- Researchers have developed and deployed sensory tags with video cameras to study how rorquals, a type of baleen whale, lunge feed and maximize their consumption despite the huge energetic cost.
- Comprehending the dynamics of lunge feeding and its energy tradeoffs could inform whale conservation and fisheries management.
- The scientists hope to develop the tags with a more compact design, more reliable sensors, and longer battery life, and they want to better understand the baleen and compare and analyze lunge feeding and its energetics across whales.
Swimming 4 meters per second, a feeding blue whale swings open its jaws and, in four seconds, swallows 140 percent of its mass—a volume of water and krill the size of a big swimming pool or school bus. The creature then rapidly decelerates to filter that first serving and gear up for the next one, repeating the entire behavior four to eight times in a single dive. Scientists are fixing rorquals, large baleen whales like the blue whale, with innovative sensory tags to begin to unravel the mechanics of this tremendous process called intermittent engulfment.
A study recently carried out with devices consisting of new video recording technology and 3D accelerometry examined this lunge feeding behavior in unprecedented detail. Accurately comprehending this behavior sheds light on how these enormous species subsist on such small prey and supports their conservation.
“These are the first tags that have both video cameras and the traditional sensor suite; it allows us to look at not only what the animal’s doing as a whole, but also what individual parts are doing,” said lead author David Cade, a PhD candidate in biology at Stanford’s Hopkins Marine Station. “We focused on what is the mouth doing while the animal is performing complex maneuvers to capture prey, when the skull and jaw are opening in comparison to peaking speed. The more information we have about how much energy does it take them to feed, the more we can make management decisions.”
Rorquals, the family of baleen whales that includes blue, humpback and minke whales, exhibit this energetically costly behavior known as lunge feeding. They have to time the lunge precisely in order to ingest such a great volume, which quickly slows them down because of drag from opening their mouths and the water they intake. Holding their breath for a dozen minutes at a time to dive down to around 300 meters to perform this activity requires even more energy.
“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,” noted co-author Jeremy Goldbogen, Assistant Professor of Biology at Stanford.
Investigating whale feeding with sensors
Scientists have been employing sensors to study whales in motion for about 15 years through biologging. Goldbogen used non-invasive suction cup sensors—which can include accelerometers, magnetometers and pressure and sound recorders—comparable to those used in the more recent research, to gain insight into blue whales’ reactions to cargo ships and minke whales’ lunge frequency. These devices can reveal whale position, movement, speed and direction but can’t hone in on the finer details.
In his 2006 examination of how much energy lunge feeding consumed, Goldbogen identified the kinematic signature of a lunging event and hypothesized that lunge-feeding whales opened their mouths as their speed peaked rather than before. Existing equipment, however, wasn’t adequate to test either theory and determine when the mouth opens.
“If the animal’s opening its mouth while it’s still accelerating, it has to push a lot harder to let water in and push against that influx of water,” explained Cade. “This new technology opened up around resolving these debates.”
Goldbogen; Ari Friedlaender, a marine mammal ecologist at Oregon State University; and John Calambokidis, a founder and marine mammal biologist at Cascadia Research, had been contemplating this question regarding the kinematics of lunge feeding for the past five years. Cade joined Goldbogen’s lab to help with data analysis, lending his experience in fieldwork and mathematics to assist the tagging trips and make sense of the information the tags collected.
These biologists collaborated with engineers from Australian company CATS to create a sensor package comprised of a miniaturized version of regular movement technology and innovative video recording that would help measure subjects’ motion and orientation. The group deconstructed and combined small HD cameras from Oregon Scientific to capture two images simultaneously and added more batteries to increase battery life.
The tags, which are slightly bigger than a hand and weigh 650 g each, can record footage for six to eight hours and data for 30 to 36 hours. They remain attached for six to 24 hours on average but could stay on for up to 48. They include a replaceable VHF transmitter with a stronger, longer-distance signal than in previous models, facilitating the task of retrieving the data by finding the tags after they pop off.
