Disembodying the invisible: electrocommunication and social interactions by passive reception of a moving playback signal.

Mormyrid weakly electric fish have a special electrosensory modality that allows them to actively sense their environment and to communicate with conspecifics by emitting sequences of electric signals. Electroreception is mediated by different types of dermal electroreceptor organs for active electrolocation and electrocommunication, respectively. During electrocommunication, mormyrids exhibit stereotyped discharge sequences and locomotor patterns, which can be induced by playback of electric signals. This raises the question: what sensory information is required to initiate and sustain social interactions, and which electrosensory pathway mediates such interactions? By experimentally excluding stimuli from vision and the lateral line system, we show that Mormyrus rume proboscirostris can rely exclusively on its electrosensory system to track a mobile source of electric communication signals. Detection of electric playback signals induced discharge cessations, followed by double-pulse patterns. The animals tried to interact with the moving signal source and synchronized their discharge activity to the playback. These behaviors were absent in control trials without playback. Silencing the electric organ in some fish did not impair their ability to track the signal source. Silenced fish followed on trajectories similar to those obtained from intact animals, indicating that active electrolocation is no precondition for close-range interactions based on electrocommunication. However, some silenced animals changed their strategy when searching for the stationary playback source, which indicates passive sensing. Social interactions among mormyrids can therefore be induced and mediated by passive reception of electric communication signals without the need for perception of the location of the signal source through other senses.