The complexity of a humpback whale’s iconic song is directly tied to how deep it dives, a new study suggests. The findings help solve a long-standing mystery surrounding what dictates the structure of whale songs – offering a rare glimpse into their underwater behaviour and potentially serving as a vital conservation tool in an increasingly busy ocean.

The study by Julia Zeh, a PhD graduate at Syracuse University’s College of Arts and Sciences, is the first to map an entire humpback whale song structure to a complete dive cycle.

Researchers attached non-invasive, tablet-sized recording tags to 16 male humpback whales off Maui, Hawaii, between 2018 and 2024 using long poles and drones.

The suction-cup devices simultaneously recorded high-resolution audio and 3D movement before detaching naturally. The data revealed extraordinary physical endurance, with some whales singing continuously for up to 16 hours. Crucially, recurring musical themes consistently aligned with different phases of the dive, expanding on earlier observations that had only identified changes in songs near the surface.

Humpback songs follow strict, repeating patterns (“themes”), but some sections are wildly complex while others are basic and predictable. Marine biologists couldn’t explain why the song was structured this way or what caused a male to switch themes. Connecting sound directly to depth helps solve this long-standing biological puzzle by showing how physical demands shape underwater vocal displays.

Themes sung at a whale’s deepest, most stable depths show the greatest acoustic variation – meaning more complex, varied singing. Scientists think this pattern points to sexual selection, where a trait evolves not because it helps an animal survive, but because it helps attract mates. Zeh said that males appear to push their acoustic capabilities at their deepest points to stand out in competitive breeding grounds.

Conversely, song elements performed during ascent and descent are far more constrained, shaped by the physical demands of managing oxygen, buoyancy and changing water pressure while vocalising.

The research could also give marine conservationists a new way to monitor humpbacks using passive underwater acoustic recorders already deployed across the world’s oceans. Researchers could establish clearer baselines for normal whale behaviour and better identify when commercial shipping, sonar or increasing ocean noise may be disrupting these protected animals.

“This gives us direction for which parts of the song to investigate if we’re interested in variation, which is likely driven by sexual selection,” said Dr Zeh, who conducted the research in Professor Susan Parks’ Bioacoustics and Behavioral Ecology Lab.

While the physiological constraints identified in the study are likely to apply to humpback whale populations globally, the researchers note that song structure differs between populations in different ocean basins. Additional tagging studies will therefore be needed to determine whether the same patterns hold elsewhere.

“Projects like this one, where we combined acoustics with movement data, gave me the tools to ask exciting questions about whale behaviour and the opportunity to be creative in how we look at well-studied phenomena like humpback whale song,” said Dr Zeh.

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