Underwater hearing and communication in the endangered Hawaiian monk seal Neomonachus schauinslandi

Abstract

Hawaiian monk seals are among the most endangered marine mammals and the most basal of the phocid seals. The auditory biology of monk seals is compelling from behavioral, evolutionary, and conservation perspectives, but we presently lack substantive bioacoustic information for this species, with no formal descriptions of underwater vocalizations and limited data concerning hearing. These seals have been isolated for more than 10 million yr and have auditory structures differing from those of related species. Additionally, unlike other aquatically mating phocids, monk seals breed asynchronously and are not known to produce social calls in water. To address existing knowledge gaps, we trained a mature male Hawaiian monk seal to perform a psychophysical task while submerged. Detection thresholds were measured for narrowband sounds across the frequency range of hearing. We also conducted a year-round characterization of the seal’s spontaneous underwater vocalizations. This individual demonstrated best hearing between 0.2 and 33 kHz, with a lower high-frequency roll-off than that of related species. Hearing at all frequencies was less sensitive than in other true seals. Despite the absence of conspecifics, the seal regularly produced 6 different underwater calls with energy below 1 kHz. Calling patterns reflected a period of annual reproductive activity lasting about 6 mo, coincident with elevated testosterone levels. This study presents the first examination of underwater vocalizations in Hawaiian monk seals, provides insight into the auditory abilities of this species and the evolution of underwater hearing among phocids, and enables improved assessments of noise effects on these vulnerable seals.

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

“…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. The distinct upward notch at 6.4 kHz, which is also evident in the underwater audiogram of this individual (Sills et al 2021 ), does not occur in other true seals. Overall, as observed for KE18’s underwater audiogram, this hearing curve does not correspond well with those of related species but best matches that of the northern elephant seal (Reichmuth et al 2013 )—the only other Monachinae seal for which auditory data are available.…”

“…We measured auditory sensitivity at 11 frequencies across the range of hearing: in octave steps from 0.1 to 25.6 kHz and at 33.2 kHz, the highest frequency to which KE18 exhibited reliable responses. Additionally, we measured hearing at 18.1 kHz to complement his underwater audiogram (Sills et al 2021 ). This frequency was of particular interest due to increased sensitivity noted under water in this region by Thomas et al ( 1990 ).…”

“…An initial report by Thomas et al ( 1990 ) suggested that monk seals perceive underwater sounds across a more restricted frequency range (~ 10–30 kHz) than other seals. In contrast, Sills et al ( 2021 ) describe a much broader range of hearing in water (< 0.1 to > 60 kHz), although with sensitivity poorer than 73 dB re 1 µPa. No in-air audiometric data are available for Hawaiian monk seals.…”

“…In this study, we evaluated the in-air hearing sensitivity of a highly trained Hawaiian monk seal whose underwater hearing had previously been described (Sills et al 2021 ). Using similar methods, we conducted in-air audiometric testing in a specialized acoustic chamber to allow for direct comparison to related species.…”

“…Hawaiian monk seals emit low-frequency (< 1 kHz) vocalizations while on shore, which include mother-pup contact calls, threats, and other social vocalizations (e.g., Kenyon and Rice 1959;Miller and Job 1992;Job et al 1995). While this species was previously thought to be silent under water, recent work has identified a moderate repertoire of low-frequency (< 1 kHz) vocalizations with apparent reproductive function that are produced by mature males (Sills et al 2021). The generation of uniformly low-frequency sounds might suggest acute low-frequency hearing abilities in this species.…”