Electrolocation of Capacitive Objects in Four Species of Pulse-type Weakly Electric Fish

II. Electric Signalling Behaviour


Zoologisches Institut, Universität Regensburg, W-8400 Regensburg, F. R. Germany.


In four species of mormyrid weakly electric fish (Pollimyrus isidori, Gnathonemus petersii, Mormyrus rume, and M. caschive) the electric signalling and motor behaviour during the solution of a conditioned electrolocation task was investigated. All fish had to discriminate between two objects of different electric properties which were identical otherwise. The behaviour of the fish before, during, and after electrolocation of the objects was recorded and analysed. The same types of stereotyped motor behaviours (‘probing motor acts’, PMAs) were observed in all fish during probing of the objects. Independent of the occurrence of PMAs, ‘probing’ was accompanied by a regular pattern (regularization) of electric organ discharges (EODs). Interval durations between successive discharges were kept constant on an individual specific level which was maintained in successive tests under the same conditions. This level was negatively correlated with the spectral frequency of peak power of a single EOD but was also influenced by additional parameters such as motivation. It did not depend, however, on the type of electrolocation target. Before and after investigation of the objects, the discharge rhythm was variable with inter-pulse intervals spanning a wider range than during ‘probing’. In contrast to discharge rhythm, there was no change in waveform or spectral composition of single EODs of a fish, regardless of behavioural context. In conclusion I suggest that regularization of electric signalling is an important factor during electrolocation in all mormyrids but its level is not critical. No special behavioural strategies exist which might explain different electrolocation abilities of the four mormyrid species.