While some electrophysiological auditory studies have been conducted on sea turtles, little is currently known about sea turtle hearing capabilities throughout ontogeny or how electrophysiological data correlate with behavioral responses, a necessary step for comprehensive hearing assessment. For this study we employed two independent but complementary approaches, i.e., behavioral and electrophysiological audiography, to assess hearing in two different size classes (i.e., post-hatchling and juvenile) of loggerhead sea turtles Caretta caretta. Behavioral trials involved first training turtles to respond to known frequencies, a multi-stage, time-intensive process, and then recording their behavior when they were presented with sound stimuli from an underwater speaker using a LabVIEW-based stimulus delivery and data acquisition system. A two-response, forced-choice approach was used, whereby the turtles selected one chute when sound was detected and another when it was not. Electrophysiological experiments involved submerging restrained, fully conscious turtles just below the air-water interface so that their ears were underwater but breathing was not restricted, and recording auditory evoked potentials (AEPs) using a Tucker-Davis Technologies system when sound stimuli were presented using an underwater speaker. Sound pressure levels (SPLs) and particle motions (i.e., particle velocity and particle acceleration) were also recorded. No ontogenetic differences in behavior-derived thresholds and sensitivity ranges were detected, and there was no difference in response speed (body lengths s-1) between hatchlings and juveniles or between suprathreshold and threshold trials. The only signifcant response speed difference was between correct and incorrect trials, with turtles swimming slower when making an incorrect choice relative to a correct choice. As was the case for behavior data, AEP-derived thresholds and sensitivity ranges were similar for post hatchling and juvenile sea turtles. At behavioral thresholds, particle accelerations and particle velocities were ~10-4 – 10-3 m s-2 and ~10-8 – 10-7 m s-1, respectively, which are at or below the detection limits of the most sensitive fishes. Based on these low particle motions, negative buoyancy of the turtle, and the anatomy of the sea turtle ear, which lacks an otolith-based accelerometer system, the pressure component and not particle motion component of sound mostly likely drove the observed thresholds, though this was not tested directly in this project. While the hearing frequency range detected in both behavior and AEP experiments were consistent (50 – 1200 Hz), both posthatchlings and juveniles had significantly higher AEP-derived (mean = 126.6 re 1 µPa over hearing range) than behavior-derived (mean = 97.1 re 1 µPa over hearing range) auditory thresholds. This is an important finding for it indicates that AEP tests are less sensitive than behavioral tests and should not be used to set the standard for sound exposure levels in the field. Collectively, data from this project help define the hearing frequency range and threshold of two ontogenetic stages of turtles and provide a means to evaluate future electrophysiological audiograms. However, more research in the areas of hearing loss/damage, hair cell regeneration, masking, and in situ behavioral responses to sound are needed to better define the impact of human-made sound sources on sea turtles.
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