Scientists have discovered that Hawaiian monk seals produce a far more complex array of underwater sounds than previously known, identifying 25 distinct call types from over 4,500 hours of recordings. This research, led by the University of Hawaiʻi, reveals novel communication strategies including combinational calls and a foraging-related whine. The findings could enhance conservation efforts for this critically endangered species amid rising ocean noise.
New research from the UH Hawaiʻi Institute of Marine Biology's Marine Mammal Research Program has expanded understanding of Hawaiian monk seal (Neomonachus schauinslandi) acoustics. Published in Royal Society Open Science, the study analyzed thousands of hours of passive acoustic recordings from five key habitats across the Hawaiian Archipelago, from Molokaʻi to the remote Northwestern Hawaiian Islands. Researchers identified more than 23,000 vocalizations, representing at least 25 distinct low-frequency call types—a significant increase from the six previously documented in seals under human care.
The seals produce these calls throughout the day, with similar patterns detected archipelago-wide and higher calling rates in areas with larger populations. Key breakthroughs include 20 previously unknown call types, evidence of combinational calls where seals link multiple types together—a first for any pinniped species—and a new 'Whine' call associated with foraging, marking only the second recorded instance of seals vocalizing while pursuing prey.
"We discovered that Hawaiian monk seals—one of the world's most endangered marine mammals—are far more vocal underwater than previously known," said lead author Kirby Parnell, a PhD candidate with the program. "By analyzing over 4,500 hours of recordings... we identified more than 23,000 vocalizations representing at least 25 distinct call types."
Parnell added, "We were surprised by the sheer diversity and complexity of monk seal vocalizations... Finding a new call type—the Whine—associated with foraging behavior was also unexpected."
The Hawaiian monk seal, the only marine mammal endemic to Hawaiʻi, holds significant cultural value. "This research provides the first comprehensive description of free-ranging Hawaiian monk seal underwater sound production," explained co-author Lars Bejder, director of the program. "Because their vocalizations overlap with the same low-frequency range as many human-generated sounds (e.g., vessel noise), this work also helps us evaluate how ocean noise may affect communication, reproduction, and behavior in this endangered species."
The study supports passive acoustic monitoring for population tracking and habitat protection. It involved University of Hawaiʻi at Mānoa students, collaborators from France, and the Pacific Islands Fisheries Science Center's Hawaiian Monk Seal Research Program, funded by NOAA Fisheries award NA19NMF4720181.