Scientists had thought that the Gardiner’s Seychelles frog—at 11 millimeters among the tiniest in the world—was deaf because it doesn’t have a middle ear, a critical component of hearing that’s found most land animals.

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So the scientists x-rayed one of the tiny frogs. The images revealed that the frogs’ pulmonary system is poorly developed, suggesting that the lungs aren’t contributing to hearing, according to the study, published September 2 in the Proceedings of the National Academy of Sciences.

So the scientists refocused their experiments on the frogs’ heads. By making various 3-D simulations of how sound travels through the frogs’ heads, the scientists found that the bones in their mouths act as an amplifier for sound waves.