Male humpback whales change their songs when they hear other males singing along.
Like a songbird calling another out, one male humpback whale may make another change his tune.
Studying humpbacks with methods adapted from bird research has uncovered the first known instances of what look like whales responding musically to each other's songs, says Danielle Cholewiak, a researcher for the Stellwagen Bank National Marine Sanctuary based in Scituate, Mass.
Cholewiak and colleagues detected melodic adjustments when a solo singer encountered another singer nearby and when researchers played their song remixes for whales.
Male whales may be using music to tell another male, "Hey, I'm talking to you," Cholewiak reported Oct. 14 at the Society of Marine Mammology's biennial conference.
Cholewiak "showed short-term acoustic interactions between males -- that was the new thing," said Adam S. Frankel of Marine Acoustics Inc., an independent consulting firm in Arlington, Va.
Among humpback whales, only males boom out long strings of repeating phrases of hums and whups and chirps. The sounds can make a boat vibrate, said Salvatore Cerchio of the Wildlife Conservation Society in New York City, who worked with Cholewiak on the new study. Scientists use the word song to describe this patterned male vocalization, just as they do for elaborate bird serenades.
Male songbirds sing at each other to claim their territory or seduce females. Though humpbacks don't defend territories, they certainly have rivalries. Typically three to eight males surround a female and battle for the position closest to her. "These guys are streaming blood," Cerchio said. "The gentle giant is a myth."
But observations so far haven't helped scientists understand whether humpbacks use songs the way birds do. Tests haven't shown male or female humpbacks consistently swimming toward or away from recorders playing songs. And scientists have yet to see humpbacks mate.
So instead, Cholewiak took a different approach, boating around a breeding ground recording and analyzing songs.
"I was drooling over what she was able to do," says Sharon Nieukirk of Oregon State University's Hatfield Marine Science Center in Newport, Ore. Whales rarely cooperate with field biologists' experimental plans.
Cholewiak undertook the song analysis while at Cornell University, which has a renowned flock of birdsong researchers. She adapted measurements used in bird studies to analyze the humpbacks' songs. For example, the whales repeat a phrase of notes several times in one block, or "theme," before moving on to another, and Cholewiak looked at how often the whales switched among these themes.
To record whales, Cholewiak spent four winters on the small island of Socorro in the Revillagigedo Archipelago off the Pacific coast of Mexico. She dropped recorders weighted with sandbags into the ocean to eavesdrop on whales. After months, she transmitted an acoustic signal that released the recorders so they popped to the surface. Analyzing the recordings, Cholewiak could determine where the singers were and reconstruct their movements.
In the sea of sound recordings, she found 14 cases in which a male sang alone for at least 45 minutes and then continued for another 45 minutes after another male started singing. Cholewiak noticed two changes in song when humpbacks sang together.
Overall, the first singers switched more often among various musical themes when a second singer hung around. Also, the first males adjusted their songs so that the pair was more likely to sing the same theme simultaneously. When males meet, Cholewiak concluded, songs change.
When she found a male singing by himself, she attempted a playback experiment. She recorded his song and used a computer program to create a simplified version incorporating three of his themes. Then she broadcast the version to him via speakers dangling below the boat.
Confronted with simplified recordings of their songs, males tended to make their singing more even. This change meant that a male came closer to spending equal amounts of time singing each theme.
Researchers don't yet know what these changes mean, but the new work opens the way for questions about what messages whales may be communicating. If humpback songs follow the pattern of birds, the messages could get pretty macho. And females could be tuning in.
That the humpbacks appeared to respond to the playback at all was a pleasant surprise, Cholewiak says. Song playbacks had fallen into disfavor after researchers found no pattern in the movements of listeners. "I was initially very reluctant to try it," she says. Yet checking song characteristics instead of whale movements made all the difference.