The mournful, curiously repetitious yet ever-changing songs of male humpback whales have long puzzled scientists. The tunes are part of the males’ mating displays, but researchers don’t know their exact function, or which males in a population are doing the singing. Now, scientists who’ve been studying the giant marine mammals in Hawaii for almost 40 years report that even sexually immature males join older males in singing, apparently as a way to learn the music and to amplify the song. The beefed-up, all-male choruses may attract more females to the areas where the songsters hang out.
Scientists generally thought that only adult male humpbacks (Megaptera novaeangliae) sing, says Louis Herman, a marine mammal biologist emeritus at the University of Hawaii, Manoa, and the lead author of the new study. “But that’s just because you can’t easily tell which ones are mature and which ones are immature,” he says. “We know that mature males are larger than immature ones, so we had to figure out an unobtrusive way to measure them in the open ocean.”
Herman and his team hit on a technique by looking at 20th century whaling records. Biologists with whaling operations in the Southern Ocean had the opportunity to measure many humpbacks killed during the commercial hunts. They determined, based on the weight of males’ testes, that the whales reached sexual maturity at a body length of 11.2 meters. Working independently, whaling biologists in Japan, who also measured killed whales, reached a similar conclusion; they described 11.3 meters as the break point between adolescents and adults.
To determine the lengths of living male humpbacks, Herman’s group analyzed digital videos that they made between 1998 and 2008 of 87 of the whale singers. The males were recorded as they sang in the waters off the Hawaiian islands of Maui, Lanai, Molokai, and Kahoolawe during their winter mating season. A swimmer carried the camera in one hand and in the other hand carried a sonar device, which measured the distance from the camera to the whale. The swimmer began filming when the whale assumed a horizontal position (singing whales are typically canted with their heads downward), so that he or she could capture the full body length of the whale while keeping the camera’s axis perpendicular to the whale and close to its midline. The researchers calculated the whales’ body lengths from these images and the distance measurements.
The scientists found that the whales varied in length from 10.7 meters to 13.6 meters. Using 11.3 meters as the boundary for sexually mature adults, the researchers counted 74 humpback singers as mature and 13 as immature. The team has been following individual humpbacks (which are identified by the unique markings on their tail flukes) for decades, and its analysis also showed that some individual males have been singing for 17 to 20 years. “It is a lifelong occupation for them,” says Herman, who notes that male calves and probably 1-year-old males don’t join in.
During the winter months in Hawaii, the male humpbacks assemble in areas that the researchers call arenas, where the males sing and compete for females. Typically, the singers are widely dispersed around their arenas, which may help amp up the reach of the chorus. Males also sing in other social situations, such as while escorting a female humpback and her calf (see video).
When chorusing at the arenas, immature and mature whales are engaged in an unintentional mutual benefits game, Herman and his colleagues argue. By singing with the big boys, the youngsters indirectly learn the songs and the social rules of the mating grounds. The older males, in turn, gain an extra voice in their asynchronous chorus, his team reports in an upcoming issue of Behavioral Ecology and Sociobiology. “We know that the females don’t respond to an individual male’s song,” Herman says. “It’s not like a songbird’s song, designed to attract a female and repel other males. The humpbacks’ songs are meant to attract females to the arena.” That is, they tell the gals where the boys are. And another voice, even one of a young, inexperienced male, may help carry the message, he says.
The findings strengthen the still controversial idea that gatherings of male humpback whales may be similar to some birds’ lek mating systems, such as those of the sage grouse, which also feature male assemblages. If proved, humpbacks would be the first whale known to have this type of system.
“This is great stuff,” says Phillip Clapham, a cetacean biologist at the National Marine Mammal Laboratory in Seattle, Washington. He applauds the idea of the males’ chorus serving to “collectively ‘call in’ the females.” He adds that “we’ve known for many years that only male humpbacks sing, but no one had ever managed to figure out a way to determine the maturational class of the singers, so this is a significant advance.”
Now all Herman and his team have to do is determine which of the male singers in the chorus a female actually mates with—an event the researchers have yet to see.
Video: A male humpback sings while escorting a female and her calf off the coast of Maui. Credit: The Dolphin Institute
This story provided by ScienceNOW, the daily online news service of the journal Science.
