, the two sets were considered separate grunt trains. Also note the longer IGI for the second grunt train. Expansion of a single grunt (C) shows the typical pulsatile waveform of a grunt. As was the case in all of the field recordings analyzed, the harmonic stack from a background hum appeared throughout the recording and is apparent in the spectrogram (the fundamental frequency, F0, is indicated). The background hum is also apparent as a baseline oscillation in the oscillogram display in C. A background growl also overlaps the beginning of the first grunt train. For the spectrogram shown here and all subsequent figures, the following settings were used: window size=3000 samples, 3 dB filter bandwidth=21.1, overlap=75%, discrete Fourier transform (dFT) size=65,529, grid spacing=0.673 Hz. Recorded on 22–23 June 1997 at 16.15°C. Eileen L. McIver et al. J Exp Biol 2014;217:2377-2389 © 2014. Published by The Company of Biologists Ltd