MEMRO 2009 - Comments, Forums, and Blogs

Marcos Gridi-Papp(1), Victoria S. Arch(2) and Peter M. Narins(1,2)
Affiliation: 1) Department of Physiological Science, University of California, 621 Charles E. Young Dr. S., Los Angeles, CA 90095 USA; 2) Department of Ecology and Evolutionary Biology, University of California, Los Angeles, CA 90095, USA.

ABSTRACT:
Amphibians were believed, until recently, not to hear frequencies above 5-8 kHz, possibly because the link between the tympanic membrane and the oval window consists of a single ossicle with cartilagenous extensions that should absorb high frequencies. Recent studies on Asian frogs revealed two species (Odorrana tormota and Huia cavitympanum) that communicate acoustically using ultrasound and are sensitive up to 38 kHz. In this study we employed laser Doppler vibrometry to describe the TM's vibration response to sound in both ultrasound-hearing Asian frogs. We also used selective muscular stimulation and video imaging to document the discovery of a mechanism of behavioral control of ET closure. Our vibrometric measurements confirmed that in both species, the vibration response range of the TM extends above 35 kHz. We also found that, in O. tormota, contractions of the submaxillary and petrohyoid muscles cause pivoting of the anterior hyoid horn over its attachment to the skull, closing the ET. Such closure shifts the acoustic response of the middle ear, producing up to 20 dB gain above 10 kHz and up to 26 dB attenuation below 10 kHz. Mathematical modeling indicates that this spectral shift is due to the extreme reduction of the volume of air behind the TM when the ET is closed and the middle ear is isolated from the mouth. ET closure was observed to always accompany the phonatory phase of calling, suggesting a protective role against self-produced intense sound. The study of hearing in Asian frogs is not only demanding a reevaluation of the spectral limits of non-mammalian middle ears, but it also has revealed a novel mechanism of behavioral middle ear tuning in vertebrates. Supported by NIH grant no. DC-00222, the P. S. Veneklasen Research Foundation and the UCLA Academic Senate grant no. 3501 to PMN.

Total time: 20 minutes