No One Knows How the Biggest Animals on Earth—Baleen Whales—Find Their Food

When it's time to eat, humpback whales head toward the ends of the earth. Their mission: feast until they are fat and happy. They must build up their energy reserves, packing on nearly a ton of blubber a week to sustain them on the voyage from their polar and subpolar feeding grounds to the balmy waters where they breed. The journey may require traveling thousands of miles over several months—and they must be ready to reproduce when they arrive. Perhaps because nature loves a paradox, these colossal predators, which can measure 60 feet long and weigh 40 tons, accumulate these fat stores by eating some of the smallest prey in the sea—including krill, shrimplike crustaceans that live in all the world's oceans but are concentrated in the cold waters found at high latitudes.

We know a lot about how humpbacks eat. They filter seawater through plates of keratin, called baleen, that line their upper jaws and resemble the frayed bristles of a worn toothbrush. They devour several thousand pounds of their tiny prey every day. To obtain that quantity of food, they must seek out dense aggregations of the crustaceans. Once they find a swarm, they may deploy a clever cooperative hunting tactic, swimming in circles while blowing columns of bubbles to create a kind of net to corral the krill. Then they feed, lunging at the tightly gathered prey with jaws agape, engulfing thousands of gallons of krill-filled water in their pleated throat pouches before straining the catch through their baleen.

Yet for all scientists have learned about these charismatic leviathans, no one knows how baleen whales (a group that includes humpback, blue, fin and sei whales, among others) find their food in the first place. Their cousins the toothed whales—sperm whales, belugas, dolphins, and the like—use ultrasonic sonar signals to detect prey, but baleen whales don't have that ability. Somehow they still manage to find their minuscule quarry in the infinite sameness of the sea.

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It's a mystery that scientists are eager to solve. In part that's because it is a huge gap in our basic knowledge of high-profile species. More urgently, the question of how baleen whales seek out their food has important conservation implications, particularly for a baleen species called the North Atlantic right whale.

The North Atlantic right whale, a dark, stocky cetacean that eats rice-size zooplankton called copepods, has the unfortunate distinction of being one of the most endangered mammals on the planet. Commercial whaling nearly extinguished this species in the early 1900s. By 1935 the League of Nations banned the hunting of all right whales. But unlike other species whose numbers plummeted because of whaling, the North Atlantic right whale has been unable to make a comeback. The animal's feeding grounds off the coast of New England and the Canadian Maritimes overlap with areas of intense human activity. Collisions with ships and entanglement in fishing gear, along with climate change–induced disturbance of their habitat and prey, have taken a terrible toll.

The most recent estimates indicate that fewer than 350 North Atlantic right whales remain, only 70 of which are females of reproductive age. According to some projections, the species could go extinct in the next couple of decades. Understanding how baleen whales track down their prey could help scientists predict where the whales will go to feed—and better manage human activities in those areas that might harm the whales.

All of this matters for more than just a single species of whale. North Atlantic right whales and other baleen whales are ecosystem engineers, feeding in deep water and then releasing nutrients near the surface through their feces, which support the growth of microscopic plantlike organisms called phytoplankton. The phytoplankton, in turn, nourish krill, copepods and other tiny drifting creatures known as zooplankton that are eaten by larger animals. The whales' tissues also trap enormous amounts of carbon dioxide that could otherwise contribute to global warming—an estimated 33 tons for the average large-bodied whale. And when whales die, their carcasses sink to the seafloor, where they sustain entire communities of deepwater organisms—from sleeper sharks to sulfur-loving bacteria—that are specially adapted to using these so-called whale falls for food and shelter. The health of baleen whale populations supports the health of a host of other species.

The most direct way to learn how a baleen whale finds its food is to tag it with a device that can record its underwater behavior and watch the animal forage. That's not possible with North Atlantic right whales, which are so stressed from human activity that any direct human contact could just make things worse. Fortunately, the right whale has cousins, such as the humpback, that are in much less peril. And one of the best places to watch them eat is on their feeding grounds at the bottom of the world.

In 2020, two weeks before the World Health Organization declared the COVID-19 outbreak a pandemic, I boarded a ship to Antarctica to follow one research group's efforts to learn how baleen whales find food. I went as a guest of the cruise operator, Polar Latitudes, to observe a study being carried out by the seven scientists they were hosting on their tourist boat and to lecture on whale evolution.

