The Sexually Dimorphic Jamming Avoidance Response in the Electric Fish Eigenmannia (Teleostei, Gymnotiformes)

Eigenmannia's jamming avoidance response (JAR) is a frequency change of its electric organ discharge (EOD) in response to an electric stimulus of similar frequency (small AF; AF = FFlgh — FStlm). It is assumed that the response to an undamped stimulus, AR = FRMponM. — FRMI, is stereotyped and non-habituating, and improves the fish's electrolocation performance in the presence of a jamming stimulus, such as the EOD of a nearby conspecific. Adult females gravid with eggs (N = 3) gave good responses (frequency decrease of at least 3 Hz) to —AF (stimulus frequency higher than fish frequency), but no response or only weak responses (<0-5 Hz) to + AF (stimulus frequency lower than fish frequency). After 2-75 years, a sexually mature female still showed the same behaviour, whereas an immature female (see below) had changed its behaviour considerably on becoming sexually mature. Large males (A^ = 4) did not give JARs to + AF, and no JARs or only weak ones to — AF (|AR| < 0-7 Hz). Increasing the stimulus intensity by +10 or +20 dB did not change this result. After 2-5 years, two large males were still found to be almost unresponsive. However, large males gave rapid frequency modulations ('short rises' and 'interruptions'), which have been described from threatening fish likely to attack, even at the weakest stimulus intensity. One group of juveniles (Ar=4; probably females) gave only a weak increase in frequency (AR2Hz) in response to —AF. Another group of juveniles (Af=4; probably males) gave strongest responses (AR > 3 Hz) to AF = 0 Hz. In these juveniles, the 'equilibrium point' of no response was at AF=— 0-6 Hz to — 1 Hz instead of at AF = 0 Hz. They thus increased, rather than decreased, their EOD frequency even at small — AFs, which would have been more economical. A decrease in frequency was weaker than an increase. A significant frequency change could even be elicited by stimuli of AF = OHz that are phase-locked to the EOD. The accuracy of assessment of AF, as determined in juvenile fish giving good + ARs and -ARs, was not better than ±0-3 Hz (at AF=-06Hz). The JAR showed strong habituation