1. 1.

    The present report shows an intraspecific, sexually dimorphic variation in harmonic content and waveform, as well as intensity, of the Electric Organ Discharges (EODs) in the green knife fish, Eigenmannia (Figs 2, 11). There is a close correlation between harmonic content of the EODs and waveform (as defined by the ratio of intervals between zero-crossings in the two half-waves of one EOD cycle; Fig. 3). The aim of the following experiments was to determine whether the fish are sensitive to differences in stimulus waveform or harmonic content.

  2. 2.

    Stimulation with electric fields of various waveforms but equal peak-to-peak amplitudes at frequencies close to the fish's frequency elicited the Jamming Avoidance Response (JAR) which is an EOD frequency change increasing the frequency difference (Watanabe & Takeda, 1963). The strength of JARs to distorted square wave and sawtooth stimuli was 25% smaller than that elicited by sine wave stimuli (Fig. 6). Undistorted square waves elicited stronger responses than sine waves, while undistorted sawtooth waves were the least effective (Table 1). The differences in response strength were proportional to differences in the intensity of the fundamental frequency or first harmonic, f1, of the stimulus waveforms.

  3. 3.

    Subharmonic stimuli of nine artificial or synthesized natural waveforms at frequencies near one-half or one-third of the EOD resting frequency elicited responses only when a strong higher harmonic, or overtone, of the stimulus was close to the EOD fundamental frequency (Fig. 7). Stimuli of different waveforms, but identical spectral amplitudes, elicited similar responses not significantly different from each other, at stimulus frequencies near one-half (Table 3) and near the EOD resting frequency (Fig. 10).

  4. 4.

    Eigenmannia's JARs to synthesized male EODs at frequencies close to the fish's frequency were weaker than those to female EODs of equal peak-to-peak amplitude. The weaker response was proportional to the weaker intensity of the first harmonic of the male EOD (Fig. 12). A stimulus frequency near one-half of the EOD baseline frequency evoked opposite results, as the second harmonic, f2, of the male EOD was relatively four times stronger than that of the female EOD (Table 4).

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