By joining a tourist expedition, the international team of researchers based in the U.S., Sweden and Japan saved on the exorbitant costs of getting to the white continent. In return for three shared staterooms, meals, and the use of two sturdy inflatable rubber boats called Zodiacs, the scientists gave the other passengers regular updates on their research, which was billed as a whale-focused expedition for citizen scientists.

The team was testing a hypothesis about baleen whale foraging that grew out of research on seabirds. Starting in the mid-1990s, Gabrielle Nevitt of the University of California, Davis, showed that dimethyl sulfide (DMS), a chemical that is released when phytoplankton are eaten by zooplankton, attracts tube-nosed seabirds—a group of carnivorous birds that includes albatrosses, petrels and shearwaters—which then eat the grazing zooplankton. It's a mutualistic arrangement: by luring the seabirds with the scent of DMS, the phytoplankton gain protection from the zooplankton. Even at the bottom of the food chain, the enemy of your enemy is your friend.

Cruise team leaders Daniel Zitterbart of the Woods Hole Oceanographic Institution, a physicist who uses remote sensing methods to study the behavior and ecology of whales and penguins, and whale behavior specialist Kylie Owen of the Swedish Museum of Natural History wondered whether whales might be similarly attracted to DMS. If so, then following the chemical toward higher concentrations should, in theory, lead whales to denser concentrations of the krill and other phytoplankton eaters than foraging randomly would. To find out, Zitterbart and Owen joined forces with whale biologist Annette Bombosch of Woods Hole; zooplankton researcher Joseph Warren of Stony Brook University; Kei Toda of Kumamoto University in Japan, who developed technology for measuring DMS, and his then graduate student Kentaro Saeki; and oceanographer Alessandro Bocconcelli of Woods Hole, who has helped pioneer the use of sophisticated digital tags to study whales.

The team planned to tag humpbacks with custom-built instruments containing pressure sensors, accelerometers, magnetic compasses and hydrophones that record their underwater behavior, along with a radio transmitter to enable tracking. Their permits allowed them to tag only a total of five whales, and they had to do it in just five days—the rest of the 12-day cruise would be spent in transit. They had little room for error.

We left from the Argentine port of Ushuaia, the southernmost city in South America, on February 28 and spent the next two days of the leap year crossing the Drake Passage, the notoriously turbulent 620-mile-wide waterway between South America and Antarctica, escorted by albatrosses and petrels. On March 1, we passed over a boundary zone known as the Antarctic Convergence and entered the calm, cold waters of the Southern Ocean. For the first time since entering the Drake, we glimpsed land off the starboard side of the ship—Smith Island, part of the South Shetland Islands of the British Antarctic Territory.

With the stomach-churning swells of the Drake behind us and the soporific effects of the motion sickness medication wearing off, I could now fully register my extraordinary surroundings. Icebergs, bergy bits and growlers—some of the many forms of ice here—joined sea and sky to display every shade of blue. Fuzzy Gentoo penguin chicks chased after their exhausted parents to demand food. Platinum blond crabeater seals lounged on divans of drifting ice, basking in the sun. I let the otherworldly beauty of the place wash over me.

On the morning of March 4, I awoke to daybreak in Paradise Bay, a scenic harbor where whaling ships once anchored. From my seat on the pontoon of a Zodiac, I watched the rising sun pierce through an opening in the cloud cover to bathe a distant glacier in golden light.

We were in whale country now, encountering groups of the mammals as they floated on the surface like logs, exhaling tall plumes of moist air. The wet whoosh of whale breath joined the thunder cracks of calving glaciers and the rumbles of avalanches.

The day before, the scientists had successfully tagged their first humpback. The passengers cheered when the scientists announced the update at breakfast. Unfortunately, the whale proceeded to sleep the entire time it was under observation. But later the same day they tagged a second whale, and this one was a model subject, making several dives up to 850 feet. Data from the sensors indicate that the whale was lunge feeding—exactly what they wanted to see.

This morning the team was attempting to tag a third individual—and hoping it behaved like number two. Zitterbart, a tall, animated man who thinks and talks with formidable speed, got up at 5:30 and headed to the ship's bridge to find out whether any whales were around and what the weather was like. The day looked promising. Whales had been spotted in the area, and the water was still—the better for retrieving tags, which are programmed to stay on a whale for just a few hours before detaching and floating to the surface.

By 6:45 the research boats were lowered into the water, and the scientists were preparing to tag a whale that had been sighted nearby. A 20-foot-long carbon-fiber pole extended beyond the bow and stern of the tag boat. They use the pole to slap the tag, which has four suction cups on its underside, onto unsuspecting humpbacks once they get to within 10 feet of the animal. Bombosch and Bocconcelli navigated across a glassy expanse of open water toward a group of whales, slowing on the approach. That bunch looked lazy, though. They didn't want to tag another slumbering whale, so Owen and Bombosch decided to target another group that looked more active.

