The evolutionary origins of electric signal complexity.

This study explores the evolutionary origins of waveform complexity in electric organ discharges (EODs) of weakly electric fish. I attempt to answer the basic question of what selective forces led to the transition from the simplest signal to the second simplest signal in the gymnotiform electric fishes. The simplest electric signal is a monophasic pulse and the second simplest is a biphasic pulse. I consider five adaptive hypotheses for the evolutionary transition from a monophasic to a biphasic EOD: (i) electrolocation, (ii) sexual selection, (iii) species isolation, (iv) territory defense, (v) crypsis from electroreceptive predators. Evaluating these hypotheses with data drawn largely from the literature, I find best support for predation. Predation is typically viewed as a restraining force on evolution of communication signals, but among the electric fishes, predation appears to have served as a creative catalyst. In suppressing spectral energy in the sensitivity range of predators (a spectral simplification), the EOD waveforms have become more complex in their time domain structure. Complexity in the time domain is readily discernable by the high frequency electroreceptor systems of gymnotiform and mormyrid electric fish. The addition of phases to the EOD can cloak the EOD from predators, but also provides a substrate for subsequent modification by sexual selection. But, while juveniles and females remain protected from predators, breeding males modify their EODs in ways that enhance their conspicuousness to predators.