Electrical communication and jamming avoidance betwen resting Gymnotus carapo

Summary

• 1. The electrical interactions of a pair of Gymnotus carapo of known dominance relationship were studied in a large semi-natural aquarium while resting during the daytime (Fig. 1).

• 2. Synchronisations of the two discharge frequencies were found to occur predominantly in bouts of 15–20 consecutive intervals although very close frequencies with beat periods lasting more than 100 intervals were recorded.

• 3. The delay of a fish's pulses with respect to those of its partner during a synchronisation showed specific dominance-related tendencies indicating that the dominant fish places its discharges in the period of maximum electroreceptive sensitivity of the submissive animal (Figs. 2, 3, and 4). Such a synchronisation effectively protects the first fish from the jamming effect of its partner's pulses.

• 4. The analysis of 4-h recordings demonstrated the existence of clear jamming-avoidance manoeuvres in Gymnotus. Examination of these responses showed that when jamming-avoidance frequency increases occurred at near discharge coincidence, it was the lower frequency fish which readjusted its frequency (Fig. 5)

• 5. Interactions at close discharge frequency (Figs. 6, 7, and 8) show that frequency following takes place in certain situations and a struggle for ‘frequency dominance’ is seen which, it is suggested, is the best strategy for the preservation of optimal electrolocation.

• 6. A model is proposed (Fig. 9) to account for the different sensitivity of Gymnotus to foreign pulses sweeping through its own discharge in either the time-positive (lower frequency partner) or time-negative (higher frequency partner) directions. The model is based on the previous finding of a phaselocked attenuation of electroperceptive information in this species.

• 7. Whether or not the jamming avoidance in Gymnotus is related to social communication remains an open question. Efficient jamming avoidance requires the determination of the sign and magnitude of the frequency difference between the interacting fish. This would also communicate information related to social dominance, previously shown to correlate well to overall discharge frequency. The present results indicate that the dominant fish was in the more favourable position, as far as efficient electrolocation is concerned, for most of the time.