The Ultimate Mouthful: Lunge Feeding in Rorqual Whales
The ocean’s depths have long shrouded the biomechanics behind the largest marine mammals’ eating methods, but new devices have brought them to light
A hungry fin whale dives deep into the ocean to perform a series of rapid accelerations with mouth agape into a dense prey patch. On each of these bouts, or lunges, the whale engulfs about ten kilograms of krill contained within some 70,000 liters of water—a volume heavier than its own weight—in a few seconds. During a lunge, the whale oscillates its tail and fluke to accelerate the body to high speed and opens its mouth to about 90 degrees. The drag that is generated forces the water into its oral cavity, which has pleats that expand up to four times their resting size. After the whale’s jaws close, the sheer size of the engulfed water mass is evident as the body takes on a “bloated tadpole” shape. In less than a minute, all of the engulfed water is filtered out of the distended throat pouch as it slowly deflates, leaving the prey inside the mouth. Over several hours of continuous foraging, a whale can ingest more than a ton of krill, enough to give it sufficient energy for an entire day.
Years ago, Paul Brodie of the Bedford Institute of Oceanography described the feeding method of fin whales as the “greatest biomechanical action in the animal kingdom.” This extreme lunge-feeding strategy is exhibited exclusively by rorquals, a family of baleen whales that includes species such as humpback, fin and blue whales. Like all baleen whales, rorquals are suspension filter feeders that separate small crustaceans and fish from engulfed water using plates of keratin—the same protein that forms hair, fingernails and turtle shells—that hang down from the top of their mouths. By feeding in bulk on dense aggregations of prey, baleen whales can support huge body sizes—they count among their numbers some of the largest animals that have ever lived. Rorqual lunge feeding is especially unusual not only with respect to the tremendous size of the engulfed water mass, but also in the underlying morphological and physical mechanisms that make this extraordinary behavior possible.
Because of the logistical difficulties in studying rorqual lunge feeding deep in the ocean, our knowledge of this ingestion process, until recently, has been limited to observations made at the sea surface. Over the past several years, my colleagues and I have made significant advances in understanding how lunge feeding works. Our collective effort has been motivated by unique data generated by digital tags attached to the backs of lunge-feeding rorquals. These tags have enabled us to quantify the particular body movements that rorquals undergo during a lunge-feeding event. With these data we have been able to determine the physical forces at play during engulfment and also to estimate the magnitude of the water mass taken in. In doing so, we have confirmed many predictions previously made by early investigators that were based only on anatomical knowledge and sea-surface observations. Moreover, our analyses have uncovered new engulfment mechanisms, which, in turn, have led us back to studying the remarkable morphological adaptations that drive the lunge-feeding process.