Among vertebrates, only microchiropteran bats, cetaceans and some rodents are known to produce and detect ultrasounds (frequencies greater than 20 kHz) for the purpose of communication and/or echolocation, suggesting that this capacity might be restricted to mammals1,2. Amphibians, reptiles and most birds generally have limited hearing capacity, with the ability to detect and produce sounds below ∼12 kHz. Here we report evidence of ultrasonic communication in an amphibian, the concave-eared torrent frog (Amolops tormotus) from Huangshan Hot Springs, China. Males of A. tormotus produce diverse bird-like melodic calls with pronounced frequency modulations that often contain spectral energy in the ultrasonic range3,4. To determine whether A. tormotus communicates using ultrasound to avoid masking by the wideband background noise of local fast-flowing streams, or whether the ultrasound is simply a by-product of the sound-production mechanism, we conducted acoustic playback experiments in the frogs' natural habitat. We found that the audible as well as the ultrasonic components of an A. tormotus call can evoke male vocal responses. Electrophysiological recordings from the auditory midbrain confirmed the ultrasonic hearing capacity of these frogs and that of a sympatric species facing similar environmental constraints. This extraordinary upward extension into the ultrasonic range of both the harmonic content of the advertisement calls and the frog's hearing sensitivity is likely to have co-evolved in response to the intense, predominantly low-frequency ambient noise from local streams. Because amphibians are a distinct evolutionary lineage from microchiropterans and cetaceans (which have evolved ultrasonic hearing to minimize congestion in the frequency bands used for sound communication5 and to increase hunting efficacy in darkness2), ultrasonic perception in these animals represents a new example of independent evolution.
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We wish to thank X. Yu and J. Gao for their assistance in the field, E. Ko and B.-P. Umunna for analysis of the sound files, C. Park and J. Stelle for 3D reconstruction of the frog's ear, A. Schaub and B. Siemers for technical assistance with the recording and playback systems, L. Fei for identification of frog species, and M. Hauser, J. Simmons, T. Smith and R. Suthers for comments on the manuscript. This work was supported by grants from the National Institute on Deafness and Other Communication Disorders (to A.S.F. and P.M.N.), a grant from the National Science Foundation to A.S.F., a grant from the State Key Basic Research and Development Plan (China) to C.-H.X., and a grant from the National Natural Sciences Foundation (China) to J.-X.S. Author Contributions A.S.F., P.M.N. and J.-X.S. were responsible for project planning. All authors (except Q.Q.) conducted the behavioural experiments, and A.S.F. and P.M.N. analysed the behavioural data. A.S.F., P.M.N., J.-X.S., Q.Q. and Z.-L.Y. conducted the electrophysiological experiments, and Q.Q. and Z.-L.Y. analysed the physiological data. A.S.F. performed the anatomical experiments and analysed the anatomical data.