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.

Credit: Michael Patrick O'Neill/OceanwideImages.com

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.

J. Exp. Biol. 217, 3862–3869 (2014)