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Predation enhances complexity in the evolution of electric fish signals

Abstract

Theories of sexual selection assume that predation is a restrictive, simplifying force in the evolution of animal display characters1 and many empirical studies have shown that predation opposes excessive elaboration of sexually selected traits2. 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 pulses3 and further amplify this sexual dimorphism nightly during the peak hours of reproduction4. Thus a signal element that evolved for crypsis has itself been modified by sexual selection.

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Figure 1: Molecular–morphological consensus phylogeny of the gymnotiform families22,30is plotted along with schematic electric organ discharge (EOD) waveforms of representative species.
Figure 2: Local and remote EODs of male and female Brachyhypopomus pinnicaudatus.
Figure 3: Biphasic EOD pulses have less energy at low frequencies than their monophasic counterparts.
Figure 4: Monophasic EODs of three species compared with each other and with biphasic EODs of congeners.

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Acknowledgements

M. Olman trained and tested the electric eel, which was provided by H. Zakon. T.Lopez donated its food. F. Torres provided me with live gymnotiforms from Peru, photographed by G.Valcarce. The Organization for Tropical Studies supported field collections with B. Rasnow. J.Alves-Gomes, W. Crampton, W. Heiligenberg, C. Hopkins, J. Sullivan, D. Taphorn and particularly M.Hagedorn contributed helpful insights. T. Collins, K. Condon, G. Read, M. Ryan and J. Trexler improved the manuscript. Support came from the NIH. Experiments complied with NIH ‘Principles of Animal Care’ No. 86-23, rev. 1985.

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Correspondence to Philip K. Stoddard.

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Stoddard, P. Predation enhances complexity in the evolution of electric fish signals. Nature 400, 254–256 (1999). https://doi.org/10.1038/22301

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