A species of fish makes grunting sounds by grinding its teeth. The French grunt fish (Haemulon flavolineatum; pictured) is thought to groan in distress by rubbing together its pharyngeal jaws, a second pair of toothed jaws located in the throat that help to process food. Frédéric Bertucci of the University of Liége in Belgium and his co-workers used X-ray cameras to capture high-speed video of the animals and found that the sound-making process was similar to chewing, but slightly slower. Electron microscope images of the teeth revealed signs of erosion.
The fish's hearing is not tuned specifically to the frequencies of the grunts, suggesting that the pharyngeal jaws first evolved for eating and were later co-opted for communication.