From my vantage point in a separate Zodiac, two humpbacks came into view. Only their small dorsal fins and the uppermost part of their sleek black backs were visible. They didn't look all that big. But like icebergs, most of their mass is below the waterline. At a distance, you only get a sense of how huge humpbacks are when they wave their great flippers in the air, raise their tail flukes ahead of a deep dive, or propel their entire bodies clear out of the water in a glorious breach.

Zitterbart gripped the unwieldy tagging pole and stood tensed, one foot on the bow box and one in the boat. Attaching the tag is a fraught operation. To ensure a strong signal from the transmitter, he had to place the tag as high on the animal's back as possible but not too close to the sensitive skin surrounding the blowhole. As the tag boat neared the whales, Zitterbart raised the pole and then, at exactly the right moment, cast it down with just enough force to plunk the tag securely onto one of the animals. The whale startled, then sank out of sight—a typical reaction—and the researchers moved quickly to stow the pole, mark the GPS location of the tagged whale and prepare to monitor the animal. They were now three for three with attaching the tags.

Once the tagged whale resurfaced, they would spend the next few hours tracking it by eye and with the aid of a VHF receiver tuned to the tag's transmitter, keeping a distance of more than 300 feet from the animal so as not to interfere with its routine. They needed to recover the tags—which store the behavior data and cost $10,000 apiece—when they automatically detached from the whales at the preprogrammed time after deployment. Now the team just had to hope they chose a cooperative subject. “Ideally we'd tag an active whale that's not feeding yet, and it would swim away to feed,” Owen explained. After that, the researchers in the prey boat would sample the water to see if krill and DMS concentrations were increasing along the whale's path. If they tagged a whale while it was already feeding, they would have no trail to follow. But the humpbacks are wild animals, with agendas of their own. “The stars really need to align for things to go the way we want them, too,” Owen said.

To visit Antarctica is to encounter forces that have shaped the fortunes of baleen whales across eons. Descended from four-legged land animals, whales underwent one of the most dramatic transformations of any vertebrate group when they transitioned to life in the water. Like all organisms, whales evolved under the influence of environmental change. They got their start some 50 million years ago in the greenhouse conditions of the Eocene epoch. Back then, the southern supercontinent of Gondwana was in the process of disbanding, and the ancient Tethys Sea reached from the Pacific Ocean to the Mediterranean. In the warm, shallow waters of Tethys, early whales underwent the first phase of their transformation: becoming seaworthy. Forelimbs morphed into flippers, noses became blowholes, ears remodeled to hear while submerged. Some 10 million years after their furry, four-legged ancestors walked along the water's edge, whales had adapted so thoroughly to aquatic life that they could no longer venture ashore.

The second phase of whale evolution unfolded as the planet transformed into a so-called icehouse world. As the Eocene gave way to the Oligocene, tectonic forces dealt a final blow to Gondwana, cleaving apart Australia, South America and Antarctica. When the separation of these landmasses was complete, the Antarctic circumpolar current swept around Antarctica, isolating it from warmer waters and pulling up nutrients from the deep that supported an abundance of phytoplankton and zooplankton. So vast and powerful was this new current, in fact, that it altered ocean circulation, temperature and productivity across the globe. From this crucible of tectonic, climatic and oceanic change, the forerunners of modern baleen whales emerged. By 35 million years ago early representatives of this lineage were patrolling the seas. Over millions of years their descendants would eventually acquire the baleen and gigantic body sizes for which this branch of the whale family is known.

Although baleen whales were molded by dramatic environmental and ecological change on evolutionary timescales, that long history did not inoculate their modern descendants against the dangers of profound change on shorter timescales. In the 20th century alone, industrial whalers armed with exploding harpoons and factory ships that could process carcasses offshore slaughtered more than two million baleen whales, pushing many populations to near extinction and degrading their ecosystems. Some species have been recovering since the demise of that industry—only to now face a new round of existential threats. Warming seas and commercial fishing are changing the availability of the zooplankton the whales depend on for food.

Four days after observing the tagging operation, I joined Warren, Zitterbart, Saeki and Julien Bonnel of Woods Hole on the prey boat. The cruise ship had to make a detour to Frei Station, a Chilean base with an airstrip on King George Island in the South Shetlands, to evacuate an injured passenger to the nearest hospital, in Chile. The researchers decided to use the unexpected stop to map the krill and DMS concentrations in a shallow embayment on the island's north side.

