Variation of electric organ discharge in Mormyrus kannume (Mormyridae) from south-western Ethiopia in relation to individual size, sex and reproductive status

Five variables of the electric organ discharge (EOD) were studied in 132 freshly caught individuals of Mormyrus kannume from the Gibe and Gojeb rivers (the enclosed Omo-Turkana system) and Baro River (the White Nile system) in south-western Ethiopia, East Africa. Most individuals exhibited the typical biphasic EODs with an initial head-positive phase followed by a head-negative phase exceeding initial one in the relative amplitude. In three repeated samples from the Gibe, stable differences in the discharge variables between females and males were found, while no sex differences were observed in other samples. In three cases, a positive correlation between the individual size and discharge duration was found: in two male subsamples from the Gibe and in a mixed sample of both sexes from the Gojeb. In most samples studied, some males exhibited the outstandingly long EOD duration. At the same time, no relationships between the discharge variables and reproductive status of individuals were found. The possible influence of individual and intra-individual variations on the results of inter-population and interspecific comparisons of the discharge variables in mormyrids are highlighted.