Hearing in Cheloniid Sea Turtles: Thresholds, Effects of Body Temperature, and Auditory Masking

Abstract

Anthropogenic noise poses a growing threat to marine organisms, particularly those specialized for low-frequency hearing. Sea turtles, whose nearshore habitats increasingly overlap with human activities such as vessel traffic, seismic exploration, and coastal construction, are vulnerable in this regard. However, major data gaps have limited our ability to assess the potential impacts of noise on the species. This dissertation addresses several of those gaps by measuring auditory sensitivity in four species of cheloniid sea turtles, examining how body temperature modulates that sensitivity, and quantifying susceptibility to auditory masking from environmental noise.In the first study, we measured underwater hearing thresholds in 47 juvenile individuals representing green (Chelonia mydas), Kemp’s ridley (Lepidochelys kempii), hawksbill (Eretmochelys imbricata), and loggerhead (Caretta caretta) sea turtles using auditory evoked potential (AEP) methods. All species showed greatest sensitivity between 200 and 400 Hz with reduced sensitivity above 600 Hz, confirming that sea turtles are susceptible to the dominant frequencies of anthropogenic noise in the marine environment. In the second study, we examined temperature-dependent hearing sensitivity in green sea turtles. Using a repeated-measures design, we tested six cold-stunned individuals across a temperature range of 10.4 to 24.1°C during their rehabilitation. Sensitivity improved with increasing body temperature, particularly between 200 and 600 Hz, with audiograms transitioning from flat or inverted U-shapes at low temperatures to more sharply tuned U-shaped profiles at higher temperatures. Body temperature had the greatest influence on hearing sensitivity at 400 Hz, where hearing thresholds decreased (sensitivity increased) by up to 4.5 dB for every 1°C increase in body temperature. These results demonstrate that hearing sensitivity in sea turtles is physiologically dynamic and influenced by thermal conditions across their habitat range. This has broad implications for interpreting previously published audiograms, which were obtained under narrow (and sometimes unreported) temperature conditions, and for assessing the seasonal vulnerability of sea turtles to noise exposure across their geographic range. In the third study, we measured critical ratios (CRs) in loggerhead sea turtles in both air and underwater. We tested six individuals in air and two individuals underwater. Aerial CRs ranged from 13 to 35 dB across frequencies, with the lowest values (i.e., greatest frequency selectivity) occurring from 200 to 400 Hz. Underwater CRs followed a similar pattern, ranging from 13 to 28 dB, with lowest values occurring at 200 Hz. In both media, CRs were highest at the periphery of the hearing range and lowest near frequencies of peak sensitivity. Based on the measured underwater CR at 200 Hz and corresponding audiogram data, we estimate masking onset at 200 Hz to occur when ambient underwater noise exceeds 82 dB re 1 µPa²/Hz. This threshold is routinely surpassed in many coastal environments, as evidenced by long-term acoustic monitoring studies. Therefore, auditory masking likely represents a widespread and chronic source of disturbance for sea turtles in areas of high anthropogenic activity. Together, these findings advance the understanding of sea turtle auditory biology by providing species-specific audiograms, demonstrating context-dependent shifts in sensitivity, and establishing quantitative benchmarks for auditory masking. They directly address priorities identified by federal agencies and provide essential input for the development of sound exposure guidelines and conservation strategies. This research highlights the importance of considering environmental and physiological variability when assessing noise impacts on marine turtles and underscores the need for adaptive mitigation approaches in an increasingly noisy ocean.