The neuroethology and underlying neurophysiology of ultrasonic communication in anuran amphibians (frogs and toads) is the focus of my dissertation research. Until recently, it was believed that only a small group of mammalian hearing “specialists” comprised of microchiropteran bats, cetaceans, and some rodents, was able to communicate in frequencies that surpass the upper-frequency limit of human sensitivity (ca. > 20 kHz). The discovery of non-mammalian vertebrates that communicate ultrasonically presents the possibility of gaining a deeper understanding of the evolutionary, ecological and morphological attributes that confer high-frequency hearing sensitivity in all vertebrate forms.
My research is focused on two Southeast Asian ranid frog species, the Chinese concave-eared torrent frog, Odorrana tormota, and the Bornean hole-in-the-head frog, Huia cavitympanum. The peripheral auditory systems of these species share a highly unusual morphological feature: the tympanic membranes (i.e., eardrums) are recessed into the skull. In contrast, the typical anuran eardrum is flush with the side of the head. Odorrana produces calls with substantial spectral energy in the ultrasonic frequency range that has been demonstrated to serve a communicative function. Complementary electrophysiological investigations have determined that the upper-frequency hearing sensitivity of O. tormota extends to ~35 kHz. Thus, this is the first known non-mammalian vertebrate to communicate with ultrasound. The species’ recessed tympana are hypothesized to play a critical role in ultrasound reception by facilitating transmission of high-frequency sound waves through the middle-ear.
During the summer of 2007 we traveled to Sarawak, Malaysia, on the island of Borneo, and recorded the vocalizations of H. cavitympanum males with ultrasonically sensitive equipment to determine whether this species’ recessed tympana are also indicative of ultrasonic communication. We found that the Bornean frogs, like their Chinese counterparts, produce high-frequency calls with substantial ultrasonic harmonic energy. In addition, we discovered that a subset of the call repertoire of H. cavitympanum consists of purely ultrasonic signals. This is the first documentation of exclusively ultrasonic signaling in an amphibian. Acoustic playback experiments performed during the subsequent field season demonstrate that the frogs are behaviorally responsive to purely ultrasonic conspecific calls in their natural environment, providing conclusive evidence that this species uses these calls for communication.
My behavioral research in Borneo is coupled with a multidisciplinary series of laboratory experiments designed to explore the mechanistic bases of ultrasound sensitivity in the peripheral and central auditory systems of O. tormota and H. cavitympanum. I have used electrophysiological techniques to characterize the full frequency-sensitivity range of H. cavitympanum, and found that their hearing sensitivity extends to 38 kHz. In addition, I am mapping the induction of the immediate early gene, egr-1, in the auditory midbrain of O. tormota to characterize differential neuronal activation patterns in response to audible and ultrasonic components of the conspecific call. Finally, I am employing immunohistochemical labeling and confocal microscopy to compare the inner ear and auditory hair cell morphology of these two “ultrasonic” frog species.