Group behaviour of a nocturnal weakly electric mormyrid fish: investigations using an electro-communicating dummy fish Sarah S. Pannhausen 1 , Martin Worm 1 , Frank Kirschbaum 2 and Gerhard von der Emde 1 1 Institute of Zoology, University of Bonn, Bonn, Germany and 2 Faculty of Life Sciences, Humboldt University of Berlin, Berlin, Germany ABSTRACT Background: Tapir fish possess organs that emit weak electric discharges. Mormyrus rume proboscirostris has been observed to show group behaviour in captivity and electrical signalling is believed to play a crucial role in group communication and coherence. Organism: Mormyrus rume proboscirostris (tapir fish). Questions: (1) What role does electro-communication play in the formation and coherence of groups of three freely swimming weakly electric fish? (2) What happens to the group when a dummy fish that emits electric discharges is substituted for one of the fish? Hypotheses: (1) Individuals will show specific patterns of electro-communication and motor behaviours when at different positions in a natural group. (2) When the fish swimming at the front is replaced by a dummy fish emitting the same series of electric signals, the structure of the group will persist and the remaining fish will behave to some extent as they would in a natural group. Methods: We recorded and analysed the natural motor and electrical behaviours of tapir fish in groups of three freely swimming fish. We focused on their electric discharge patterns, inter- active communication, and group coherence. Then we replaced the first fish with an artificial dummy fish that mimicked the electrical behaviour of the substituted individual. We analysed the coherence of the group, as well as the electrical signalling behaviour of the two remaining fish. Results: Fish behaved similarly in the presence of an electro-communicating dummy fish as when following the first fish in a natural group. When the dummy came to a standstill but continued to emit electric signals, the fish remained significantly closer to the dummy than to a real fish. The electrical behaviour (regularization, double-pulses) of fish in mixed groups was similar to that of fish in natural groups. The interactive electro-communication patterns of the fish were directed towards the dummy in a similar way as to real fish. They were even more abundant in mixed groups, presumably because the dummy could not respond dynamically to the fish. Keywords: electro-communication, group behaviour, robotic fish, weakly electric fish. Correspondence: Sarah S. Pannhausen, Institute of Zoology, Rheinische Friedrich-Wilhelms-Universität Bonn, Poppelsdorfer Schloss, Meckenheimer Allee 169, 53115 Bonn, Germany. email: s6sapann@uni-bonn.de Consult the copyright statement on the inside front cover for non-commercial copying policies. Evolutionary Ecology Research, 2018, 19: 591–617 © 2018 Sarah S. Pannhausen