Sex-specific difference in agonistic sounds depends on size of sonic organs in fishes: Testing the hypothesis in the croaking gourami (Labyrinth fishes).

In most vocal fish species, females possess smaller sound-generating organs and vocalize less than males. In certain cases females lack sonic organs, in others differences between sexes are unknown. This study analyzes in detail the relationship between sexual dimorphism of sonic organs and the characteristics of agonistic behavior and of sounds recorded under the same behavioral conditions in a vocal fish species, the croaking gourami Trichopsis vittata. During agonistic contests both sexes stretch and pluck two enhanced (sonic) tendons when beating pectoral fins alternately, resulting in a series of double-pulsed bursts, termed croaking sound. The following anatomical, behavioral, and acoustic variables were analyzed: diameter of enhanced tendons in each specimen, duration of same-sex dyadic contests, number and duration of lateral display bouts and of sounds, number of single- and double-pulsed bursts, burst period, peak-to-peak amplitudes of pulses, dominant frequency and sound pressure level (SPLrms). Female sonic tendons were approximately one-fifth smaller than male's of the same size. Six out of seven behavioral variables did not differ between sexes. Sound characteristics were similar in both sexes except for SPLs, which were on average 5 dB lower in females. The degree of sexual dimorphisms in sonic organs may explain differences in sound characteristics. Sounds differ only in one sound characteristic (SPLrms) in T. vittata, in contrast with the congeneric Trichopsis pumila which possesses a more pronounced sexual dimorphism in sonic organs and in which agonistic sounds differ in all sound properties between sexes.