We wore jackets, hats and gloves against the morning cold, but just a few weeks earlier Antarctica had logged an all-time high temperature of 64.94 degrees Fahrenheit. The Antarctic Peninsula, where we had been exploring, is one of the fastest-warming regions on the planet. As a result, it's losing large amounts of ice, which is bad for krill, Warren said. Juvenile krill depend on winter sea ice for shelter and are thought to eat algae that grow on the underside of the ice.

Rising temperatures are not the only source of pressure on krill. Demand for the small crustaceans has surged over the past two decades, mostly from the nutritional supplements industry, which promotes krill oil as a rich source of omega-3 fatty acids for humans, and the aquaculture industry, which uses krill in feed for farmed fish. Whether the krill fishery is being managed sustainably is a contentious question. But a 2020 study of krill predators found that even with conservative catch limits for Antarctic krill in the waters around the Antarctic Peninsula—less than 1 percent of the stock in the southwestern Atlantic sector of the Southern Ocean—penguins in this region are declining, perhaps because the fishing vessels are focusing their efforts in areas that the penguins also favor. As the distribution and biomass of krill and other prey species change, predators—including whales—have to adapt their foraging routines accordingly.

As the Zodiac chugged away from the cruise ship, the researchers set up their equipment. They use an echo sounder to send sound waves down into the water, where they bounce off krill and any other animals they encounter, generating a picture on Warren's laptop of the creatures drifting in the water column. The lower the frequency of the ping, the deeper the transducer can “see.” Higher-frequency pings, in contrast, can see smaller targets. The team uses two frequencies, one low and one high, to search for aggregations of the tiny krill, which typically hang out in the upper 650 feet of the water column. Warren's lab mascot, a small squeaky-toy pig named Sir Pings-a-Lot II, was overseeing the proceedings. “This is as exciting as it's going to get,” Warren joked as he dropped the echo sounder overboard.

Krill are not as thrilling to track as their predators, but in recent years the science that happens in the prey boat has produced the greatest gains. As the boat traveled along its transect lines, Saeki reached over the side to scoop seawater from the surface every two minutes for analysis. Two plastic cases about the size of a handbag and carry-on suitcase contained the equipment for measuring any DMS in the water samples. A bubbler pushes air into the sample to get the DMS into the gas phase; a dryer removes any lingering moisture; an ozonator creates elementary sulfur from the DMS gas; and a photomultiplier measures light emitted by the sulfur—the amount of light is proportional to the amount of DMS present. Previously this sort of analysis was done in the lab; Woods Hole researchers were able to miniaturize Toda's DMS measurement setup to fit into a small boat. “The fact that we can run the DMS sniffer in the Zodiac is the big accomplishment this season,” Zitterbart said. Among other things, it allows them to analyze a water sample on the spot. “We don't know how long the DMS signal from the water sample is viable,” he explained. “To be cautious, we process it within two minutes.”

Monitor the echo-sounder data, scoop the water, process the sample. Repeat. There were no whales here to distract from the monotony, just achingly blue sky, a raw wind and the drone of the outboard. We were more than halfway through the survey before the echo sounder detected any krill—a patch of the crustaceans suspended above the seafloor in the shallow waters of the embayment. What the work lacked in adrenaline it made up for in potential scientific impact. “Nobody has surveyed much of these bays, so any data we can get are valuable,” Warren said. They returned to the ship with two krill patches detected and dozens of water samples analyzed—data that will help researchers understand how krill and DMS are distributed in the Southern Ocean and establish a baseline for measuring future change.

By March the brief austral summer was already drawing to a close. Daylight was ceding time to darkness, and the sea ice was starting to advance. Soon the humpbacks would head north to breed in the warm waters off the western coasts of South and Central America. Maybe that's why they weren't cooperating. Although the researchers had successfully tagged the five whales they had permits for, only two of the creatures went on to feed while they were being monitored. The other three snoozed or milled around the bays relaxing. To Zitterbart, the whales' lack of interest in foraging means that next time the team needs to shift the timing of their research. “By March [the humpbacks] are already so big that they're sleeping too much,” he said. “Earlier in the season is better because the whales are still building their body reserves and are more active.”

The water chemistry strategy may need tweaking, too. Preliminary analysis of the samples obtained by the researchers as well as additional samples gathered by passengers through the ship's citizen science program showed lower than expected signals from the DMS. Perhaps there just wasn't a lot of DMS in the water. But another possibility, Warren speculated, is that a layer of melted freshwater atop the seawater diluted the signal. “The physics of water complicates things,” he said. To get a clearer picture of the chemistry, the researchers may need to sample deeper water.

