Abstract
Huia cavitympanum, an endemic Bornean frog, is the first amphibian species known to emit exclusively ultrasonic (i.e., >20 kHz) vocal signals. To test the hypothesis that these frogs use purely ultrasonic vocalizations for intraspecific communication, we performed playback experiments with male frogs in their natural calling sites. We found that the frogs respond with increased calling to broadcasts of conspecific calls containing only ultrasound. The field study was complemented by electrophysiological recordings from the auditory midbrain and by laser Doppler vibrometer measurements of the tympanic membrane's response to acoustic stimulation. These measurements revealed that the frog's auditory system is broadly tuned over high frequencies, with peak sensitivity occurring within the ultrasonic frequency range. Our results demonstrate that H. cavitympanum is the first non-mammalian vertebrate described to communicate with purely ultrasonic acoustic signals. These data suggest that further examination of the similarities and differences in the high-frequency/ultrasonic communication systems of H. cavitympanum and Odorrana tormota, an unrelated frog species that produces and detects ultrasound but does not emit exclusively ultrasonic calls, will afford new insights into the mechanisms underlying vertebrate high-frequency communication.
Highlights
The nearly universal ability of mammals to hear high-frequency sounds has led this taxon to be considered uniquely specialized among vertebrates
We presented H. cavitympanum males with a putative competitive stimulus comprised of the purely US call of a conspecific male
Males of H. cavitympanum, are the only non-mammalian vertebrates currently known to produce and detect vocalizations consisting of spectral energy entirely above the upper limit of human sensitivity
Summary
The nearly universal ability of mammals to hear high-frequency sounds has led this taxon to be considered uniquely specialized among vertebrates. Humans are among the poorer performers in the class; our putative high-frequency cut-off is 20 kHz, and we have labeled frequencies above this boundary ‘‘ultrasound’’. This anthropocentric designation ignores the fact that the vast majority of mammals tested hear well into the ultrasonic range, with specialists, such as echolocators, hearing up to and beyond 100 kHz. Non-mammalian vertebrates are comparatively restricted in their high-frequency hearing sensitivity. For the sake of generality, we adhere to the anthropocentric designation of extraordinarily high frequencies as those in the ultrasonic range, and consider ‘‘high frequencies’’ to be those exceeding 10 kHz
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