Whales are the biggest animals to ever have existed on Earth, and yet some subsist on tiny creatures. How they do this is only just being uncovered.
A 160-ton blue whale feeds nearly continuously when prey conditions are good, but for biologists, the exact mechanism remains a confounding feat of gigantic proportions.
By attaching new sensor technology to whales just before they dive, researchers from Stanford’s Hopkins Marine Station have captured this energetically expensive activity in more detail than ever before, according to their report in Current Biology.
Lunge Feeding of Blue, Humpback and Minke Whales
This type of feeding, called lunge feeding, is unique to rorquals, a family of baleen whales that includes blue whales, humpbacks and minke whales. Having a precise understanding of this process gives us some clues as to how these massive animals survive on such tiny prey, which in turn will help biologists develop more effective conservation efforts.
“This feeding process is facilitated by a complex suite of biomechanical and anatomical adaptations that together allow the whales to engulf a volume of water and prey that is larger than their own body,” said co-author Jeremy Goldbogen, assistant professor of biology at Stanford. “For a large blue whale, this represents a volume of water and prey that is approximately the size of a large swimming pool or a school bus, and this is engulfed in a matter of seconds.”
To gulp down a mouthful of krill or fish, rorquals have to time their lunge just right. The enormous intake slows them down rapidly due to the drag caused by opening their mouths and the added burden of the water they take in.
Performing this behaviour at deep depths can make it even more energetically costly.
New sensor package reveals all
Engineers developed a new sensor package that housed miniaturised versions of 3D accelerometry plus new video recording capabilities. The researchers attached these tags to whales in South Africa, Patagonia, and off the west and east coasts of the United States. The resulting video is what you would see if you were riding on the whale’s back.
The researchers could see skull movement in concert with the whale’s orientation and motion.
The researchers found that whales that fed on krill followed a distinct pattern of activity. They opened their mouth at peak speed and closed it around the time they were back to normal speed. Humpbacks that fed on fish, however, varied their timing. This is likely in response to the more advanced escape abilities of fish compared to krill; the whales may be performing lunge feeding that is less energetically ideal when the trade-off is eating prey that can supply them with more energy.
How this helps Whales
From next month most populations of humpback whales will be removed from the endangered species list. Even accounting for that change, three of the eight species of rorquals are endangered. There is insufficient data to determine the status for three other species in this family. Any attempt to ensure the survival of these giants will require us to know much more about them, including the particulars of their mealtime activities.
“Because they operate on an energetic knife-edge, any changes in the environment related to their food supply could have profound impacts on individual and population health,” said Goldbogen.
Kinematic Diversity in Rorqual Whale Feeding Mechanisms. Cade, David E. et al. Current Biology