Whether defending a harem or a monogamous mate in the wild, forcepsfish and longnose butterflyfish warn competitors away with a single percussive popping sound followed by a flick of the head. Kelly Boyle and Timothy Tricas from the University of Hawai'i at Mānoa say, ‘Sounds may reflect physical attributes of the sound production mechanism, be constrained by body size and therefore control signal reliability during agonistic behaviours.’ However, it was unclear how both species produced their sound effects and what information the threatening pop conveys (p. 3829). Introducing a competitor into the territory of a forcepsfish or longnose butterflyfish in a laboratory aquarium, Boyle and Tricas filmed and recorded the defender's response and found that the fish began generating the sound well before tossing the head back: instead of producing the popping sound, the head flick was a by-product of the sound production mechanism. Analysing the fish's muscle activity, the duo saw that muscles in the jaw and neck are activated to pull back the pectoral girdle, ribs and rear half of the swim bladder to initiate the sound, before releasing the head so that it is jerked back even faster than is seen when snapping at prey. The duo also found that the largest forcepsfish produced the loudest and longest pops. ‘Forcepsfish sounds may be accurate indicators of size and condition that are related to resource holding potential during social encounters,’ they say.

K. S.
T. C.
Sound production in the longnose butterflyfishes (genus Forcipiger): cranial kinematics, muscle activity and honest signals
J. Exp. Biol.