Fig. 5. Comparison of relative power spectra between the first (blue) and second (green) acoustic peaks recorded as mantis shrimp struck a three-axis force sensor. The curved surface (A) and flat surface (B) yielded similar spectral distributions. Second peaks (green) typically contained more energy at higher frequencies in the ultrasonic range (above 20 kHz) than first peaks, which is consistent with cavitation being the source of the second peak (Brennen, 1995; Lush and Angell, 1984; Martin et al., 1981). The peak amplitudes of the acoustic recordings were scaled to 1.0 prior to conducting Fast Fourier Transforms, thereby allowing comparisons of relative power/frequency of first and second peaks within a given strike and across strikes.