Humpback whales have evolved into spirited acrobats to maximize their feeding prowess, marine scientists have learned.
A predatory strategy involving "360-degree barrel rolls" was described last week at the annual meeting of the American Association for the Advancement of Science in Boston.
"They do these acrobatic maneuvers in order to successfully exploit dense aggregations of krill," said Jeremy Goldbogen, a biologist with the Cascadia Research Collective in Olympia, Wash. "And the density of how much krill they can engulf in a single gulp is really key to the energetic efficiency of how these whales feed, and it’s instrumental to the life history of these animals."
When the whale spots a patch of krill or other prey, it lunges at it from below and simultaneously flips on its back, Goldbogen said.
"The mouth is on the underside of the animal," he said. "So they rotate 180 degrees so that they are inverted and the mouth is in the right position to exploit those dense aggregations."
He spoke at a news conference and later panel discussion Feb. 17 at Boston’s Hynes Convention Center.
While the migration patterns of humpbacks are fairly well understood, less is known about what they do when they are underwater. But that is changing thanks to electronic tags that record diving data and then uplink it to a satellite.
Typically, the whales dive to a depth of 820 feet (250 meters), then execute a series of roller-coaster maneuvers before returning to the surface, Goldbogen said in a session on so-called rorqual whales, which include eight species such as humpbacks, blue whales, fin whales and minke whales.
All favor a sneak-attack lunge from dark depths.
"Coming from below is what I think of as an ambush strategy," said Goldbogen. "If you can imagine yourself as krill looking down, a whale coming up from below is not going to be contrasted from ambient light from behind, versus the whale was coming from above. So those type of ambush strategies increase the efficiency of feeding, which is instrumental to them having these huge bodies."
In a paper dated Nov. 28 in Biology Letters, a British journal, Goldbogen said he also recorded the rolling behavior "when whales were in a searching mode between lunges," suggesting that helps them assess the prey field and find the densest aggregations.
Another panelist, Daniel Palacios, said the population of humpbacks in the North Pacific is 18,000 and growing by 7 or 8 percent per year.
"That is believed to be close to recovering," said Palacios, a researcher with the National Oceanic and Atmospheric Administration in Pacific Grove, Calif. Decades of whaling had "decimated, literally," their ranks, he said.
But not all the news is good.
The amount of ambient noise in the ocean from freighter traffic is doubling every decade, said Megan McKenna, a researcher with the National Park Service. That is equivalent to 3 decibels per decade, she said.
"What some of the large whales are facing is this rise of background noise in the ocean, this chronic source," she said.
Studies with blue whales and North American right whales have found them calling louder as ocean noise increases.
"If you and I were talking in a louder room, we might talk at a louder level, and we’re seeing that with whales," she said. "That they are increasing their call level with background noise. There’s likely a physiological limit to that, and if we keep that trend, of 3 decibels per decade, is there going to be a time where they stop calling because it’s just too noisy in their environments?"
