Why does a calmly sitting frog's throat vibrate? 0 12

Amphibians do not use intercostal muscles for breathing, and frogs (like other tailless amphibians) do not have ribs at all. To draw air into their lungs, they use a specialized sublingual apparatus that moves the lower wall of the pharyngeal cavity.

When this wall lowers, the pharyngeal cavity expands — and air enters it through the nostrils; then the nostrils close with valves, the wall rises — and air is pushed into the lungs. It is these movements of the lower wall that we observe.