New Publication: Hawaiian monk seal underwater hearing and communication
Updated: Feb 21
Written by Kirby Parnell
We are pleased to share a new publication in Endangered Species Research, which describes underwater sound reception and sound production in Hawaiian monk seals. This research was completed by Kirby Parnell and colleagues at the University of California Santa Cruz during Kirby’s Master’s thesis, prior to starting her Ph.D. at the University of Hawai’i. The manuscript is entitled:
Authors: Jillian Sills, Kirby Parnell, Chloe Lew, Brandi Ruscher, Traci Kendall, Colleen Reichmuth
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.
Little is known about the acoustic biology of Hawaiian monk seals (HMS), which are one of the most endangered marine mammal species. Their underwater vocalizations and how they “speak” to each other under water have never been described in the scientific literature. With regard to their hearing abilities, prior to this study only one individual had completed an underwater audiogram (“hearing curve”) to describe sensitivity to sounds across a range of frequencies (i.e., to sounds of different pitch). The results—which suggested that HMS have relatively poor hearing abilities and cannot detect their own vocalizations in air—were a bit perplexing. To shed light on both hearing and communication in Hawaiian monk seals, one adult male seal at the University of California Santa Cruz’s Long Marine Laboratory was trained to cooperatively participate in underwater hearing tests (Video 1). Additionally, his underwater vocalizations were passively recorded and described in relation to his annual molt (i.e., shedding of fur and skin) and testosterone values (Video 2). This monk seal produced at least 6 different low-frequency vocalizations under water throughout the year (Figure 1). Vocal behavior and testosterone peaked simultaneously before the annual molt, suggesting that HMS males produce vocalizations for reproductive purposes (e.g., to attract or compete for mates). Results from the hearing tests showed that monk seals have decreased sensitivity to sound compared to other seal species – that is, they can’t hear as well as other seals. However, this work also demonstrated that HMS can hear over a wider range of frequencies than previously thought, and might therefore be more susceptible to low-frequency noise (e.g., boat noise) in their environment. This study provides information that can be used to protect Hawaiian monk seals in the wild. It’s important that we know what monk seals can hear and how they communicate so that we can determine how susceptible these vulnerable seals are to the effects of noise in both air and water.
Video 1: Video exemplar of underwater auditory testing with Hawaiian monk seal Kekoa (KE18). This video clip illustrates a portion of a testing session to evaluate underwater hearing sensitivity at 400 Hz. Credit: http://www.int-res.com/articles/suppl/n044p061_supp/
Video 2: Overview of the acoustics research that Kekoa (KE18) participates in at UC Santa Cruz’s Long Marine Laboratory. Credit: PinnipedLab, YouTube https://www.youtube.com/watch?v=1NpU2_456r4&feature=emb_logo
Figure 1: Spectrograms for the 6 different call types and one vocal bout produced under water by Kekoa. A spectrogram is a visual representation of a sound where the x-axis is time, the y-axis is frequency (or pitch), and the color represents the amplitude of the sound (how loud or soft the sound is). Click here to hear these sounds in Audio S1.
Technical Caption: Spectrograms for the 6 discrete call types produced under water by Hawaiian monk seal Kekoa (upper panels); also shown is a representative vocal bout for this individual (bottom panel). Corresponding waveforms—in the shaded regions below the spectrograms—represent the relative amplitude of each vocalization. The spectrogram amplitude scalebar applies to all spectrograms. The percentage of total vocalizations for each call type produced in a 1 yr period (n = 3858 calls) is shown in the upper right corner of each panel. Note that in the ‘moans’ panel, an ascending moan is shown following the second moan. Spectrogram settings: Hann window, window length 4096 points, 90% overlap, 3 dB filter bandwidth 16.9 Hz
Full citation details: Sills JM, Parnell K, Ruscher B, Lew C, Kendall TL, Reichmuth C (2021) Underwater hearing and communication in the endangered Hawaiian monk seal Neomonachus schauinslandi. Endangered Species Research 44:61-78. https://doi.org/10.3354/esr01092
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