Jim Darling has struggled for 25 years to crack the code of the humpback song. The vocalization, uttered only by males, is perhaps the longest and most elaborate known among animals. Its formal structure is built from a succession of themes, or melodies, that have a striking range of tones from piccolo chirrups to low-pitched foghorn blasts. Some scientists say they can detect rhymes. Considering how much time and energy go into producing this submarine aria, most people assumed the purpose must be to lure mates. That theory took a blow in 1997, when Darling, a Whale Trust researcher, and Flip Nicklin discovered that singers in the 'Au'au Channel were drawing not the opposite sex but other males.

Darling and the rest of the Whale Trust team have been using an underwater speaker to play recordings of the song. The first experiments appear to confirm that females aren't attracted to the singing, whereas males seem eager to investigate the source. Maybe the song isn't for wooing but for broadcasting a challenge, as when a bull elk bugles across the mountainsides. If so, you would expect a contest to erupt when another male comes to check out the claim.

Yet when a new male joins a singer, Darling notes, the two whales often circle each other without obvious aggression. They may even swim off together like bachelor buddies, often to join other whales. Perhaps singers are recruiting male allies to help find a female and displace the primary escort at her side. If the female tries to bolt, a fast-swimming, flipper-banging competitive group may then take shape.

Or maybe the songs are far more than simple calls to allies or rivals. Hit tunes and national anthems could be better analogies, for all we know. All the humpbacks within one region, the North Pacific, for instance, sing the same song. Only an expert like Darling can detect minor variations among subpopulations, such as the humpbacks wintering off Hawaii and those off the Philippines. Yet researchers have found that the humpback populations in other parts of the world sing distinctly different songs. The songs also change over time—from one year to the next, and even over a single breeding season.

A decade ago, the humpbacks in the channel ended their song with a rising series of whoops just before coming up for breath. The next year, the finale switched to a series of ribbits. Two years ago the song had only four themes, down from as many as eight in earlier years, and even a novice could pick out a new growly tone dominating a particular section. As of 2006, there were six themes, one with a recently added flourish of four loud squeaks, and the final noises before surfacing were more like a buzz.

Lately, researchers listening in on humpbacks along northern feeding grounds have picked up singing during late autumn and again in spring and even early summer. Navy hydrophones deployed on the sea bottom detect humpbacks singing during their long migrations as well. Could it be that the whales sing to establish their identity as a group or possibly as individuals? That they are telling others about who they are and where they come from? Or sharing lore about the currents and fish and maybe the stars?

Years of study lie ahead. "Why do I do it?" Darling wonders aloud. "Human beings like puzzles. I want to know. Period."

And perhaps the urge to know goes both ways. Nicklin recalls snorkeling some distance from a humpback when it approached within a few yards. Curiosity about humans is not uncommon among humpbacks, especially young ones. But this adult animal gently carried Nicklin toward its eye with a fin. Who's to say this wasn't a case of a fellow big-brained mammal reaching out in wonder and curiosity, as in the electric moments when a chimpanzee or gorilla first touched a researcher's hand?