Many animals communicate with acoustic signals. Crickets rub a hind leg along a row of protruding spikes; rattlesnakes shake a set of beads in their tail. But vocalization is by far the most common mechanism of acoustic communication. Found only among vertebrates, vocalization involves forcing air through a tube across one or more thin membranes located in a tube between the lungs and the mouth. These membranes vibrate, producing rapid fluctuations in air pressure in the outgoing air stream that can be detected as sound by a receiver that is tuned to the appropriate frequencies.

Vocalization results from the interaction of four forces acting on each membrane. The first force is air pressure, which is generated by expelling air from a flexible sac, such as a lung. The second force is the elasticity of the membrane, which returns the membrane to its original position after it is disturbed...