Fig. 1. (A) Experimental design; the short, vertical lines indicate the position of the recording electrodes in the 9.5 l tank, the additional short, vertical line in the back right corner indicates the position of the ground electrode. (B) Oscillogram of a representative Type 2 chirp recorded from an isolated fish. Note that its duration, indicated by the period of amplitude modulation, is 15–20 ms. (C) An instantaneous frequency plot corresponding to the same Type 2 chirp as illustrated in B. Instantaneous frequency values were derived by taking the inverse of the cycle length, calculated as the duration between consecutive downstroke zero-crossings. (D) A spectrogram displaying the same Type 2 chirp as illustrated in B,C). (E) A spectrogram showing a representative Type 2 chirp (on the left) and a Type 1 chirp (on the right) recorded during a dyadic interaction. The higher frequency of two fish (877 Hz) is modulating its electric organ discharge frequency (EODf) to produce these chirps, whereas the lower frequency fish (714 Hz) does not modulate its EODf during this segment of the interaction. Note that the Type 2 chirp is associated with a much smaller frequency excursion than is the Type 1 chirp. (F) Spectrogram showing two abrupt frequency rises (AFRs) produced in succession by the lower frequency of two interacting fish; the third harmonics are shown and during this segment of the recording the EOD of the higher frequency fish is relatively weak, allowing for a clearer representation of the EOD modulations of the other. Both of the AFRs shown consist of multiple distinct and consecutive small frequency rises. For display purposes, the low amplitude components of each spectrogram were removed and only the strongest (10–20%) amplitude components are shown.