Intraspecific agonistic interactions in freely swimming mormyrid fish, Marcusenius macrolepidotus (South African form)

South African bulldogs (Marcusenius macrolepidotus, Mormyridae) generate brief (less than 1 ms) electric organ discharges (EODs), separated by much longer and highly variable inter-discharge intervals (IDIs). The diurnal and nocturnal overt behaviour and electrical activity were studied under various conditions: in isolated fish, in pairs of fish, and in a group of four fish that were kept in a “natural” large aquarium. EODs from up to four individuals were recorded simultaneously and identified. While resting during the day, isolated fish showed a broad inter-individual variability of IDI patterns, with distribution histogram modes ranging from 85.7 ms to 325.8 ms. When foraging during the day, IDI modes were shorter and less variable (36.3–48.3 ms). Behaviour patterns displayed during nocturnal agonistic encounters were retreating, parallel swimming, anti-parallel display, attack, and fleeing/chasing. High-discharge-rate (HD) displays were observed at several stages of these encounters, for example, during antiparallel display (a period of low overt motor activity), or following attacks. IDI durations as short as 11 ms occurred during HD displays, which followed low-rate inter-HD activity almost without transition. IDI distribution histogram modes when fish showed anti-parallel display were 15.4 ms and 24.8 ms, and 30.0 ms during nocturnal nonagonistic interactions. No overt fighting was observed once a dominance relationship was established. In a large aquarium, an approaching dominant male evoked a simultaneous discharge arrest in a group of three subdominant males