In-air hearing in Hawaiian monk seals: implications for understanding the auditory biology of Monachinae seals

Abstract

The auditory biology of Monachinae seals is poorly understood. Limited audiometric data and certain anatomical features suggest that these seals may have reduced sensitivity to airborne sounds compared to related species. Here, we describe the in-air hearing abilities of a Hawaiian monk seal (Neomonachus schauinslandi) trained to participate in a psychophysical paradigm. We report absolute (unmasked) thresholds for narrowband signals measured in quiet conditions across the range of hearing and masked thresholds measured in the presence of octave-band noise at two frequencies. The behavioral audiogram indicates a functional hearing range from 0.1 to 33 kHz and poor sensitivity, with detection thresholds above 40 dB re 20 µPa. Critical ratio measurements are elevated compared to those of other seals. The apparently reduced terrestrial hearing ability of this individual—considered with available auditory data for a northern elephant seal (Mirounga angustirostris)—suggests that hearing in Monachinae seals differs from that of the highly sensitive Phocinae seals. Exploration of phylogenetic relationships and anatomical traits support this claim. This work advances understanding of the evolution of hearing in amphibious marine mammals and provides updated information that can be used for management and conservation of endangered Hawaiian monk seals.

Acoustic Monitoring of Professionally Managed Marine Mammals for Health and Welfare Insights

“…Behavioral hearing tests have been successfully trained in bottlenose dolphins, (e.g., Tursiops sp. [ 55 ]; killer whales ( Orcinus orca, [ 56 , 57 ]); false killer whales ( Pseudorca crassidens, [ 58 ]); Pacific white-sided dolphins ( Lagenorhynchus obliquidens , [ 59 ]); Tucuxis ( Sotalia fluviatilis , [ 60 ]); harbor porpoises ( Phocoena phocoena, [ 61 ]); California sea lions ( Zalophus californianus, [ 62 , 63 ]); Stellar sea lions ( Eumetopias jubatus , [ 64 , 65 ]); harbor seals ( Phoca vitulina, [ 62 , 63 ]); Harp seals ( Pagophilus groenlandicus, [ 66 ]); spotted seals ( Phoca largha , [ 63 ]); ringed seals ( Pusa hispida , [ 67 ]); bearded seals ( Erignathus barbatus; [ 68 ]); northern elephant seals ( Mirounga angustirostris , [ 62 ]); northern fur seals ( Callorhinus ursinus , [ 69 ]); Hawaiian monk seals ( Neomonachus schauinslandi, [ 70 ]); Pacific walrus ( Odobenus rosmarus divergens , [ 71 ]); belugas ( Delphinapterus leucas , [ 72 ]); Florida manatees ( Trichechus manatus latirostris , [ 73 ]); polar bears ( Ursus maritimus , [ 74 ]); and sea otters ( Enhydra lutris , [ 75 ]). Behavioral hearing tests are the “gold-standard” of tests to determine individual animal sensitivity and range.…”

Mechanisms of auditory masking in marine mammals

“…3 A). Among pinnipeds, critical ratio measurements are available for nine species: Callorhinus ursinus (Moore and Schusterman, 1987 ), Erignathus barbatus (Sills et al 2020 ), Mirounga angustirostris (Southall et al 2000 , 2003 ), Neomonachus schauinslandi (Ruscher et al 2021 ), Pagophilus groenlandicus (Terhune and Ronald 1971 ), Phoca largha (Sills et al 2014 ), Phoca vitulina (Renouf 1980 ; Southall et al 2000 , 2003 ; Terhune 1991 ; Turnbull 1994 ; Turnbull and Terhune 1990 , 1993 ) , Pusa hispida (Sills et al 2015 ; Terhune and Ronald 1975 ), and Zalophus californianus (Southall et al 2000 ; 2003 ). Critical ratios measured for seals and sea lions (Fig.…”