Abstract
Rising levels of anthropogenic noise throughout the world’s oceans have created growing concern about the impact of sound on many marine species. Sea turtles do not appear to vocalize or use sound for communication, but may use sound for navigation, locating prey, avoiding predators, and general environmental awareness. Endangered leatherback sea turtles (Dermochelys coriacea) have the largest latitudinal distribution of all sea turtles, foraging in high-latitude sub-polar waters and nesting on low-latitude tropical beaches. Much of their habitat overlaps with sound-producing activities, exposing them to anthropogenic sounds such as: oil and gas exploration and extraction, shipping, construction, and sonar.
To determine if leatherbacks are capable of detecting these sounds, we measured the hearing sensitivity of hatchlings in water (n=11) or air (n=12) by recording auditory evoked potentials (AEPs). AEPs are produced by the synchronous discharge of neurons in the auditory pathway of the central auditory nervous system after acoustic stimulation detectable by the ear. Before testing, we isolated hatchlings from noise and vibrations and lightly restrained them to prevent movement that would mask AEP signals. To further reduce myogenic artifacts, we sedated (underwater: n=11; air: n=7) or anesthetized (air: n=5) hatchlings. For underwater measurements, we submerged hatchlings 14 cm and presented stimuli with an underwater speaker (Clark Synthesis, Inc. AC339), calibrated with a hydrophone (High Tech, Inc. HTI-96-MIN). We recorded AEPs during 45-60 second intervals, raising hatchlings to the surface to breathe between intervals. For aerial measurements, we placed hatchlings on foam pads to reduce the opportunity for detection of vibratory stimuli, and presented stimuli with an aerial speaker (Definitive Technology, Inc. DI6.5R), calibrated with a microphone (LinearX Systems, Inc. M31). An Evoked Potential Workstation run by laptop computer with SigGenRP and BioSigRP software (Tucker-Davis Technologies, Inc.) generated stimuli and recorded AEP responses. Using a three-electrode array, we recorded responses to 50 ms pulsed tonal stimuli between 50 and 1600 Hz, beginning at the loudest producible level and attenuating in 6 dB steps until no AEP response could be detected. We determined that an AEP response was present if the recorded signal showed a peak in the frequency domain twice that of the stimulus frequency and ≥6 dB above the noise floor 100 Hz on either side of the peak. We defined threshold as the lowest level we detected an AEP response. We monitored hatchling respiratory and heart rates throughout the experiment and measured blood gas values at the completion of the experiment.
Results showed that leatherback sea turtle hatchlings are able to detect sounds underwater and in air, responding to stimuli between 50 and 1200 Hz in water and 50 and 1600 Hz in air, with maximum sensitivity between 100 and 400 Hz in water (84 dB re: 1 μPa-rms at 300 Hz) and 50 and 400 Hz in air (62 dB re: 20 μPa-rms at 300 Hz). These represent the first measurements of leatherback hearing sensitivity and, like other species of sea turtle for which hearing has been measured, they appear to have a relatively narrow, low-frequency range of hearing sensitivity. Sedation or anesthesia proved to be a successful technique for facilitating the collection of AEPs. Anesthesia had little effect on measured hearing sensitivity with average thresholds for anesthetized hatchlings