How does the largest animal on earth survive on a diet of the smallest of prey? By having a jaw that spans a quarter of its body length, an enormous mouth that goes from the head to the belly button, and by doing lots of “lunges,” according to UBC zoology PhD candidate Jeremy Goldbogen.
Goldbogen is studying a family of baleen whales called rorquals that include the fin, humpback and blue whales which, at 30 metres long and weighing 150 tons, are the largest animals that have ever lived.
Rorquals are characterized by a special, accordion-like blubber layer that goes from the snout to the navel. The blubber expands up to several times its resting length to allow the whales to engulf large quantities of prey-laden water, sort of like filling up a balloon with water.
“These gigantic animals -- bigger than any of the dinosaurs -- feed almost exclusively on krill, tiny shrimp-like crustaceans about 1-2 centimetres long,” says Goldbogen. “Despite their majestic stature, we know very little about their foraging habits, which is crucial to conservation efforts.”
Up to now, no one knew just how much food a fin whale needed to eat to sustain its average 20-ton body mass. To help answer this basic question, Goldbogen and a team of scientists from the Scripps Institution of Oceanography at the University of California, San Diego, and Cascadia Research Collective, a non-profit organization in Washington, used digital tags that, when attached to the whale’s back by suction cups, log how fast it swims and how deep it dives.
“For the first time, scientists have a clear picture of these whales’ feeding behaviour beyond what we see when they surface,” says Goldbogen, who has spent the last two years deciphering data from seven fin whales and nine humpbacks in the north Pacific Ocean.
“Fin whales routinely dive to depths of more than 200 metres to feed on aggregations of krill,” says Goldbogen. “Once they get there, they execute an average of four ‘lunges,’ where they quite literally drop their jaw while swimming 11 kilometre per hour.
“The mechanics of this unique behaviour is similar to opening a parachute at high speed. The result for the whale is an increase in water pressure, which rapidly expands the mouth as huge volumes of prey-laden water rush inside.”
In collaboration with his UBC advisor Robert Shadwick and Nick Pyenson from the University of California’s Museum of Paleontology, Goldbogen combined the tag data with measurements of jaw bones from museum specimens to determine how much water and prey are engulfed during lunge-feeding. “Our results demonstrate that fin whales can take in about 70 cubic metres of water in one gulp,” says Goldbogen. “That’s bigger than their own body and roughly the size of a school bus.”
After the jaws close around this huge volume of water and prey, a fin whale must then expel the water while retaining the prey. To do this, the whale uses baleen -- a comb-like structure composed of the same substance that makes up human hair and nails – to filter krill from the expelled water. Based on published accounts of krill density at these whale foraging sites, Goldbogen was able to conclude that each lunge provides the fin whale with about 10 kilograms of krill.
“Now that we know how much krill is ingested per lunge, we can estimate that a fin whale must forage for approximately three hours a day to meet its daily energetic requirements,” says Goldbogen. “That’s about the same amount of time humans spend cooking and eating a day. Considering their size and what they eat, lunge-feeding appears to be quite an efficient strategy for these rorquals. This makes us wonder what role lunge-feeding has played in the evolution of their extremely large bodies.”
The new knowledge will inform efforts to conserve these endangered animals. “Even though lunge-feeding enables a whale to take big gulps of prey-laden water, it does require a lot of energy. As a result, whales rapidly deplete their oxygen stores and must return to the surface to breathe after taking only a few lunges. If prey patches aren’t dense enough or are located too deep in the water, rorquals will have to spend a larger proportion of the day searching for food.”
Goldbogen and colleagues are now comparing the jawbones and skulls of all baleen whales -- which range from the six-metre-long pygmy right whales to 30-metre-long blue whales -- to determine the physics of these massive structures during feeding and how large whales evolved from smaller ancestors.
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