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The individual frequencies of the continuous, wave-like electric organ discharges (EODs) of the South American knife fish Eigenmannia (Gymnotiformes, Teleostei) are remarkably constant at stable temperatures [1, 2] within a species-specific range of 240--600 Hz [3]. Playback experiments using tape-recorded EODs, presentations of sinusoidal electric stimuli and field observations seemed to indicate that the EOD frequency is a cue in species recognition [3, 4]. However, a later study of sympatric fish communities revealed, a signal's frequency is of limited use in species recognition due to extensive overlapping of species-specific ranges in EOD frequencies [5]. We tested whether Eigenmannia is sensitive to the finer detail of its EOD, especially the sexual dimorphism in waveform and harmonic content [6, 7]. The male EOD resembles short pulses superimposed on a head-negative D.C. baseline containing higher harmonics or overtones which are strong compared to the fundamental (or EOD) frequency, while the female EOD almost resembles a sine wave containing weak, higher harmonics (Fig. 1 a, b). Four E. lineata (11.6-13.2cm), obtained from a tropical fish dealer, were tested during the day. The test aquarium (75 x 40 x 42 cm high; water conductivity 100 + 5 gS cm1 ; temperature 27_+1.5 ~ was provided with a centred porous pot (length 33 cm; inner diameter 6.5 cm; wall 1.2 cm) the long axis of which was parallel to the smaller sides of the aquarium. An isolated fish remained in the porous pot most of the day (L:D, 12:12). One electric dipole made of carbon rod electrodes (diameter 0.5 cm; length 1.8 cm; separation of the two vertically oriented carbon rods 7.8 cm) was placed parallel and close to each end of the aquarium, that is, symmetrically to the left and right of a fish hiding in its shelter. Imitations of natural EODs were digitally synthesized by a specially designed microprocessor system [8] and presented through one of the dipoles. A function generator provided sine and sawtooth waves. Stimulus amplitudes (p-p) were kept constant among two signals within 5%, in a range of 240-690 gV cm1 at the position of the fish's head in its shelter (measured with the fish removed; at least 50 dB above threshold of the conditioned response). Stimulus frequency was 50 Hz above the fish's EOD frequency in order not to evoke a jamruing avoidance response (JAR, [1, 2]). Male EODs and sawtooth waves with their strong overtones contrasted with female EODs and especially sine waves with little or no harmonic content (Fig. 1, right). The first step in a fish's training consisted of forming the association of "presence of an electric signal" with a food reward (one Chironomus mosquito larva) obtained at a feeder nearby the active dipole. Learning progress was monitored by measuring the time (s) from the onset of a sine wave signal to the fish's touching a feeder. The next step in the fish's training consisted of discriminating a rewarded signal (for example, sine waves) from an unrewarded one (for example, sawtooth waves). Mild punishment was necessary to establish discrimination because of the fishes' tendency to approach an active dipole independent of whether it presented a rewarded or an unrewarded signal. When appropriate, a fish was "punished" by approaching it gently with a plastic rod (without touching). A fish punished in this way returned to its shelter. Discrimination conditioning was conducted in a randomized-blocks design [9]. A training block consisted of four training trials (signal no. 1, left; signal no. 1, right; signal no. 2, left; signal no. 2, right; sequence determined by random permutation) with equal numbers of presentations of each out of two signals actually in use. A subsequent test trial without food reward nor punishment completed one randomized block. The sequence of test trials was also determined by random permutation. The number of trials per day was between 30 (24 training and 6 test trials) and 40 (32 training and 8 test trials). The inter-trial interval was 5 rain. When the fish did not respond a signal was turned off after 150 s. Stimulus amplitude was smoothly turned on or off by hand during 1/2 s. Under these conditions discrimination was established during the first day of training in all fish. The fish discriminated successfully between all pairs of signals tested: rewarded sine waves and unrewarded sawtooth waves (P<0.01 in each of four fish tested; Fig. 2a); rewarded sine waves and unrewarded male EODs (P<0.001 in each fish; Fig. 2b); rewarded female EODs and unrewarded male EODs (P<0.01 in each fish; Fig. 2c). A fourth experiment (not shown) consisted of reversing the roles of rewarded and unrewarded signals, as used in the second experiment, and immediately following it: now it was the dipole playing back rewarded male EODs which was approached faster than the one presenting unrewarded sine waves (P<0.025 in three out of four fish; N > 12 test trials per fish and waveform). A simple cue enabling a fish to discriminate between two signals, as used in the present report, might be their differences in fundamental frequency intensities (compared to a sine wave. of 0 dB, fundamental frequency intensifies were: -4 .1 , -3 .0 , and 0dB for the sawtooth wave, the male and female EODs, respectively). A similar type of signal analysis was shown in Eigenmannia's JAR: the strength of JAR was correlated with the intensity of that stimulus harmonic which was close to the EOD fundamental frequency, and independent of stimulus waveform or harmonic content [7]. Therefore, in a fifth experiment the first experiment was repeated in a modified way, using an attenuated sine wave to match the weaker intensity of the fundamental frequency component of the sawtooth wave. All four fish tested still discriminated the rewarded sine wave from the unrewarded sawtooth wave of stronger peak-to-peak amplitude (P<0.01 in each fish;

Conditioned discrimination of electric waves differing only in form and harmonic content in the electric fish, Eigenmannia