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.
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.
The auditory effects of single- and multiple-shot impulsive noise exposures were evaluated in a bearded seal (Erignathus barbatus). This study replicated and expanded upon recent work with related species [Reichmuth, Ghoul, Sills, Rouse, and Southall (2016). J. Acoust. Soc. Am. 140, 2646–2658]. Behavioral methods were used to measure hearing sensitivity before and immediately following exposure to underwater noise from a seismic air gun. Hearing was evaluated at 100 Hz—close to the maximum energy in the received pulse, and 400 Hz—the frequency with the highest sensation level. When no evidence of a temporary threshold shift (TTS) was found following single shots at 185 dB re 1 μPa2 s unweighted sound exposure level (SEL) and 207 dB re 1 μPa peak-to-peak sound pressure, the number of exposures was gradually increased from one to ten. Transient shifts in hearing thresholds at 400 Hz were apparent following exposure to four to ten consecutive pulses (cumulative SEL 191–195 dB re 1 μPa2 s; 167–171 dB re 1 μPa2 s with frequency weighting for phocid carnivores in water). Along with these auditory data, the effects of seismic exposures on response time, response bias, and behavior were investigated. This study has implications for predicting TTS onset following impulsive noise exposure in seals.
The Hawaiian monk seal (Neomonachus schauinslandi) is an endangered marine mammal and the subject of significant conservation concern. Limited bioacoustic information was available for this species until recently. The adult male Hawaiian monk seal Kekoa (KE18) was removed from the wild after repeated problematic interactions with conspecifics; he was then transferred temporarily to UC Santa Cruz, where he participated in studies to increase understanding of monk seal auditory biology. Compared to other seals, Kekoa’s behavioral hearing data suggest that monk seals have less sensitive hearing and a reduced functional frequency range of hearing in air and under water. A year-round characterization of his spontaneous underwater vocalizations revealed at least six low-frequency call types with a simultaneous peak in calling behavior and testosterone levels during the breeding season. Kekoa’s huaka‘i, or journey, has provided the first description of underwater communication for this protected species and contributed much-needed perspective about amphibious hearing abilities. Kekoa’s work has also inspired ongoing research with captive and wild individuals to confirm species-level traits in sound reception and production. These efforts have applications to studies of free-ranging monk seals through passive acoustic monitoring, development of automated call detectors, and the use of multi-sensor biologging devices.
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