Echo response to chirping in the weakly electric brown ghost knifefish (Apteronotus leptorhynchus): role of frequency and amplitude modulations

Teleost fish of the order Gymnotiformes are distinguished by their ability to produce electric discharges by means of specialized organs. These electric organ discharges serve various behavioral functions, including communication. During such electric interactions, male brown ghost knifefish (Apteronotus leptorhynchus (Ellis in Eigenmann, 1912)) generate several types of transient frequency and amplitude modulations (chirps) of the otherwise nearly constant discharges. Previous studies have shown that the chirps generated by one individual follow those of the other with a preferred latency of approximately 500-1000 ms. As demonstrated in the present study, signals consisting of either frequency modulations or amplitude modulations are able to trigger this echo response. Signals composed of just amplitude modulations are effective in triggering an echo response only if the reduction in amplitude is large (approximately 40%, relative to baseline of the electric organ discharge of the emitting fish). By contrast, in frequency-modulated signals, a maximum frequency increase as small as 1.2% relative to baseline frequency is sufficient to trigger an echo response. This remarkable sensitivity might be an adaptation for the detection of so-called type-2 chirps, as chirps of this type are composed of rather small frequency increases and negligible amplitude modulations. In line with this hypothesis is the observation that during electric interactions of two fish, the generation of these chirps dominates the production of any of the other five chirp types known.