Ultrasonic communication in frogs

While previous work had indicated that a species of frog and a songbird, both from China, were capable of producing calls with ultrasonic frequencies (> 20 kHz), here Feng and colleagues (2006) provide evidence that the frog (Amolops tormotus) uses those ultrasonic calls to communicate. Before this discovery, the production and detection of ultrasound was only known from a few groups of mammals.

In order to test whether the frogs could detect the ultrasonic components of conspecific calls, the authors conducted playback experiments. Wild frogs increased their call rate in response to the presentation of the ultrasonic stimuli. Additionally, the auditory processing center of the midbrain of anesthetized frogs in the lab produced action potentials in response to tones from 1 to 34 kHz. A similar sensitivity (up to 22 kHz) was shown for a sympatric stream-dwelling species, but not for a pond-dwelling species of frog. By experimentally blocking their ear canals, the authors confirmed that stimulation of the ear is responsible for the frogs’ ultrasonic sensitivity.

The authors suggest that ultrasonic production and detection is a response by stream-dwelling frogs to avoid signal interference by the broadband noise of the water. Although most amphibians can produce and detect sounds only up to about 12 kHz, the fast streams that are inhabited by A. tormotus create a broad energy spectrum of noise all the way up to 22 kHz. The authors analyzed the anatomy of the frog and encountered several adaptations responsible its unique sensitivity: the presence of ear canals (unlike most other frogs) with a complex shape (which may increase the resonance of higher frequencies), as well as lighter-than-normal ossicles and exceptionally thin tympana (which improve the transmission of high-frequency sounds).

Feng et al. (2006) provide extensive evidence of the ultrasonic abilities of a species that has evolved to solve problems of the production, transmission, and reception (the main focus of the paper) of vocal signals in a noisy environment.

Feng, A. S., P. M. Narins, C.-H. Xu, W.-Y. Lin, Z.-L. Yu, Q. Qiu, Z.-M. Xu, and J.-X. Shen. 2006. Ultrasonic communication in frogs. Nature. 440:333–336.