Researchers have uncovered a hidden world of underwater communication among Hawaiian monk seals, an elusive species that are now known to produce a far richer and more complex array of sounds than previously imaged by science.

The discovery – published in a breakthrough study in Royal Society Open Science – was made by a team of researchers from the University of Hawai‘i’s Institute of Marine Biology (HIMB), led by the Marine Mammal Research Program (MMRP).

The study analysed more than 4,500 hours of passive acoustic recordings collected across the Hawaiian Archipelago. The findings are striking: over 23,000 underwater vocalisations were detected, representing 25 distinct call types – a four-fold increase from the six previously documented in seals under human care.

“We discovered that Hawaiian monk seals – one of the world’s most endangered marine mammals – are far more vocal underwater than previously known,” says lead author Kirby Parnell, a PhD candidate at HIMB. “This acoustic diversity reveals a level of communication complexity we’ve never seen before in this species.”

The research team deployed underwater recorders at five key seal habitats, from the island of Moloka‘i to the remote Northwestern Hawaiian Islands, capturing a remarkable acoustic portrait of this endangered species’ daily life.

Among the discoveries, researchers noted a vastly expanded vocal repertoire, including 20 previously unknown call types; “combinational calls”, where seals link distinct vocalisations together (a form of acoustic complexity never before documented in any pinniped species); and a new ‘whine’ call, associated with foraging activity (only the second known case of a seal species vocalising while hunting prey).

“The discovery of combinational calls was particularly surprising,” Parnell said. “It suggests that monk seals may use a more sophisticated communication system than we ever thought possible. Finding a call type tied to foraging also raises fascinating questions about how they use sound while hunting in the dark depths.”

For a species found nowhere else on Earth, these findings carry significant implications. The Hawaiian monk seal – a cultural icon and one of the most endangered marine mammals – is highly vulnerable to the growing cacophony of human-generated ocean noise.

“This is the first comprehensive description of free-ranging monk seal underwater vocal behaviour,” explained Professor Lars Bejder, Director of MMRP and co-author of the study. “Because their calls overlap with low-frequency sounds from vessels and other human activity, understanding their acoustic world is essential to assessing how noise pollution might interfere with communication, reproduction, and survival.”

The results lay a crucial foundation for using passive acoustic monitoring to track monk seal populations and protect their habitats more effectively. Future work will focus on linking specific call types to behaviours such as mating, social interaction, and foraging – steps that could transform how conservationists monitor this rare species across the Hawaiian Islands.

The ambitious project involved an international team of graduate and undergraduate researchers, with collaboration from NOAA’s Pacific Islands Fisheries Science Center and support from the Hawaiian Monk Seal Research Program, which deployed recorders in the Papahānaumokuākea Marine National Monument.

“Manually annotating over 23,000 calls was no small feat,” said Parnell. “Our interns and collaborators were instrumental in making sense of this massive dataset.”

Supported by NOAA Fisheries through the Cooperative Ecosystem Studies Unit (CESU), the research offers not only a window into an ancient species’ acoustic world, but also a vital tool for ensuring its future.

“Every call we record brings us closer to understanding – and protecting – the last remaining voices of Hawai‘i’s monk seals,” said Bejder.

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