Shifts in frequency tuning of electroreceptors in androgen-treated mormyrid fish

Summary Several species of mormyrid electric fish have a sex difference in the pulse waveform of their electric organ discharge (EOD). Field studies in Gabon, West Africa have shown for one such species,Brienomyrus brachyistius (triphasic), that the sexually mature male EOD differs in shape and is nearly twice the duration of the EODs of females and juveniles. Fourier analysis reveals that differences in EOD duration correlate with those in the EOD power spectrum which has a peak at 0.3 kHz in males and 1.3 kHz in females and juveniles. We find a corresponding sex difference in the frequency tuning of at least one class of electroreceptors known as Knollenorgans. The average best or characteristic frequency of Knollenorgans is lower in males compared to females and juveniles. This correlates with a lower peak in the power spectrum of the male's pulse. When females are treated with gonadal androgens, their EODs increase 2–3 fold in duration, and the power spectra of their pulses are correspondingly lowered to match that of mature males. The average best frequency of Knollenorgans decreases by nearly 1 kHz which matches the downward shift of their EOD's power spectrum.For a second species ofBrienomyrus (sp. 2) which is commercially imported from Nigeria, we have not detected a sex difference in the power spectrum or duration of the EOD. The power spectrum peaks at about 4.2 kHz in males, females, and juveniles. Androgens, however, do cause a coincident downward shift in the average peak of the EOD power spectrum (from 4.2 to 1.3 kHz) and the average best frequency of Knollenorgans (from 2.3 to 1.4 kHz).Specimens ofBrienomyrus (sp. 2) that have been electrically silenced by surgical means are tuned, on the average, only 0.2 kHz higher than control animals. Silenced animals that have been treated with androgens are tuned, on the average, 0.2 kHz below controls. The results suggest that electroreceptor tuning is only partially modifiable during androgen treatment if the electroreceptors arenot being stimulated by an external electrical stimulus, i.e. the animal's own EOD. Since androgen treatment has a dramatic effect on receptor tuningonly in intact fish, it seems likely that retuning isnot due to a direct action of androgens on receptors, but rather due to the action of the principal electrical stimulus upon the receptors, i.e. the EOD. The implications of such results for the development of species and sex differences in electro-receptor tuning is discussed.