Mechanism, behavior and evolution of calling in four North American treefrogs
Abstract
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.
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