Do female oyster toadfish grunt as readily as males, despite having smaller sound-producing muscles?

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As in insects, frogs and birds, vocal activity in fishes tends to be more developed in males than in females. Sound-producing swimbladder muscles can differ between sexes: they may be larger in males or present only in males.

Male oyster toadfish (Opsanus tau L) produce a long, tonal advertisement call known as a boatwhistle. Both sexes also grunt—a short, more pulsatile call used in agonistic interactions. Both sexes have sound-producing muscles, but these are larger in males.

Researchers tested the hypothesis that males would call more than females by inducing grunts in oyster toadfish of various sizes held in a net. They measured how often fish called and examined developmental changes in grunt parameters. A small number of fish were recorded twice to examine call repeatability.

Both sexes were equally likely to grunt. Sound pressure level (SPL), each individual's range in SPL, number of grunts and fundamental frequency were similar in females and males. SPL increased with fish size before leveling off in fish weighing >200 g. Fundamental frequency and the other parameters did not change with fish size.

In fish recorded twice, the number of grunts in a train, grunt duration and the interval between grunts were highly variable. This suggests that grunt parameters reflect internal motivation rather than different messages.

Grunt production may explain why females have well-developed sound-producing muscles. The findings also suggest that females have an active but unexplored vocal life.