Sound Production and Reproductive Ecology of Strongly Acoustic Fish in Africa: Pollimyrus isidori, Mormyridae

We have studied the reproductive ecology and bioacoustic signals of weakly electric mormyrid fish in Africa in order to discover the natural conditions under which acoustic signals are used and to gain insight into the evolution of their acoustic behaviour and hearing. Pollimyrus isidori migrated from a tributary of the Niger River (Mali) into a shallow (2- 3 m) flood plain during the onset of the flooding seasons (August, 1991 and 1994). The fish were fully primed for reproduction upon entering the plain, females were significantly larger than males, and the sex ratio was skewed toward males (about 2 males per female). Males established territories (d ≈ 1 m) within the roots of floating rafts of grasses, about 0.5 m below the water surface. Males produced conspicuous acoustic displays consisting of Grunts and Moans during the night (130 dB peak re 1 μLPaat approximately 10 cm). These sounds had a fundamental of 340 Hz, but the band-width of the grunt extended to over 3 kHz. Experiments at the field site with captive animals showed that male sound production was stimulated by the presence of conspecific females. The P. isidori repertoire included 3 additional sounds. Analysis of environmental noise showed that these communication sounds fall within a distinct spectral window, thereby minimizing potential interference from other aquatic animals and abiotic noise sources. Waveform analyses showed that the sounds remained coherent over short distances (0.5 m) but lost amplitude more rapidly than would be predicted in a deep water free-field.