Monk Seals May Be Stringing Sounds Together Like Sentences

Despite their quiet appearance, Hawaiian monk seals have a lot to say underwater. A new study published in Royal Society Open Science found that the endangered mammals produce 25 distinct underwater calls – 20 never heard before – and link their sounds into sequences, a communication strategy never seen in any other seal species.

Researchers also discovered a new foraging call used during hunting, suggesting that seals use sound not only to socialize but also to survive. The study highlights a soundscape that has remained unpublished until now.

“We found that Hawaiian monk seals – one of the most endangered marine mammals in the world – are much noisier underwater than previously thought,” Kirby Parnell, lead author of the study, said in a press release. “By analyzing more than 4,500 hours of recordings across the Hawaiian archipelago, we identified more than 23,000 vocalizations representing at least 25 distinct call types.”

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Hawaiian monk seals, an endangered species, depend on sound

The Hawaiian monk seal (Neomonachus schauinslandi) is Hawaii’s only native marine mammal and one of the rarest seals on Earth. There are fewer than 1,600 left in the wild, making each new discovery about their behavior crucial for conservation.

“This research provides the first comprehensive description of the underwater sound production of free-ranging Hawaiian monk seals, an important step toward understanding how they use sound for critical events in their biological history,” Lars Bejder, co-author of the study, said in the press release.

“As their vocalizations overlap the same low-frequency range as many human-generated sounds (e.g., ship noise), this work also helps us assess how ocean noise may affect the communication, reproduction, and behavior of this endangered species,” Bejder continued.

Using passive acoustic recorders to decode seal communication

Researchers placed passive acoustic recorders in five key habitats, stretching from Moloka’i to the remote northwest Hawaiian islands, to capture the full range of monk seal sounds. Over thousands of hours of recording, the team identified 25 call types, six times more than previously known for seals cared for by humans.

In addition to the astonishing number of calls, the recordings revealed that monk seals can connect sounds in sequence, forming what researchers describe as combinatorial calls – a level of vocal complexity never before documented in any seal species.

They also detected a new high-pitched sound, dubbed Whine, produced when foraging, suggesting that monk seals may use their vocalizations not only for mating or socializing, but also for hunting. This is the second time a seal species has been recorded vocalizing in pursuit of prey.

“We were surprised by the diversity and complexity of monk seal vocalizations,” Parnell said. “The discovery of combinatorial calls, where seals link multiple call types, suggests a previously unknown level of complexity in pinniped acoustic communication. The discovery of a new call type – the Whine – associated with foraging behavior was also unexpected and suggests that monk seals may use sound not only for mating or socializing, but perhaps also for foraging purposes.”

Listening could help protect Hawaii’s seals

The new findings give scientists a baseline for understanding how monk seals use sound in the wild and how human noise might interfere with it. Using passive acoustic monitoring, researchers can now track these elusive seals across the Hawaiian Islands without disturbing them, thereby identifying where and when they are most active.

Next, the team plans to link specific calls to behaviors such as foraging, reproduction and social interactions, and develop automated systems that can detect their vocalizations in real time.

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