Mānoa: Hawaiian monk seals are far more ‘talkative’ than previously known

A new study from the University of Hawaiʻi at Mānoa’s Hawaiʻi Institute of Marine Biology has uncovered that endangered Hawaiian monk seals posses a far more complex underwater vocal repertoire than previously known, revealing 25 distinct call types used across the Hawaiian Islands.

Until now, researchers believed monk seals had only a few simple underwater calls, based mostly on seals in human care. But by analyzing more than 4,500 hours of underwater recordings from across the Hawaiian Archipelago, the institute identified more than 23,000 individual vocalizations, including 20 newly documented calls produced throughout the day.

The findings, published on Wednesday in Royal Society Open Science, offer new insight into one of the world’s rarest marine mammals. Scientists say it could reshape how we can monitor and protect them.

Monk seal vocalizations

The study deployed passive acoustic recorders at five key monk seal habitats from Molokaʻi to the Northwestern Hawaiian Islands.

In addition to uncovering 25 distinct underwater calls, the research provided evidence that monk seals can combine different vocalizations together to create “combinational calls,” a communication strategy never before reported in any pinniped (seals, sea lions and walruses) species. 

The team also discovered a novel elemental call type — “the whine” — produced during foraging, representing only the second known example of a seal species using vocalizations while pursuing prey.

“We were surprised by the sheer diversity and complexity of monk seal vocalizations,” said Kirby Parnell, lead author of the study and a Ph.D. candidate with Marine Mammal Research Program. “The discovery of combinational calls, where seals link multiple call types together, suggests a previously unknown level of complexity in pinniped acoustic communication. Finding 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 possibly for foraging purposes as well.” 

Conservation implications

The authors of the study said their research lays the foundation for using passive acoustics to monitor monk seal populations.

Future research will aim to link these documented vocalizations to specific Hawaiian monk seal behavoirs, such as foraging, swimming, social interactions and reproductions, which could help evaluate how human activity affects the species’ behavior.

“This research provides the first comprehensive description of free-ranging Hawaiian monk seal underwater sound production, an important step toward understanding how they use sound for critical life-history events,” said Lars Bejder, the director of the Marine Mammal Research Center and co-author of the study. “Because their vocalizations overlap with the same low-frequency range as many human-generated sounds (e.g. vessel noise), this work lays the foundations to evaluate how ocean noise may affect communication, reproduction and behavior in this endangered species.”