Grunt variation in the oyster toadfish Opsanus tau: effect of size and sex.

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As in insects, frogs and birds, vocal activity in fishes tends to be more developed in males than in females, and sonic swimbladder muscles may be sexually dimorphic, i.e., either larger in males or present only in males. Male oyster toadfish Opsanus tau L produce a long duration, tonal boatwhistle advertisement call, and both sexes grunt, a short duration more pulsatile agonistic call. Sonic muscles are present in both sexes but larger in males. We tested the hypothesis that males would call more than females by inducing grunts in toadfish of various sizes held in a net and determined incidence of calling and developmental changes in grunt parameters. A small number of fish were recorded twice to examine call repeatability. Both sexes were equally likely to grunt, and grunt parameters (sound pressure level (SPL), individual range in SPL, number of grunts, and fundamental frequency) were similar in both sexes. SPL increased with fish size before leveling off in fish >200 g, and fundamental frequency and other parameters did not change with fish size. Number of grunts in a train, grunt duration and inter-grunt interval were highly variable in fish recorded twice suggesting that grunt parameters reflect internal motivation rather than different messages. Grunt production may explain the presence of well-developed sonic muscles in females and suggests that females have an active but unexplored vocal life.