How a chorus frog sings

Pacific Treefrogs, or chorus frogs, are the state's smallest frog and its loudest. They vary from green to gray but always have a white underbelly and a dark band running through the eyes. Here is how males call to attract a mate.

To call:

1. The frog opens its nostrils, inhales and fills its lungs with air.

2. Then it closes its nostrils and forces the air from its lungs across the vocal chords in the larynx and into the vocal sac. Vocal chords vibrate as air passes over them.

3. The vocal sac acts as a resonator (like a organ pipe) and amplifies the sound. Air is shunted back and forth from the lungs to the sac to produce repeated calls without having to refill the lungs. The rate of calling depends on how fast air is moved between the lungs and the sac.

Sources: USGS Northern Prairie Wildlife Research Center, "The Way Nature Works"; Macmillan Publishing, University of Washington, Seattle Times research

MICHELE LEE MCMULLEN / THE SEATTLE TIMES