A predicted humpback whale hearing curve based on modified behavioral observation audiometry data

Despite decades of research, little capability exists to reliably predict the impact of ocean noise on marine mammals.¹ Noise impact predictions require knowing the frequencies marine mammals can hear and their sensitivity to those sounds (e.g., the hearing threshold, or the susceptibility to noise-induced hearing loss). For odontocetes and pinnipeds, these are well documented using behavioral methods, where the animal is trained to respond to an audible signal,² or auditory evoked potential (AEP) methods, where the neural response of the brain is measured.² Both the hearing range and sensitivity for all species of baleen whales are currently unknown. We used a behavioral observation audiometry (BOA) hearing test method in free-ranging humpback whales (Megaptera novaeangliae). BOA is an established hearing test method used in human children incapable of understanding and carrying out the verbal instructions necessary to perform a hearing test.³ A minimum response level (MRL) is calculated from the lowest sound levels to which the subject shows a response.^3,4,5 It offers a useful estimate of hearing sensitivity when other options are unavailable. Here, this method was used to generate a predicted hearing curve for humpback whales, which was compared with an anatomically derived curve based on studies of the cochlear and middle ear.^6,7,8 The predictive hearing curve presented here, along with new evidence of higher frequency hearing, will assist regulators and stakeholders in assessing the potential impact of anthropogenic sound on large whales.