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

Brandi Ruscher,Beau P Richter,Colleen Reichmuth,Jillian M Sills

doi:10.1007/s00359-021-01498-y

Brandi Ruscher, Beau P Richter + Show 2 more

Open Access

https://doi.org/10.1007/s00359-021-01498-y

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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.

Highlights

The Hawaiian monk seal, Neomonachus schauinslandi, is a phocid species endemic to the Northwestern and main Hawaiian Islands
The audiogram lacked the characteristic U-shape of mammalian hearing curves and instead was relatively flat with a distinct elevation at 6.4 kHz
Hearing range was constrained in both the low- and high-frequency regions of the audiogram relative to that of Phocinae seals evaluated under the same conditions; best sensitivity was approximately 50 dB higher

Summary

Introduction

The Hawaiian monk seal, Neomonachus schauinslandi, is a phocid (true seal) species endemic to the Northwestern and main Hawaiian Islands. Monk seals are unique in comparison to seals living at higher latitudes as they experience relatively stable environmental conditions and resources year round. For this reason, monk seals do not breed synchronously during a brief, predictable period each year like temperate and polar seals that show much stronger seasonality in behavior. While parturition tends to occur during spring and summer, females can give birth throughout much of the year (Kenyon and Rice 1959; Johnson and Johnson 1984; Johanos et al 1994). This enables males to continuously compete for access to dispersed females that apparently come into estrous within a few weeks of weaning their pups (Johnson and Johnson 1984; Atkinson and Gilmartin 1992; Johanos et al 1994)

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