Theories of sexual selection assume that predation is a restrictive, simplifying force in the evolution of animal display characters and many empirical studies have shown that predation opposes excessive elaboration of sexually selected traits. In an unexpected turnaround, I show here that predation pressure on neotropical, weakly electric fish (order Gymnotiformes) seems to have selected for greater signal complexity, by favouring characters that have enabled further signal elaboration by sexual selection. Most gymnotiform fish demonstrate adaptations that lower detectability of their electrolocation/communication signals by key predators. A second wave phase added to the ancestral monophasic signal shifts the emitted spectrum above the most sensitive frequencies of electroreceptive predators. By using playback trials with the predatory electric eel (Electrophorus electricus), I show that these biphasic signals are less detectable than the primitive monophasic signals. But sexually mature males of many species in the family Hypopomidae extend the duration of the second phase of their electric signal pulses and further amplify this sexual dimorphism nightly during the peak hours of reproduction. Thus a signal element that evolved for crypsis has itself been modified by sexual selection.