Over the past two years, the team deployed these tags on whales off the coast of South Africa, Chilean Patagonia and four sites in California during two-to-four-week fieldtrips. The researchers would board a zodiac and track down the creatures, find a time they were resting or swimming slowly to avoid chasing or distressing them and maneuver the vessel to where they predicted the animals would surface. When the whales came up, the biologists tapped a tag on each whale with a pole from six meters away, between the dorsal fin and pectoral fins. Then they left the animals alone, sometimes conducting a focal follow from 100 to 200 meters away to observe an individual’s movement and noteworthy surface behavior correlating with the data the tag would record. Once the tag fell away, the researchers used VHF pings it emanated to locate it in the ocean.
After synchronizing the video and data from the tags, the scientists saw that humpback and blue whales that ate krill opened their mouth at peak speed and closed it at regular speed, as Goldbogen had predicted; however, timing appeared more varied in humpbacks that consumed fish. This was possibly because fish are more capable of escaping than krill; the whales could be engaging in less energetically optimal lunge feeding if doing so allows them to increase their likelihood of catching the more energy-rich, agile prey.
“The animal has to be more responsive to the prey so they have more diversity in when they’re opening their mouth,” said Cade. “That has an effect on the energetics of the system. We haven’t calculated that energetic cost yet but knowing they lunge differently on different prey types is a new find.”
Unlike the Woods Hole Oceanographic Institution’s acoustic DTAG—“the gold standard for suction cup tagging,” used to study marine animals’ echolocation, vocalizations and responses to acoustic disturbance—the tags developed for this research don’t have a programmable release. This feature increases bulk, drag, fragility and cost, so the new tags come with the VHF transmitter that pings its coordinates via GPS to the ARGOS system, through which data like this can be downloaded on scientists’ computers around the world. Even so, ARGOS has its own shortcomings; it only provides signals at certain times when the satellite is orbiting nearby, its coverage is decreasing and it faces termination due to meager funding.
Another limitation of the technology is that, although quick to process, the video its small cameras capture isn’t high-quality enough for large-scale sharing.
Better technology, better knowledge, better conservation
“They’re [Whales are] so inaccessible. They’ve never been held in captivity, despite centuries of study. We know a lot about their anatomy because of whaling but don’t know a lot about their behavior,” pointed out Cade on the paucity of information on whale ethology.
Three of the eight species of rorquals are endangered; three others’ status can’t be determined due to lack of data. The conservation of these creatures hinges on a broader understanding of their biology, including the specifics of their feeding.
“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.
Investigations like this one could also improve our awareness of whales’ impact on fish populations, which is important because people have long blamed the decimation of fish stocks on cetaceans. According to Cade, beyond estimates, we don’t have a clear knowledge of how much whales actually consume. These predators have fulfilled crucial roles in the ocean ecosystem for millions of years, and we don’t fully know the ecological consequences of their anthropogenic decline. Learning more about rorqual feeding could promote both whale conservation and insight into ecological processes that have implications for our fisheries.
Therefore, the biologists at Hopkins plan to keep delving into the mysteries of whale feeding. So far, they’ve deployed the tag on three rorqual species—blue, humpback and fin whales—as well as grey whales and one killer whale.
“We’re looking to expand and compare across all lunge-feeding whales, for instance, and look for the energetics of those systems and if differences in body size or prey type accounted for [energetic] differences,” said Cade.
The researchers also want to uncover the fluid mechanics of the baleen, the internal system that filters the animals’ enormous quantities of small prey.
Further enhancements to data-collecting tags, including a more compact design, more reliable sensors, and longer battery life, and continued support for ARGOS, would assist such gains in knowledge.
Banner image: Humpback whales that feed on fish show diversity in when they open their mouths, sacrificing energy efficiency to raise foraging flexibility and optimize for the defter prey type. Photo credit: Mike Baird.