Tuesday, August 10, 2010
After three months of work, a team of San Francisco State scientists has amassed more than 4,000 recordings of underwater moans and bubbling chatter made by blue whales off the California coast - a collection that could help explain how the largest of ocean animals communicates.
Yet the scientists still don't know whether the whales' calls represent a kind of cetacean groupthink, a beacon for possible mates, or perhaps a social signal that an entire group is migrating or moving toward some new source of food.
Roger Bland, an acoustical physicist at San Francisco State University, and his colleagues have analyzed 4,378 blue whale songs recorded as the animals swam past an undersea observing station on the Pioneer Seamount, 50 miles out from Mavericks, the famed surfing spot north of Half Moon Bay.
Four hydrophones captured the loud and eerie sounds. Each is a burst of warbles, a little like someone gargling underwater, followed exactly 130 seconds later by a loud, long, deep-toned and sad-sounding moan.
Like humpbacks and fin whales, only the male blues are believed to vocalize. Yet unlike most whales, which have widely varied song repertoires, the blue whales all communicate at the same pitch, Bland said.
"We can only speculate what they mean and wonder just what adaptive advantage the (songs) may give the whales in their evolution," he said.
The songs Bland and his colleagues recorded seemed most often to be associated with either fast travel that might have happened during their migration or while they milled about near abundant masses of krill.
So far, the scientists analyzed only the long, mournful moans the whales make - known as their "B calls" - while their bubblings remain to be scrutinized.
But each of the calls made by the whales sounded exactly the same - precisely four octaves below middle C on the human scale. And where the calls did vary occasionally, their pitch differed by barely half of 1 percent. By comparison, a tiny change in human musical pitch between the notes middle C and C sharp would mark a change of fully 6 percent, a change that might go unnoticed by tone-deaf humans but would be clear to musicians.
One possibility for such mass accuracy, Bland and his colleague suggest, is that female blues may be able to locate a group of males by their sounds, which may be slightly higher or lower in pitch when the males are swimming toward the females or away from them. That effect is called the Doppler Shift - the change in pitch heard as a siren or a train whistle is nearing or speeding away.
Blue whales live in all the oceans of the world, and in each region the species and subspecies vary. But all are endangered because their numbers were decimated by worldwide hunting before international protections imposed in 1966.
They are the largest animals ever known to have lived: more than 100 feet long and weighing up to 200 tons for the south Pacific species, and somewhat less in the north Pacific.
Bland's colleagues working on this latest report include Michael D. Hoffman, a former S.F. State student, and Newell Garfield, an S.F. State oceanographer and director of the university's Romberg Tiburon Center for Environmental Studies. They published the results of their study in the July issue of the Journal of the Acoustical Society of America.