Polarity Discrimination and Quantitative Analysis of Agonistic Rasp Signaling in the Weakly Electric Fish, Brienomyrus brachyistius (Mormyridae)

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Mormyrid electric fish extract species, gender, dominance, and orientation information from the waveform of their electric organ discharges (EODs). Orientation information was examined in this study by alternating the polarity of 4 different EODs delivered by a dipole playback electrode (PBE). Fish were found to show a strong head-butt and circle response to the head (+) lead of the electrode, confirming previous studies. A second experiment was performed with the same playback apparatus to quantify the temporal characteristics of electrical responses to agonistic signals called 'rasps'. Rasps consist of two bursting patterns: a fast burst (FBr) with inter-spike intervals (ISIs) of 8-20ms, followed by a medium burst (MBr) with ISIs of 30-70ms and are performed when males fight for dominance status, as well as during courtship. Experimental stimulus rasp patterns were gathered from dominant and subordinate males in 3 different contexts, and a looped pattern of natural nocturnal swimming discharges was used as a control. Testing 14 fish, it was found that rasps quickened in response to the experimental stimuli along three different temporal parameters: rasps/second, rasp duration (seconds) and the shortest ISI achieved (ms). Many dominant individuals were observed 'rasp matching' the experimental stimuli, while several subordinates went silent during playback. The dominance status of experimental fish was also measured and revealed that dominant males performed rasps at a higher rate and with shorter ISIs than subordinate males. Subordinate males were often observed delivering more medium bursts and continuous discharges than dominant males.
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