It began with a straightforward question: How much do baleen whales eat?
Because baleen whales—humpback whales, right whales, blue whales, and others—mostly feed hundreds of feet deep, we can’t easily observe their behaviors. And it would be neither desirable nor possible to try to answer the question by keeping such huge animals (blue whales, up to a hundred feet long, are the largest on Earth) in captivity to monitor their daily eating patterns. What’s more, some species eat voraciously for several months, then fast for the rest of the year, which further complicates tracking their food intake.
“This is such a basic question, I'd assumed we'd figured it out 30, 40, 50 years ago, but no one had ever measured it,” says Matthew Savoca, a postdoctoral fellow at Stanford University’s Hopkins Marine Station, in California, and a National Geographic Explorer.
For Savoca, the question went deeper than basic science and burning curiosity. How much baleen whales eat is directly proportional to how much they defecate. And whale feces is a major part of the ocean’s productivity, providing valuable energy and nutrients to a vast array of marine life-forms.
Recently, Savoca, with help from international collaborators, set out to find the answer. The team outfitted baleen whales—named for the bristly material in their jaw plates, which traps tiny prey such as krill and zooplankton—in the Atlantic, Pacific, and Southern Oceans with advanced tracking technology. They also used drones to measure krill concentrations.
The results, published November 3 in Nature, are striking: Baleen whales eat much more than previously estimated. An individual blue whale, for example, eats an average of 16 tons of food every day—about three times more than scientists had thought. (Learn more about the hidden world of whale culture.)
“This study shows that baleen whales play a much more important role in our ecosystem than we thought,” says Sian Henley, a marine scientist at the University of Edinburgh, who was not involved in the study. That’s because the 14 known baleen whale species are crucial to moving vital nutrients such as carbon, nitrogen, and iron through the ocean, primarily via their excrement.
The new information, Henley says, also “tells us that we need to improve ocean protection and management at the largest scale possible, especially in the Southern Ocean.” The waters off Antarctica are particularly vulnerable to human impacts, largely because of warming temperatures from climate change and overfishing that disrupt the normal circulation of nutrients, which could also harm krill and other food sources of baleen whales. This would be especially damaging, as these whales are still recovering from centuries of whaling.
As whales continue to rebound, their role in recycling nutrients should reset the nutrient cycle—and boost krill—once again, he says.
‘Better than nothing’
To estimate how much baleen whales eat, scientists previously analyzed their metabolic needs based on their size and activity level using a closely related or similarly sized animal as reference. For instance, by measuring how much orcas (or killer whales) eat, biologists extrapolated what a humpback whale or a blue whale would consume.
“When you get down to the behavior, ecology, and physiology of these animals,” Savoca says, “a blue whale and a humpback are very, very different from a killer whale.” He allows that the earlier attempt is “better than nothing, but it’s not actually a very good guess at all.”
For their investigation, Savoca’s team tagged 321 whales spanning seven baleen species: humpback whales, blue whales, fin whales, bowhead whales, Antarctic minke whales, Bryde’s whales, and North Atlantic right whales. (Watch the world’s biggest animal lunge for its dinner.)
Savoca describes the tags—devices equipped with accelerometers, magnetometers, GPS, light sensors, gyroscopes, and cameras attached to the backs of whales with special glue—as “whale iPhones.” Just as our phones can tell us how many steps we take in a day, whale iPhones can measure how many lunges a whale makes and at what depths. Baleen whales often catch food by lunging, or accelerating through the water in quick bursts, horizontally or vertically, with mouths agape.
The team also used drones to measure the size of the whale’s mouth, which enabled them to calculate the volume of water it can capture during such lunges. Sonar to measure the density of krill living in the whale’s habitat let them determine how many of these tiny shrimp-like animals the whale could swallow with each lunge.
Putting all of this together, they found that the tagged animals ate anywhere from 5 to 30 percent of their body weight in krill each day. Previous estimates suggested baleen whales consume less than 5 percent of their body weight daily.
Mystery of the missing krill
This discovery also helps solve another riddle, Savoca says: Why the oceans off Antarctica aren’t brimming with krill. Baleen whales, the tiny crustaceans’ primary predators, were nearly eradicated during the 1900s, an era of industrial whaling Savoca calls “one of the most effective and efficient extermination campaigns in Earth’s history.”
Though people increasingly are extracting krill for fish food and for their nutrient-rich oil, that industry isn’t large enough to explain why the polar waters aren’t brimming with this vital food source for whales, seals, and more, according to Savoca.
In the late 1980s, marine biogeochemist John Martin hypothesized that a lack of iron in the Southern Ocean was limiting the number of phytoplankton, which is a primary food source for krill. Plants and animals need only trace amounts of iron, but they can’t survive without it. (Read why the Southern Ocean is the fifth—and newest—ocean.)
Subsequent experiments showed that whale feces are some of the most iron-rich material in the ocean. Along with dust from the Sahara Desert and other terrestrial sources, this iron source formed the backbone of the Southern Ocean’s iron cycle. By eating, digesting, and disposing of krill, whales take iron from deep in the ocean and bring it to the surface with their floating feces, making it usable for tiny phytoplankton, krill’s main prey. More feces creates a positive feedback loop as more phytoplankton means more krill, which can support more whales.
With populations of Antarctic baleen whales, particularly southern fin and minke whales, still increasing, it makes sense that the krill haven’t yet recovered, Savoca says. But there are positive signs: Numbers of humpback whales in the western South Atlantic have increased to 25,000 from just 450 in the mid-20th century.
Not so simple
Emma Cavan, a marine biogeochemist at Imperial College London, praises the study but points out that it’s “too simple a solution” to say that “krill numbers are down just because whale numbers are down.” Climate change and fishing also play their part.
For instance, the climate is changing more quickly in the polar regions, and the resulting shifts—such as warmer, more acidic water—could reduce populations of phytoplankton.
Even so, Cavan says, the study is a strong reminder that healthy oceans need whales—and their waste.