Going forward, Zitterbart wants to move away from cruise-ship sightseeing schedules and focus on building a detailed picture of the activity in a single bay. The plan is to hitch a ride on a cruise to one of Antarctica's research bases and stay there with just the Zodiacs. They'd map the whales, the krill and the water chemistry in the same place multiple days in a row and see how they change, then catch the mother ship on its way back.

First, though, they need to find a boat that can take them back to the bottom of the world. The cruise industry has a backlog of paying customers from the past few years who were unable to go on their planned voyages because of the pandemic. Trips the team might ordinarily be able to tag along on are fully booked. “We anticipated needing five years of data, and now three years are gone,” Zitterbart said of the pandemic's effect on the project. He's hopeful they may be able to get passage in 2024. In the meantime, he has turned his attention to research on the other side of the planet that could hasten help for the whales that need it most.

During the past three years, while waiting for the next Antarctic opportunity, Zitterbart, Owen and their colleagues have been studying the relationship among DMS, zooplankton and baleen whales in the waters off Massachusetts. Because they can't tag North Atlantic right whales, they're looking for correlations between DMS hotspots and right whale aggregations in Cape Cod Bay. The idea is to see if the chemical can be used as a proxy to predict where the whales will show up. The team surveys the whales by boat and plane, no tags required. Whereas the Antarctic research aims to identify the precise mechanism by which the baleen whales find their prey—whether it's by following DMS gradients to swarms of krill or some other means—the Cape Cod work seeks only to establish whether these whales tend to show up in parts of the ocean where DMS concentrations are higher. If so, then regardless of whether the whales are actually detecting DMS or following some other cue that just happens to be linked to DMS, the scientists can theoretically use DMS values to predict where and when whales will appear.

Current efforts to protect North Atlantic right whales involve seasonal speed restrictions for ships and visual and acoustic monitoring systems. For example, from January 1 to May 15 in Cape Cod Bay, an important feeding ground for the right whales, all vessels 65 feet long or longer have a speed limit of 10 knots to reduce the likelihood of serious injuries to whales from collisions. If whales are seen or heard in the area at any time of year, then boats of all sizes are asked to slow down and watch out for the creatures. A free app called Whale Alert displays seasonal management areas and whale-detection data on a map in near-real time.

But these management approaches lack predictive power, says research ecologist David Wiley of the National Oceanic and Atmospheric Administration's Stellwagen Bank National Marine Sanctuary, who works with Zitterbart on the DMS research. And big boats in congested shipping lanes often can't change course fast enough to avoid collisions with slow-moving whales. “With a predictive tool like DMS, we can plan rather than react.”

In 2021 Owen, Zitterbart, Wiley and their collaborators published a paper based on the Cape Cod research showing that higher levels of DMS correspond to higher concentrations of zooplankton, so if baleen whales do track DMS, it will, in fact, lead them to prey. Now the researchers are looking at whether baleen whales actually aggregate in these DMS hotspots. Preliminary results indicate that both North Atlantic right whales and sei whales (another baleen species that eats copepods) do.

To strengthen their case, starting this year, the researchers will measure DMS concentrations in Cape Cod Bay and Massachusetts Bay every two weeks along standardized track lines before the right whales get there, when they arrive and when they leave. Their goal is to figure out how much DMS has to be in the water for the whales to show up. “We need to find out the thresholds, what's biologically relevant to the whales,” Wiley says of the study, which he estimates will take around two years.

The dream is to be able to monitor places where DMS levels are building—and thus likely to be gathering spots for North Atlantic right whales—from space using satellite imaging. Wildlife managers could reroute ships around those areas or temporarily shut down fisheries or wind energy sites that might disturb the whales until the DMS levels subside and the whales move on. Climate scientists have long been interested in DMS because it promotes cloud formation. They have already found that the chemical can be detected from space. But it will take higher-resolution satellite data than is now available to predict the movements of whales.

For North Atlantic right whales and all the organisms whose fates intertwine with theirs, insights can't come fast enough. “If things don't change, right whales will go extinct in our lifetime,” Wiley says. He believes the plight of this keystone species is the conservation issue of our time. Maybe with the help of hungry humpbacks in Antarctica and some curious scientists, North Atlantic right whales and other imperiled baleen whales will one day reclaim their place as rulers of the ocean realm.