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

The EOD Sound Response in Weakly Electric Fish

1. A spontaneous EOD response to sound is described in two gymnotoids of the pulse Electric Organ Discharge (EOD) type, Hypopomus and Gymnotus, and in one mormyrid, Brienomyrus (Figs. 2-4). 2. In all three species, the EOD response to the sound onset was a transient EOD rate increase. In the low EOD rate Hypopomus (3-6 EODs/s at rest) the first, second, or third EOD interval following sound onset was significantly shorter than the average EOD interval before stimulation. The shortest latency found was 100 ms, the longest ca. 1.2 s. Gymnotus (around 50 EODs/s at rest) responded similarly, but the third interval after sound onset was the first to be affected even at highest intensities (shortest latencies approx. 60 ms; latencies >0.5 s at low sound intensities). In Brienomyrus (4-8 EODs/s at rest) the response occurred already at the first EOD interval after sound onset. 3. An EOD sound response was recorded in Hypoporous and in Gymnotus up to 5,000 Hz sound frequency (in one Gymnotus individual: up to 7,000 Hz). Due to technical limitations the low frequency limit of the response could not be exactly determined: the fishes responded well even below 100 Hz. Hypopomus had its maximum sensitivity around 500 Hz (Fig. 5), Gymnotus around 1,000 Hz (Fig. 6). 4. In all three species the EOD sound response was graded with sound intensity (Hypopomus: Fig. 7). 5. No EOD response to sound was found in two gymnotoids of the wave type, Eigenmannia and Apteronotus, and in the gymnotoid pulse fish Rhamphichthys. A criterion is proposed by which it should be possible to predict whether or not a weakly electric fish species will show the EOD sound response. 6. It is concluded that the EOD response to sound is similar to EOD responses to other kinds of stimulation (light, touch, vibration, food, and even electrical). The possible biological function is discussed