Modifications of female Xenopus laevis calls in the frequency domain. (a) Spectrogram (top, amplitude in colour scale; dynamic range, 35 dB; window size, 2048 points; filter bandwidth, 20 Hz) and oscillogram (bottom) of a natural rapping call. (b) Oscillogram (left) and power spectrum (right, window size, 2048 points) of a natural female click, (c) a synthetic female click, (d) a synthetic male click, (e) a filtered white noise click and (f) a broadband white noise click, used to build experimental modified calls.