Hawaiian Monk Seals Produce ~23,000 Complex Underwater Calls — Novel 'Combinational' Vocalizations Discovered

Hawaiian monk seals are far more vocal than scientists expected

A team from the University of Hawaiʻi at Mānoa, led by the Hawaiʻi Institute of Marine Biology, has discovered that endangered Hawaiian monk seals produce an unexpectedly large and complex variety of underwater sounds. The study, published in Royal Society Open Science, analyzed more than 4,500 hours of passive acoustic recordings and identified roughly 23,000 individual seal sounds sorted into at least 25 distinct call types.

What the researchers found

Prior to this study, researchers had documented only six monk seal call types, most from animals in human care. The new analysis uncovered 20 previously undocumented call types, expanding the known repertoire substantially. The team also documented a new call dubbed the Whine, which appears to be associated with foraging behavior.

We discovered that Hawaiian monk seals — one of the world’s most endangered marine mammals — are far more vocal underwater than previously known, said Kirby Parnell, the study’s lead author and a PhD candidate with the Marine Mammal Research Program.

Combinational calls: a surprising complexity

One of the study’s most striking findings is the occurrence of "combinational calls," where seals link different call types into sequences. This type of combinatorial linking has not previously been reported for seals, sea lions, or walruses and suggests a higher level of acoustic complexity in pinniped communication than scientists had assumed.

Methods and implications

Researchers deployed stationary acoustic recorders from Molokaʻi to the Northwestern Hawaiian Islands. Call rates were higher in locations with larger seal populations, indicating that vocalizations likely play a role in social interactions. Because recordings were passive, the approach offers a noninvasive way to monitor seals and their behavior over large areas without disturbing the animals.

Understanding monk seal acoustics is especially urgent because many of their calls occupy the same low-frequency band as common human-made ocean noise. Better knowledge of seal vocal behavior can inform conservation measures to reduce harmful noise overlap and help protect critical life-history events such as feeding and reproduction.

Collaboration and effort

The project involved UH students and alumni as well as collaborators from France and NOAA’s Pacific Islands Fisheries Science Center. Manually annotating over 23,000 calls was a major effort; Parnell acknowledged a team of interns who assisted with the analysis. Lars Bejder, director of the Marine Mammal Research Program and co-author, called the work the first comprehensive description of free-ranging Hawaiian monk seal underwater sound production.

Why it matters: The findings broaden our understanding of monk seal behavior, provide new tools for noninvasive monitoring, and highlight the need to mitigate low-frequency ocean noise that could interfere with vital seal communications.