A new groundbreaking study from the University of Hawaiʻi Institute of Marine Biology’s Marine Mammal Research Program has unveiled an astonishing complexity in the underwater vocalizations of the endangered Hawaiian monk seal (Neomonachus schauinslandi). Published in Royal Society Open Science, this research redefines our understanding of acoustic communication in these elusive marine mammals, expanding the known vocal repertoire from a mere six call types previously identified in captive individuals to an impressive 25 distinct underwater vocalizations documented in the wild. By leveraging thousands of hours of passive acoustic monitoring scattered across the Hawaiian Archipelago, the study marks a pivotal advance in cetacean and pinniped bioacoustics, unveiling sophisticated communication strategies that have profound implications for conservation biology.
The principal investigator, Kirby Parnell, a doctoral candidate with the Marine Mammal Research Program, meticulously analyzed over 4,500 hours of continuous acoustic recordings collected from multiple key monk seal habitats ranging from the inhabited island of Molokaʻi to the sparsely populated Northwestern Hawaiian Islands. His team identified more than 23,000 individual vocal events classified into at least 25 unique call types, thereby demonstrating a far richer and nuanced vocal repertoire than previously anticipated. This upturn in detected vocalizations challenges the longstanding assumption that monk seals relied on relatively simple acoustic signals, suggesting that underwater sound production plays a far more integral role in their social and ecological interactions.
The deployment of passive acoustic recorders allowed this unprecedented insight into an otherwise cryptic behavioral aspect of Hawaiian monk seals, which spend most of their lives underwater where direct human observation is impractical. The study revealed not only a significant expansion of call diversity but also unexpected combinational vocal behaviors. Unlike previous understandings, the seals are capable of linking discrete call elements in sequences, effectively creating “combinational calls.” This phenomenon, previously undocumented in any pinniped species, points to a hitherto unknown level of complexity in marine mammal communication systems, akin to syntax in human languages, and opens new avenues for exploring cognitive capacities in these marine mammals.
Among the newly characterized vocalizations is a novel call type designated the Whine, uniquely produced during foraging activities. Until now, the behavioral context of seal vocalizations has been predominantly associated with social interaction, mating, or territorial displays. Observing an elemental call correlated with active prey pursuit is especially significant, as this is only the second recorded example across seal species that links vocal activity explicitly to foraging. This finding suggests that Hawaiian monk seals may employ acoustic signals not only for reproductive or social purposes but may also utilize sound as an ecological tool for hunting, potentially aiding in prey detection or coordination in complex underwater environments.
These findings carry substantial ecological and conservation implications, particularly considering that the Hawaiian monk seal is one of the most endangered marine mammals globally and Hawaiʻi’s only endemic seal species. The discovery that their vocalizations predominantly occupy low-frequency acoustic bands—overlapping significantly with anthropogenic noise sources like vessel traffic—raises urgent concerns about acoustic interference, which could impede critical communication behaviors. Mitigating noise pollution and protecting acoustically favorable habitats become essential for ensuring the survival and reproductive success of this vulnerable population.
Lars Bejder, Director of the Marine Mammal Research Program and a co-author of the study, emphasizes that this research establishes the first robust baseline for underwater acoustic behavior in free-ranging Hawaiian monk seals. Understanding how these animals utilize sound during crucial life-history stages—including foraging, territoriality, social bonding, and reproduction—is foundational for developing effective management strategies. This research enhances the ability to detect subtle behavioral changes in response to increasing ocean noise, thereby informing policymakers and conservationists aiming to preserve soundscapes vital to the seals’ ecological needs.
The long-term goal of the research team is to develop sophisticated, automated acoustic detection algorithms capable of real-time monitoring of monk seal vocalizations across their range. Automated systems would dramatically improve data acquisition efficiency, allowing continuous, non-invasive surveillance of population dynamics, behavioral states, and stress responses induced by environmental changes. This technological advancement represents a paradigm shift in marine mammal monitoring, facilitating conservation actions that are both timely and spatially comprehensive.
From a methodological standpoint, this study exemplifies the power of interdisciplinary collaboration, combining expertise from marine biology, bioacoustics, data science, and ecology. Graduate and undergraduate students from the University of Hawaiʻi at Mānoa led the labor-intensive annotation of over 23,000 calls, a process that involved meticulous manual classification supplemented by developing initial automated tools. Additionally, international cooperation was fostered with colleagues from France and NOAA Fisheries’ Pacific Islands Fisheries Science Center, highlighting the global importance of Hawaiian monk seal conservation.
The deployment of acoustic recorders in remote and sensitive habitats such as the Papahānaumokuākea National Marine Sanctuary was facilitated by NOAA Fisheries’ Hawaiian Monk Seal Research Program. These protected areas provide invaluable natural laboratories for studying these animals away from intense human activity, allowing the capture of authentic behavioral acoustics essential to rigorous ecological inference. However, as human maritime traffic and ocean noise increase worldwide, ongoing monitoring within these sanctuaries will serve as a critical indicator of anthropogenic impact on marine ecosystems.
This research not only enriches scientific understanding but also resonates culturally within Hawaiʻi, where the monk seal is a treasured endemic species intertwined with local tradition and environmental identity. The progressive uncovering of their hidden acoustic world invites deeper appreciation and a renewed call for stewardship. The study’s revelations about the complexity of seal vocal communication enhance public engagement and underscore the urgency of protecting marine ecosystems from noise pollution and other anthropogenic threats.
In summary, this pioneering study dramatically expands the known vocal repertoire and communication ecology of Hawaiian monk seals, revealing a sophisticated acoustic network characterized by intricate call combinations and foraging-related sounds. It sets a new benchmark for marine mammal bioacoustics and offers vital insights for the protection of one of the ocean’s most endangered pinnipeds. Future research will further elucidate the behavioral contexts of these vocalizations, paving the way for innovative conservation technologies and enhanced ecosystem management approaches.
Subject of Research: Animals
Article Title: [Not provided]
News Publication Date: 11-Nov-2025
Web References:
- Marine Mammal Research Program, University of Hawaiʻi: https://www.mmrphawaii.org
- NOAA Hawaiian Monk Seal Research Program: https://www.fisheries.noaa.gov/species/hawaiian-monk-seal/science
- Papahānaumokuākea National Marine Sanctuary: https://sanctuaries.noaa.gov/papahanaumokuakea/
Image Credits: Krista Jaspers
Keywords: Hawaiian monk seal, Neomonachus schauinslandi, marine mammal bioacoustics, underwater vocalizations, endangered species, combinational calls, foraging vocalizations, passive acoustic monitoring, ocean noise pollution, marine conservation, Papahānaumokuākea, pinniped communication
