Females whisper briefly during sex: context- and sex-specific differences in sounds made by croaking gouramis

Croaking gouramis, Trichopsis vittata (family Osphronemidae), possess enhanced pectoral fin muscles and tendons, enabling the fish to generate series of short or long broadband bursts while beating the fins. Agonistic disputes over territories result in lateral displays during which both sexes produce croaking sounds. During mating, a female approaches a male's nest at the water surface and emits purring sounds in a vertical, head-up position. This behaviour is followed by spawning bouts. I investigated whether aggressive and prespawning sounds differ in acoustic characteristics and whether they might convey different information. Agonistic sounds of both sexes lasted longer than the females' prespawning sounds, consisting of four or five bursts/sound compared to 2.4 bursts in purring sounds, and included a smaller percentage of short bursts. Sound pressure levels were higher in agonistic than in prespawning sounds. The dominant frequency of male agonistic signals was lower than that of female prespawning sounds. Both sexes produced long, high-intensity croaking sounds during agonistic disputes; these probably help them assess the fighting ability of opponents. On the other hand, only females emitted short, low-intensity purring sounds, which probably initiate and coordinate spawning behaviour. The low sound level and short duration of prespawning sounds indicate that they function as mating displays at very short distances; this probably reduces the chance of attracting competitors or predators. The croaking gourami is the only fish species in which females are known to initiate spawning acoustically.