Male túngara frogs can add a distinctive note ("chuck”) to their mating call. Production of the chuck involves vibrating a pair of laryngeal fibrous masses that is attached to the vocal cords. The muscular control of this mechanism remains unknown. Recent studies revealed a split in the laryngeal dilator muscle, which unveiled the deep dilator as a novel laryngeal muscle with unique attachments, innervation, and (likely) function. The deep dilator may position the fibrous masses for chuck production. The goals of this study were 1) to confirm the innervation of the novel muscle through electrophysiology; and 2) to determine the action of each laryngeal muscle (including the deep dilator), in isolation and in combination with one another, to elucidate the control of laryngeal function. I stimulated 32 combinations of the five laryngeal muscles electrically with 3-5 repetitions. Using suction glass electrodes, I stimulated the branches of the laryngeal nerves in excised larynges maintained in saline solution and filmed the resulting movements to measure their displacement due to stimulation. The results showed that the novel muscle is exclusively innervated by the short laryngeal nerve, a condition equivalent to that of the mammalian posterior cricoarytenoid muscle, responsible for opening the vocal cords. Also, contraction of the deep dilator muscle is required and sufficient to produce lateral displacement of the fibrous masses and, therefore, to create a chuck. This identifies the deep dilator as a key element in the evolution of call complexity in túngara frogs. Clarifying the mechanism that controls the addition of chucks to the túngara frog call is an important step in understanding the evolution of signal complexity in animal communication systems. The recognition of the mechanism may allow comparative studies to be made that can reveal why complex calling evolved in the túngara frog lineage while not in others.

Sound does not transmit well across the interface of two media. Therefore, most organisms communicate using one medium. Some anurans vocalize at the interface of air and water, though reception of these vocalizations is generally unknown. The túngara frog ( Engystomops pustulosus ) may be the first anuran to have evidence suggesting simultaneous acoustic communication both above and below the air-water interface. This thesis addresses whether the female túngara frog would be receptive to underwater acoustic signals and if males project their advertisement calls at biologically relevant intensities underwater. Females floated and swam with their eardrums and body walls constantly submerged. Using laser Doppler vibrometry, peak vibrations of female eardrums were found to be centered at about 3.5 kHz in air, but dropped to about 1.4 kHz underwater. The peak velocity of the eardrum was about 0.2 mm/s in air and 0.04 mm/s in water when stimulated with tones at 80 dB relative to 20 µPa. Males projected their advertisement calls with a sound pressure level of 121 dB (at 10 cm, re. 20 µPa) in water and 98 dB (at 10 cm, re. 20 µPa) in air. In relation to air, the dominant frequency of the advertisement call (0.8 kHz) was the most intense spectral band underwater whereas the dominant frequency of the chuck (2.5 kHz in air) was less intense. The advertisement signal for the male túngara frog was broadcasted underwater with more energy than in air at its main frequencies. Female eardrums were sensitive to frequencies within the male advertisement call both in air and in water, if the frequencies were transmitted at amplitudes plausible to be encountered in nature. These results strengthen the available evidence of underwater communication, and indicate the presence of auditory specializations in the acoustic communication of this species.