Differentiation of courtship songs in parapatric sibling species of dwarf stonebashers from southern Africa (Mormyridae, Teleostei)

Abstract

We describe the nocturnal courtship songs of male dwarf stonebashers, Pollimyrus castelnaui, from the Okavango River and its inland delta. We examined the question of whether the songs are sufficiently differentiated from those of its parapatric sibling species, the only recently discovered P. marianne from the Upper Zambezi River, to form a potential cue for mate choice. Both species vocalised two sound types in courtship, the moan and the grunt, which they combined into long songs in similar fashion. However, one sound type was clearly differentiated: while P. castelnaui moans were of a husky quality and composed of three or four broadband formants, P. marianne moans were more tonal, with a single spectral line dominating the first and any higher formants (and a smaller bandwidth BW−10 dB for the dominant frequency of the first formant). Moan and Grunt Duration and the moan Pulse Group Period (mPGP) were longer, and the latter more variable, in P. castelnaui compared to P. marianne (range of mPGP: 10-30 ms in P. castelnaui, 7-16.7 ms in P. marianne). P. castelnaui grunts were of longer duration and composed of more pulses than those of P. marianne. A single male from the contact zone between the Okavango and the Zambezi, the lower Kwando River, resembled P. castelnaui in moan BW−10 dB but P. marianne in Moan Duration and mPGP. Both southern African species thus vocalise in a species-specific fashion. Since in both species several characteristics of both moans and grunts show high between- and low within-male variability, mate choice may be selective for individual high-quality males characterised by acoustic features.