1. Department of Molecular and Integrative Physiology & Beckman Institute, University of Illinois, Urbana, Illinois 61801, USA
2. Departments of Physiological Science and Ecology & Evolutionary Biology, University of California, Los Angeles, California 90095, USA
3. Shanghai Institutes of Biological Sciences, Chinese Academy of Sciences, Shanghai 200032, China
4. State Key Laboratory of Brain and Cognitive Science, Institute of Biophysics, Chinese Academy of Sciences, Beijing 100101, China
5. *These authors contributed equally to this work

Correspondence to: Albert S. Feng^1,5 Correspondence and requests for materials should be addressed to A.S.F. (Email: afeng1@uiuc.edu).

Abstract

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 mammals^{1, 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 range^{3, 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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