Agonistic behaviour and electric signalling in a mormyrid fish, Gnathonemus petersii

1. Agonistic motor behaviour and concurrent electric signalling were studied in individually held, residential Gnathonemus petersii. Aggressive behaviour was elicited by presenting a specimen of a closely related species, Mormyrus rume, for 3 min a day. 2. The principal agonistic motor patterns are described (Fig. 2). Among them head butt, approach and lateral display were further analysed. 3. The electrical activity displayed during agonistic behaviour was found to differ fundamentally both from isolated resting and swimming conditions. The mean discharge rate recorded during aggressive behaviour (31 Hz, Fig. 3 c) is approximately twice the rate observed in an isolated swimming fish (Fig. 3b) and three times the rate displayed by a resting animal (Fig. 3a). An attacking G. petersii exhibits a much greater range of electric organ discharge (EOD) intervals than isolated swimming or resting individuals. EOD-interval histograms recorded from attacking fish show two sharp modes at high discharge rate; there are no intermediate intervals. 4. During the course of an attack, the initially low and variable discharge rate increases fairly linearly as the distance from the attacked fish diminishes (Fig. 9). The EOD rate associated with physical contact (head butt) comprises between 60 and 80 Hz in 24 of 28 attacks analysed; the dominant mode of the distribution is 61 Hz (Fig. 8). 5. During subsequent lateral display, G. petersii emits a high discharge rate pattern consisting of two types of ldquosteady-staterdquo activities which may last up to a few seconds: the first is a fairly regular alternation of approx. 16 and 8 ms intervals (paired pulses); this pattern gives rise to the two peaks of high discharge rate in the interval histogram (Fig. 3c). The second is a regular sequence of either 16 or 8 ms intervals (Fig. 4A). The only female among the animals used in our study showed the same display but did not exhibit the highest possible discharge rate (i.e. a regular sequence or 8 ms intervals; Fig. 4B). The high discharge rate is terminated by a sudden discharge break (Figs. 4A and 6). 6. It is suggested that the attack-associated EOD rate increase is a remnant of an ordinary locomotory pattern which has changed its function to a ritualised aggressive signal that occurs in a socially significant and well-defined context. The high discharge rate might serve three functions: (i) behavioural isolation of closely related, sympatrically living mormyrids (perhaps by character displacement); (ii) recognition of sexes; (iii)_synchronisation of mates during courtship

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

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