The electric organ discharge (EOD) of the mormyrid Pollimyrus isidori is a short pulse with three phases: (1) weak head positive (PI); (2) strong head negative (N); (3) weak head positive (P2).
1. At a stable water conductivity (100 pS/cm), which is near the upper end of the natural range in tropical Africa, there was a statistically significant difference between the sexes only in one of five EOD parameters, the P-ratio. The Pl-amplitude was lower than the P2-amplitude (i.e. P1/ P2 < l) in males (N = 10), while, on average, the opposite (Pl/P2 > 1) was true for females (N = 14). Because of wide overlapping we do not consider this sex difference to be a sexual dimorphism. The difference between males and females could be due to well-known biophysical and physiological reasons (discussed later) and need not be the result of intraspecific selection (such as female choice).
2. Water conductivity seriously affected the EOD
waveform. The P-ratio decreased in 2/3 of our fish (16 out of 24), as conductivity increased from 5 to 200 @/cm, causing 6 out of 14 females to change from a P-ratio of > 1 to a P-ratio of < 1, becoming more "male-like". P1 amplitude increased with decreasing conductivity in the EODs of 5 out of 10 males to a more "female-like" shape (P-ratio > 1). The P-ratio changed only slightly when above a conductivity of 200 pS/cm. The N-wave duration increased with decreasing conductivity, while the peak amplitude frequency of an EOD amplitude spectrum decreased.
3. Long-term stability was found to be poor in the EOD of l female (better in 2 other fish), which changed from a "female-like" waveform (P-ratio > l) to a "male-like" waveform (P-ratio < 1 over the whole conductivity range) without apparent reason within 120 days.
4. The EOD waveform of Petrocephalus bovei did not show a sex difference. Decreasing conductivity affected the EOD of P. bovei in a similar way to most P. isidori: the PI-wave increased and the P2-wave decreased, while the N-wave broadened strongly.
5. The occurrence of multiple discharges per primary neural command signal at very low conductivities, indicates that P. isidori is adapted to conductivities above 17 pS/cm, and P. bovei to those above 5 pS/cm.
6. Because of both the lack of a distinct sexual dimorphism,
and the dependence of EOD waveform on water conductivity, the EODs of both species do not appear to be the unequivocal signatures needed for sex recognition, or for the distinction from other species (if the EODs are of similar spectral amplitudes). Alternative mechanisms are known that would be better for these purposes.