Sound production in female Trichopsis schalleri (Labyrinth fishes): comparison to males and evolutionary considerations

Croaking gouramis (genus Trichopsis, Anabantoidei) generate series of two-pulsed bursts (croaks) during agonistic interactions. Sex-specific differences are minor in T. vittata which raises the question whether sexes differ in the other two species. The current study analyses sounds recorded in female T. schalleri, compares the sound characteristics to those of males investigated earlier and correlates these characteristics to female body size. Sex-specific differences were found in three out of six sound characteristics. In females, sounds were lower in burst number, burst period and SPL. Pulse period, dominant frequency and peak-to-peak amplitude ratios of pulses did not differ between sexes. Burst period and SPL increased significantly with female body weight, whereas dominant frequency decreased. The present acoustic data indicate the sex-specific differences are more pronounced in T. schalleri than T. vittata. The results also demonstrate that both sexes are vocal, which remains to be shown for females of the third species, T. pumila, which have poorly developed sonic organs. The evolution of the pectoral sound-producing mechanism in Trichopsis is most likely based on an exaptation process during which acoustic signals are generated by fin tendons initially related to other functions as is evident in closely related genera lacking this organ.