Assessment of the signal value of certain discharge patterns in the electric fish,Gymnotus carapo, by means of playback

Summary

The signal value of 4 distinct discharge patterns inGymnotus carapo was investigated by means of a model electric fish. The model produced an electrical approximation to theGymnotus discharge and field but bore no physical resemblance to the real animal (Figs. 1, 2). Three of the sequences used: Bursts, Offs and Shifts (Fig. 3) were simulations of patterns known to be involved in the agonistic behaviour ofGymnotus. The fourth was that characteristic of a resting fish and was used as a control.

• 1. No aggressive behaviour was shown towards the model when it was electrically inactive. Components directed to the head region during normal interaction (Head Butt, Bite, etc.) were invariably directed to the electrical “head” of the otherwise symmetrical model. Polarity inversion resulted in a corresponding reversal of the end attacked.

• 2. Comparison of the frequency distributions and rate of occurrence of behaviour components during interaction with the model and conspecifics revealed great similarity-the differences found being due to the immobility of the model (Fig. 4).

• 3. A repeated measures analysis of variance (AOV) was used to examine the 6 replications of the experiment. The factors in the analysis were: Playback Sequence (Conditions), Amplitude (Level), Dominance and Days. Two groups of agonistic components, Approaches and Contacts were examined.

• 4. The Days factor was significant for Contact scores only, due to the rapid habituation of high intensity (Contact) components; Approach scores remained at the same level (Fig. 5).

• 5. Animals in a given dominance group showed the highest level of aggression towards the model when the amplitude was closest to their own (Fig. 6).

• 6. High score for dominant and low for submissive in the Shifts condition, compared with the control, suggest that this is a “dominance” signal.

• 7. Contact scores in the Offs condition were very low (Fig. 7) while Approaches remained at a high level (Fig. 6)-demonstrating the extreme effectiveness of the Off as a submissive signal.

• 8. Contact scores in the Bursts condition showed a highly significant difference for the dominant group between the two playback amplitudes. Highest scores were for high level playback, the fish showing little interest in an apparently small fish (low amplitude) behaving aggressively (producing Bursts). These scores were even lower than for submissive animals at either amplitude (Fig. 7).