Fully inflated vocal sac becomes a sound resonating chamber like the wooden case of a musical instrument. A tiny blood-sucking fly which may carry parasites lands on the sac. In a study of this frog's behavior and evolution by Michael J. Ryan and A. Stanley Rand, researchers at the Smithsonian Tropical Research Institute in Panama (inset) prepare to watch a real-time video of a tungara frog in a darkened room when calls are broadcast into the room.

© Gregory G. and Mary Beth Dimijian