Bridging the hearing data gap in the endangered Australian sea lion (Neophoca cinerea) through biomechanical modelling

The endangered Australian sea lion ( Neophoca cinerea ) divides its functional ecology between land (for mating and pup-rearing) and sea (for foraging). Given this amphibious lifestyle, it is susceptible to natural and anthropogenic sound exposure in both air and water. Hearing data for the species are limited to a small sample size (two individuals) of behavioural measurements in air at only a few frequencies, and no underwater hearing thresholds are currently available. Understanding its auditory sensitivity is important for assessing potential risks from anthropogenic sound across its habitat. This study used computed tomography-based finite-element modelling of three post-mortem Australian sea lion heads to simulate frequency-dependent sound reception across broad frequency ranges in air and under water. Transfer functions were calculated as the ratio of stapes-footplate velocity to incident sound pressure, providing a quantitative measure of mechanical sound transmission to the inner ear. The models indicated differences in sound reception between the two media. The transfer functions were subsequently converted into estimated audiogram curves by anchoring the minimum thresholds to published values for Australian sea lions and closely related otariid species. The resulting curves were broadly consistent with published otariid audiograms. This study provides a mechanistic description of aerial and underwater sound reception in Australian sea lions, together with an anatomically informed basis for estimating hearing sensitivity and identifying frequency ranges relevant to anthropogenic sound exposure. More broadly, it provides the first mechanistic estimate of underwater hearing in ASLs and demonstrates the potential of CT-based finite-element modelling as a conservation physiology tool for inaccessible marine mammals.