Electric and Motor responses of the Weakly electric fish, Gnathonemus petersii (Mormyridae), to play-back of social signals

Abstract

1. Seven isolated G. petersii resting in their daytime hiding-places were stimulated via a dipole model (Fig. 1a) with previously tape-recorded electric organ discharge (EOD) patterns in an attempt to determine whether G. petersii distinguishes two different intraspecific EOD patterns, rest and attack.
2. Rest pattern A was characterized by a broad distribution of EOD intervals, a low mean discharge rate (8 Hz, Fig. 3), and a long period of significantly positive autocorrelation (2 s, Fig. 4a). Accordingly, the spectrum of EOD rate fluctuations showed a low frequency range (0.005 to 0.12 Hz, Fig. 5a). Attack pattern B was a considerably different EOD interval distribution of high mean discharge rate (25 Hz, Fig. 3), showing a short period of significantly positive autocorrelation (0.8 s, Fig. 4b), only. Here, the spectrum of EOD rate fluctuations was at a considerably higher frequency range (0.09 to 0.47 Hz, Fig. 5b).
3. Play-back of attack pattern B elicited significantly more bodily startle responses from the experimental fish (Fig. 6) than did the rest pattern A (Table 1). Also the number of attacks directed at the dipolemodel was significantly greater during stimulation with attack pattern B (Table 2, Fig. 6).
4. The EOD responses of the experimental fish differed in several respects depending on which stimulation pattern was used. The modes of the pulse rate histograms as well as their spans were lower during play-back of rest pattern A than during stimulation with attack pattern B (average 12.3 vs 16.2 Hz, and average 47 vs 56 Hz, respectively; Fig. 11). Shortterm (0.2 s) EOD rate correlations were stronger when the fish were stimulated with rest pattern A than when they were stimulated with attack pattern B (average correlations 0.67 and 0.61, respectively; Figs. 10 and 11). Significant positive correlations were maintained for longer periods of time during rest pattern stimulation than during attack pattern stimulation (average 1.94 and 1.24 s, respectively; Figs. 10 and 11). The spectra of EOD rate fluctuations of the stimulated fish were at lower frequency ranges during rest pattern stimulation than during attack pattern stimulation (average amplitude-spectrum peak frequencies 0.02 and 0.07 Hz, respectively; Figs. 12 and 13).
5. Although maximal cross-correlations from the EOD rates to the stimulus pulse rates were weaker during rest pattern stimulation (average 0.2) than during attack pattern stimulation (average 0.33), significant cross-correlations were maintained for longer periods of time during rest pattern stimulation than during attack pattern stimulation (average 1.78 and 0.92 s, respectively). The lags of maximal cross-correlations were greater during rest pattern stimulation than during attack pattern stimulation (average 2.6 and 0.8 s, respectively; Figs. 14 and 15).
6. The results clearly showed that at least two specific EOD time patterns encode different lsquomessagesrsquo in the intraspecific communication system of G. petersii.