Extratympanic hearing in salamanders: A comparative assessment of structural variation and terrestrial function of an atympanic ear

Abstract

The auditory system mediates the detection of acoustic cues and enhances survival within complex environments by enabling organisms to construct an auditory scene of their surroundings. The tympanic middle ear evolved multiple times in all terrestrial tetrapod lineages to overcome the impedance mismatch encountered by sound pressure at the air-skin boundary, indicating its significance for aerial hearing; however, fossil evidence demonstrates that the earliest terrestrial tetrapods retained aquatically-adapted ears that were unspecialized for detecting airborne sound. How did these unspecialized ears function on land? Comparative study of extant atympanate vertebrates can provide key insights into the ancestral state and early evolution of the terrestrial tetrapod auditory system following the water-to-land transition. In this dissertation, I use atympanate salamanders as a model to investigate the structural and functional parameters underlying terrestrial hearing with unspecialized ears. In chapter one, I review the biology of the salamander auditory system. In chapter two, I characterized morphological variation of the salamander ear and found evidence for habitat-related specialization, suggesting underlying physiological variation. In chapter three, I measured auditory sensitivity to sound pressure and seismic vibration, and observed variation in sensitivity that corroborates the ecomorphological trends reported in chapter two. I assessed the contributions of hypothesized extratympanic pathways for hearing, including seismic sensitivity, cavity resonance, and bone conduction. I determined that aerial auditory sensitivity is mediated by bone conduction of sound as head vibrations that are detectable to the inner ear. In chapter four, I evaluated the sound localization capabilities of an atympanic ear. I found that bone conduction hearing in salamanders supports a figure-eight pattern of directional sensitivity to airborne sound. I contextualize my findings with other studies of tympanate and atympanate taxa and suggest that bone conduction may represent a general mechanism enabling aerial sound detection and localization in terrestrial species with atympanic ears.