Fish courtship songs species-specific

It's not just male birds that sing courtship songs to attract females of their species, now scientists have found that certain species of African fish do it too.

Male dwarf stonebashers of two different species "sing" courtship songs at night that are distinct and specific to their own species to attract the opposite sex.

Michael Lamml and Bernd Kramer studied two different members of the mormyrid stonebasher genus Pollimyrus that occur together in the Okavango River and its inland delta in southern Africa and found that both species produced songs that were a characteristic of their species, in much the same way as the songs of birds.

The two species studied, Pollimyrus castelnaui and the relatively recently discovered Pollimyrus marianne (described by Kramer, van der Bank, Flint, Sauer-Gurth and Wink in 2003) are small members of the elephantfish family Mormyridae and produce electrical signals to aid navigation in murky waters.

P. marianne, which reaches around 7cm in length, is found predominantly in the Upper Zambezi and lives in fast-flowing stretches of rapid water over a rocky bottom, as well as in the more typical mormyrid habitats of weedy backwaters. P. castelnaui is more widespread, being found in the Okavango, Zambezi, Kafue and Cunene systems, as well as in Lake Malawi and it is typically found in slower moving river margins or still areas with dense aquatic vegetation.

Moaning and gruntingThe two fish occur together in some parts of their range and sing different songs so that they can attract females of the correct species. Lamml and Kramer say that the songs produced can be characterised into either moans or grunts:

"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)."

The scientists also discovered that the grunts and moans produced by the two fish differed in length and the number of pulses in the signal:

"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.

Kramer and Lamml believe that their findings showing species-specific moans and grunts in the species might be important in mate choice with males being selected by females based on the acoustic qualities of their singing.

For more information see the paper: Lamml M and B Kramer (2006) - Differentiation of courtship songs in parapatric sibling species of dwarf stonebashers from southern Africa (Mormyridae, Teleostei). Behaviour, Volume 143, Number 6, 2006, pp. 783-810.