When Is Temporary Threshold Shift Injurious to Marine Mammals?

Evidence for synaptopathy, the acute loss of afferent auditory nerve terminals, and degeneration of spiral ganglion cells associated with temporary threshold shift (TTS) in traditional laboratory animal models (e.g., mice, guinea pigs) has brought into question whether TTS should be considered a non-injurious form of noise impact in marine mammals. Laboratory animal studies also demonstrate that both neuropathic and non-neuropathic forms of TTS exist, with synaptopathy and neural degeneration beginning over a narrow range of noise exposures differing by similar to 6-9 dB, all of which result in significant TTS. Most TTS studies in marine mammals characterize TTS within minutes of noise exposure cessation, and TTS generally does not achieve the levels measured in neuropathic laboratory animals, which have had initial TTS measurements made 6-24 h post-exposure. Given the recovery of the ear following the cessation of noise exposure, it seems unlikely that the magnitude of nearly all shifts studied in marine mammals to date would be sufficient to induce neuropathy. Although no empirical evidence in marine mammals exists to support this proposition, the regulatory application of impact thresholds based on the onset of TTS (6 dB) is certain to capture the onset of recoverable fatigue without tissue destruction.

Automated summary

Evidence suggests that both neuropathic and non-neuropathic forms of temporary threshold shift (TTS) exist in traditional laboratory animal models, with consequences for auditory nerve terminals and spiral ganglion cells. Studies indicate that TTS in marine mammals may not reach the levels observed in laboratory animals, raising questions about the potential impacts of noise exposure on marine mammal auditory systems.