Meeting Abstract

P2.77  Wednesday, Jan. 5  Allometry of the Middle Ear Cavity in Trachemys scripta elegans WILLIS, Katie L*; POTTER , Kimberlee; CARR, Catherine E; Univ of Maryland; Armed Forces Institute of Pathology; Univ of Maryland kwillis@umd.edu

One notable characteristic of many chelonian species is a large middle ear cavity. An early hypothesis for the function of this cavity was that it resonated at ecologically important frequencies, enhancing sound perception. Wever (1978), however, calculated the best resonance frequency for the cavity in air to be 6kHz, above the upper limit (1200 Hz) of the turtle’s audiogram (Patterson, 1966). Auditory brainstem responses and laser vibrometry in lightly anesthetized turtles, however, revealed a best sensitivity to underwater sound pressures at 300-500 Hz (Christensen-Dalsgaard et al., 2010). The ear was about 10dB less sensitive to underwater sound pressures than in air, which in terms of sound intensity shows that thresholds in water are lower than in air, indicating a specialization for underwater hearing. We hypothesize that the large middle ear cavity is adapted for hearing underwater and that its best resonance frequency underwater will reflect the animal’s auditory needs. We used MRI (7 or 9T) to obtain accurate information about the cavity. Animals from 4 to 8 inches in length were imaged. Image stacks were reconstructed in 3-D and analyzed using NeuroLucida by tracing the edge of each cavity and using the volume calculator. The middle ear cavity has an approximately elliptical form for most of the space, scaling with head size, which indicates a small decrease in resonance frequency with increasing head size. The outer boundary of the cavity is a cartilaginous disk, covered by skin. The extracolumella is connected to the cartilaginous disk and the columella. This structure allows the cartilaginous disk to be vibrated from either outside or inside the cavity. Thus motion of the air in the middle ear cavity could drive the tympanic disk underwater.