Mechanism, behavior and evolution of calling in four North American treefrogs
Acoustic communication in frogs is an important model system for the study of behavioral evolution in vertebrates. This study aimed to extend the current model on the mechanism of calling in frogs in order to strengthen the bridge between behavioral and physiological knowledge in the field. The limits and tradeoffs of sound production by the passive larynx were examined with artificial laryngeal activation, the pressure-volume relations of the vocal sac and the lungs were determined by inflation, and a comparison of acoustic output was made through laryngeal activation between calling with the mouth open or the mouth closed. The pressure differential across the passive larynx of the four species in this study showed a positive linear correlation with sound amplitude, frequency and airflow. The species examined maximize the intensity of their advertisement calls at the potential cost of producing high frequency calls. These findings indicate that the frequency structure of treefrog advertisement calls might be constrained by the structure of the larynx and selection for producing intense calls. During inflation, air pressure in the vocal sac and in the lungs had a negligible increase at low volumes, and increased exponentially otherwise. Pressure-volume curves and direct measurement showed that the vocal sac can deflate passively, reinflating the lungs. Interspecific differences indicate that the elasticity of the vocal sac might evolve to match calling rates. The vocal sac radiated most of the energy in the call and its tuning matched the dominant frequency of natural calls. The acoustic radiation system allowed frogs to produce more intense and better tuned calls with the mouth closed than with the mouth open. Inflation of the lungs by buccal pumping is probably too slow to support the repetition rate of natural advertisement calls. Both selection for intense calls or fast call rate could be important for the maintenance of the behavior of calling with the mouth closed. This study illustrates that, while selection for call traits can produce evolution of the calling apparatus, functional relations in call production can bias trait evolution.