Sexual selection by female choice prevents speciation reversal in a hybridizing trio of mormyrid fish in southern Africa: evidence from playback experiments of electric organ discharges

We studied the question of whether or not female choice among variant forms of species-specific male advertising signals, electric organ discharges (EODs), is a factor in preventing panmixy in a parapatric sibling complex of three species of mormyrid fish, inhabiting three parallel rivers in southern Africa. The three species’ EODs are characteristically differentiated in waveform. The Upper Zambezi River is inhabited by Pollimyrus marianne Kramer, van der Bank, Flint, Sauer-Gürth & Wink, 2003, the Okavango River by P. castelnaui (Boulenger, 1911), and the smaller Kwando River in their middle by P. cuandoensis Kramer, van der Bank & Wink, 2013, which is their hybrid species of unidirectional origin. P. castelnaui females ( out of 5) and P. marianne females ( of 5) responded stronger to playback of the EODs of male conspecifics compared to those of male P. cuandoensis. Pollimyrus castelnaui and P. marianne females neither preferred nor discriminated against the male EODs of each other’s species, respectively (one exception). The single P. cuandoensis female available preferred a P. marianne male EOD over one of its own species, and was neutral in all other tests. This suggests that female resistance in the two main system species to P. cuandoensis male EODs is an evolved one, effectively limiting hybridization to the Kwando. The females of the two main system species, P. castelnaui and P. marianne, thus prevent panmixy in the Okavango and the Zambezi, respectively, thereby keeping up the three-sibling species complex by discriminating female choice against P. cuandoensis males in the Okavango-Kwando-Zambezi system.