The captivating effect of electric organ discharges: species, sex and orientation are embedded in every single received image

Abstract

Some fish communicate using pulsatile, stereotyped electric organ discharges (EODs) that exhibit species- and sex-specific time courses. To ensure reproductive success, they must be able to discriminate conspecifics from sympatricspeciesinthemuddywaters they inhabit. We have previously shown that fish in both Gymnotus and Brachyhypopomus genera use the electric field lines as a tracking guide to approach conspecifics (electrotaxis). Here, we show that the social species Brachyhypopomus gauderio uses electrotaxis to arrive abreast a conspecific, coming from behind. Stimulus image analysis showsthat, even in a uniform field, everysingle EOD causes animageinwhichthegradientandthelocalfieldtimecoursescontain enough information to allow the fish to evaluate the conspecific sex, and to find the path to reach it. Using a forced-choice test, we show that sexually mature individuals orient themselves along a uniform field in the direction encoded by the time course characteristic of the opposite sex. This indicates that these fish use the stimulus image profile as a spatial guidance clue to find a mate. Embedding species, sex and orientation cues is a particular example of how species can encode multiple messages in the same self-generated communication signal carrier, allowing for other signal parameters (e.g. EOD timing) to carry additional, often circumstantial, messages.This ‘multiple messages’ EOD embedding approach expressed in this species is likely to be a common and successful strategy that is widespreadacrossevolutionary lineages andamongvaried signaling modalities.