Page 73 - Winter 2020

consequences of the delayed development of the tympanic pathway are unknown.
Our understanding of sound processing in bullfrog tadpoles is based on inferences from ear structure and on recordings of neural activity from auditory nuclei in the central nervous system. Neural responses to underwater sounds (particle motion) can be recorded in bullfrog tadpoles early in life, even before the appearance of the operculum (Simmons, 2019). These responses may originate from direct activation of saccular hair cells by sound waves passing through the body, in a “fenestral” route (Hetherington, 1987). Once the operculum has developed, all three auditory organs are likely activated by sounds, as shown by the wider range of sound frequencies that evoke good neural activity. With the appearance of the tympanum after the completion of metamorphosis, neural responses to airborne sounds become more similar to those in adults (Simmons, 2019).
“Croaking” Forward
This short review highlighted some general trends in middle and inner ear structure and function in adult and developing frogs, but our picture is incomplete. Frogs are a diverse order, with considerable variation in habitat and in the role of sounds in their lives. Much of our knowledge of frog bioacoustics is based on research on a few key species. The recent discovery of ultrasound-detecting species highlights the surprises that are waiting to be uncovered (Feng et al., 2006). Even in well-studied species, there is more to learn about the function of the operculum and the role of lung transmission. The frog middle ear can show malformations (Horowitz et al., 2005), and understanding their origins can contribute to our knowledge of developmental and environmental factors affecting hearing.
Our current view of ear development across metamorphosis is based on a small number of species. It is not clear if the developmental time course of hearing identified in bullfrog tadpoles is the same as in other species with different ear morphologies. When does the tonotopy of the amphibian papilla develop? When does transmission through the lungs emerge? What is the maturational sequence in totally aquatic frogs? Finally, we know very little about the tadpoles’ acoustic worlds. Frog bioacoustics is a rich line of research that can contribute significantly to our understanding of hearing across the life span and in response to changing environmental demands.
Acknowledgments
Work from Andrea Megela Simmons’ laboratory reviewed here has been funded by the National Institute on Deafness and Communication Disorders, National Institutes of Health, Bethesda, MD. MicroCT images were obtained with the assistance of Aaron Nakasone, Micro-CT and X-ray Microscopy Imaging Facility, Boston University, Boston, MA.
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