Hormonal control of sex differences in the electric organ discharge (EOD) of mormyrid fishes

Summary Field studies have demonstrated that several species of mormyrid fish from Gabon, West Africa have a sex difference in the pulse-like waveform of their Electric Organ Discharge (EOD). Administration of androgen hormones (testosterone or dihydrotestosterone) to a female or juvenile can induce the EOD typical of a sexually mature male. Data for two such species —Brienomyrus brachyistius (triphasic) andStomatorhinus corneti — are presented, showing that transformation of a female's or juvenile's EOD to a male-like EOD involves a 2–3 fold increase in EOD duration and a downward shift in the peak frequency of the EOD's power spectrum (as determined by Fourier analysis). ForBrienomyrus brachyistius (triphasic), estradiol can also induce changes in the EOD waveform, although not as dramatic as that for androgens. Changes in EOD duration and power spectra are often accompanied by an alteration of the wave-shape or morphology of the EOD pulse, i.e., the relative amplitude of its peaks and the presence of inflection points in its negative and positive phases.A third species,Hippopotamyrus batesii (triphasic), not previously known to have an EOD sex difference, also responds to testosterone treatment with an increase in EOD duration. Preliminary field data indicate this species may have a sexual dimorphism in its EOD, suggesting that the response to a steroid hormone may be an indicator of a sex difference in a species' EOD waveform. Such findings are discussed in relation to the affects of steroids on vertebrate neurons and muscle, and the evolution of electric communication systems.Abbreviations DHT 5-dihydrotestosterone - EOD electric organ discharge - F mature female - HTI interval between peaks (H and T) in EOD's first derivative - IF juvenile female - IM juvenile male - M mature male - PPW peak frequency of power spectrum