Underwater Hearing in the Loggerhead Turtle (Caretta caretta): A Comparison of Behavioral and Auditory Evoked Potential Audiograms

Methods for collecting behavioral audiograms are often time consuming and require trained, captive subjects. It is more practical to measure hearing sensitivity using electrophysiological methods, such as auditory evoked potential (AEP) testing, in which electrodes measure action potentials in response to acoustic stimuli. These data can be collected in a matter of hours. However, results should be verified through behavioral testing. Current knowledge of marine turtle auditory abilities is based on a few electrophysiological tests. The purpose of this study was to collect and compare behavioral and auditory evoked potential audiograms in a captive adult loggerhead turtle (Caretta caretta). The behavioral audiogram was collected using a go/no-go modified staircase method utilizing 2-second pure-tone stimuli. AEP thresholds were measured underwater using subdermal electrodes placed beneath the frontoparietal scale, dorsal to the midbrain. Action potentials were measured in response to 50 ms tonal stimuli and averaged over a maximum of 1,000 responses. Evoked potential testing yielded thresholds from 100 - 1131 Hz with peak sensitivity at 200 and 400 Hz (110 dB re 1 µPa). Behavioral testing yielded thresholds from 50 - 800 Hz with peak sensitivity at 100 Hz (98 dB re 1 µPa). Behavioral thresholds averaged 8 dB lower than AEP thresholds from 100 to 400 Hz and 5 dB higher at 800 Hz. Results indicate that behavioral and evoked potential techniques are suitable for determining marine turtle hearing sensitivity. AEP testing is a good alternative when dealing with wild or untrained animals and when time is a critical factor