A hungry whale dives down into the ocean depths to feast on a meal of krill. To maximize dining efficiency on the tiny prey, the whale opens its mouth to about 90 degrees and, in a matter of seconds, engulfs about ten kilograms (22 pounds) of krill, along with some 70,000 liters (18,500 gallons) of water. After several hours of such "lunges" the whale might ingest more than a ton of krill, which provides enough energy for an entire day.
Such a physiological and biomechanical feat is a marvel of the animal kingdom. And now the entire process has been carefully described in an article authored by Jeremy Goldbogen, a new postdoctoral researcher at Scripps Institution of Oceanography at UC San Diego. The article is the cover story of the March-April 2010 issue of American Scientist magazine.
"For a long time it's been a mystery how these whales are really doing what they are doing. When they feed they inflate, pretty much doubling their body mass," said Goldbogen, who describes the feeding whale image as a "bloated tadpole." "It's an interesting feeding method and no one's really known how they do it: the physics, the mechanism behind it."
For years scientists struggled to learn about the process because of the logistical difficulties of observing and studying lunge feeding.
But new data about the feeding of rorquals-a family of baleen whales that includes the largest marine mammals on the planet, including humpback, fin, and blue whales-emerged in recent years from digital tags deployed on the backs of whales by Scripps Oceanography Professor John Hildebrand and members of his laboratory, as well as John Calambokidis
of the Cascadia Research Collective based in Olympia, Wash. A variety of sensors within the tag, including a hydrophone and a set of accelerometers and pressure transducers, generated data related to the motions of the whale's body at depth. For example, the hydrophone enabled an accurate estimate of the whale's swimming speed through each foraging dive. Such information allowed the researchers to determine the physical forces at play during a lunge feeding event.
Highly elastic blubber on fin whales can span from underneath their chins to their belly buttons, or roughly half the animal's body length. The tremendous drag created by an open mouth at high speed stretches the throat pouch open, thereby allowing tremendous amounts of water to rush into the body cavity that is bounded by the extremely stretchy blubber. To gain further insights on the physics of blubber inflation during this process, Goldbogen looked for insights beyond the world of marine biology.
"The realization that rorqual lunge-feeding involves incredibly high amounts of drag led to the most unlikely collaboration with Jean Potvin, a parachute physicist at Saint Louis University in Missouri," Goldbogen writes in the American Scientist article. "Together, we developed a new, more-detailed model of rorqual lunge feeding inspired from decades of parachute-inflation studies."
Such details give new insights about rorqual feeding in the American Scientist article, which is a synthesis of research advanced in recent years by Goldbogen and other whale scientists.
Goldbogen is continuing work at Scripps with Hildebrand at the Scripps Marine Physical Laboratory as well as Paul Ponganis of the Scripps Center for Marine Biotechnology and Biomedicine in research on the heart rates of whales during their deep foraging dives.