Kitabı oku: «Spying on Whales»

I sat transfixed by a sea littered with a million fragments of ice, all rising and falling in time with the slow roll of the waves. We had spent the morning looking for humpback whales in Wilhelmina Bay, threading our rubber boat between gargantuan icebergs that were tall and sharp, like overturned cathedrals. Now we stopped, cut the engine, and listened in the utter stillness for the lush, sonorous breath of an eighty-thousand-pound whale coming to the water’s surface. That sound would be our cue to close in. We had come to the end of the Earth to place a removable tag on the back of one of these massive, oceangoing mammals, but we took nothing for granted in Antarctica. As we sat waiting on the small open boat, I came to feel more and more vulnerable, a speck floating in a sea of shattered ice. “Don’t fall in,” my longtime collaborator and friend Ari Friedlaender deadpanned.

I struggled to remember how long we had been away from the Ortelius, our much larger oceanic vessel with its ice-hardened steel hull. In every direction, we were enclosed by a landscape of nunataks, jagged spires of rock that pierced the creamy tops of surrounding glaciers. Where the glaciers met the sea, they ended in sheer, icy cliffs towering over the bay. Without a human structure for scale, these landforms seemed both near and far at the same time. This otherworldly scene of ice, water, rock, and light warped my sight lines, bending my sense of distance and the passage of time.

If you hold your fist with your left thumb out, your thumb is the western Antarctic Peninsula; your fist, the Antarctic continent’s outline. The Gerlache Strait is part of a long stretch of inner passageway along the outer side of Antarctica’s left thumb, and Wilhelmina Bay cuts a rough cul-de-sac off the Gerlache. The Gerlache is a hot spot for whales, seals, penguins, and other seabirds, and Wilhelmina Bay is the bull’s-eye. All come here to hunt for krill, small crustaceans that form the centerpiece of Antarctic ocean food webs. Consider your hand again: individual krill are about the length of your thumb, but whales pursue them because they explode into great aggregations, or swarms, during the Antarctic summer. With the right mixture of sunlight and nutrient-rich water, dense clouds of krill form a sort of superorganism that can stretch for miles and concentrate in hundreds of individuals per cubic foot. By some measures, there is more biomass of krill than of any other animal on the planet. Calorie-rich swarms of them lurked somewhere, not far, just under our boat.

Where there are krill in sufficient quantities, there will be whales, but the fundamental problem with studying whales is that we almost never see them, except when they come to the surface to breathe or when we dive, in our own limited fashion, in search of them. Whales are inherently enigmatic creatures because the parameters of their lives defy many of our tools to measure them: they travel over spans of whole oceans, dive to depths where light does not reach, and live for human lifetimes—and even longer.

In Wilhelmina Bay our goal was to attach a sleek plastic tag to the back of a whale to record audio, video, depth, changes in the whale’s speed, and even pitch, yaw, and roll. Our tags would provide crucial context for how humpbacks interact with their environment by relaying, in a time-stamped way, how they feed on krill. Ari and his colleagues have tagged and tracked whales along the Antarctic Peninsula for nearly two decades, charting their movements against the backdrop of changes in krill-patch density, water temperature, daylight, and other variables. As climate change warms the poles faster than the rest of the planet, every year counts.

I sat on the gunnel of the boat while Ari scanned from the bow. We were several days into a multiweek expedition with high hopes to tag as many humpbacks as possible—ideally, a pod feeding together—but thus far we had been skunked. Ari stood rigid like a figurehead, holding a twenty-foot-long carbon-fiber pole in folded arms. The pole flexed in synchrony with the swell, the teardrop-shaped tag bobbing at one end. I watched clouds shift slowly overhead, mirroring the dappled quicksilver of the water, and wondered whether any other place on Earth could feel as alien as Antarctica. Then a loud gurgle interrupted my daydreaming, followed by a trumpeting blast of water vapor bursting from flaring, paired nostrils. A whale’s blow.

We knew to expect more plumes of water immediately thereafter. A pod of whales synchronize when they surface to breathe—sometimes nearly simultaneously or split seconds apart. They usually breathe a few times in a row, in quick succession, before resubmerging—unless they’re asleep or truly exhausted, whales tend to act like surfacing is a nuisance and they’d rather be deep underwater. Their tight coordination in breathing likely has a lot to do with maximizing time spent below the surface engaged in the cooperative tasks of finding food and avoiding predators. Some species travel or hunt in pods that are tightly knit family genealogies, while others, such as the humpbacks before us, form short-lived associations, seemingly a matter of happenstance.

“Oh, that’s right,” Ari called out. The vapor from the blow lingered in the cold air. Ari pointed to a small patch of water a dozen yards from the boat, perfectly calm against the waves at the surface. This patch, called a flukeprint, betrayed the whale’s movement, unseen deep below our tiny boat. The single flukeprint bloomed into several, each the size of our boat, rising up from the depths, whirling and spreading into the smooth geometry of a lily pad. We were right. “He’s got buddies,” Ari said. Without the aid of an echo sounder—which would also tip off the whales about our location—we used the ephemeral patterns on the surface to read their path.

We started up the engine, throttling ahead slightly to a spot beyond the last flukeprint. Within seconds, right on cue, a pair of enormous nostrils bubbled at the surface, releasing a thundering tone and then a spray that carried past us as we kept up with the whale. A dorsal fin surfaced, and a second and third blow exploded nearby. “Pull up behind this last one. We have about three more breaths until they go down,” Ari shouted.

We trailed the laggard of the group, maneuvering the boat into position. Ari lowered himself across the bow and held the pole against his torso, extending the tip with the tag just ahead of the dorsal fin as we motored close to the behemoths moving yards away. Then, in a decisive motion, Ari launched the tip of the pole toward the whale’s back, where the tag’s suction cups hit the skin with a satisfying thwack. The whale rolled beneath the surface as we pulled back to pause and wait for it to return. We spotted its sleek, shiny back as it arose again, marked with a neon tag, and we cheered. The whale took one last breath before it raised its monstrously wide tail flukes out of the water and slipped down into emerald darkness with the others. Ari radioed back to the Ortelius. “Taaaaag on,” he said with a hint of swagger as he grinned at me.

Tag on

The whole rodeo of tagging is a bit like sticking a smartphone on the back of a whale, complete with the logistics of getting close enough to a forty-ton mammal in the first place. Just as your smartphone can record movies, track where you go, and automatically rotate images, the same technology—miniaturized and cheap, combining video, GPS, and accelerometers all in one device—has fueled a revolution in understanding how animals move throughout their world. Scientists call this new way of recording organismal movement biologging, and it has captured the interest of ecologists, behavioral biologists, and anatomists, all interested in knowing the details of how animals move through space and over time. Biologging has been especially important for revealing the daily, monthly, and even annual meanderings of animals that are extremely difficult to study. Stick a tag on a penguin, a sea turtle, or a whale, and there’s a chance to know how it swims, what it eats, and everything else it does whenever you’re not around to watch it—which is most of the time, for animals at sea.

The logistics of studying whales places them in a realm truly apart from every other large mammal on land or at sea. To know anything about them in the wild takes time on a boat, sticking a tag on their back, sliding a camera underwater, or spying overhead with a drone—if you’re lucky enough to come upon them in the first place. Biologging is helping us overcome this challenge by giving us a remote view into their lives, an extension of our senses more intimate and sometimes more detailed than any telephoto lens. In the case of humpbacks, tag data have revealed how these gulp-feeding whales lunge at large schools of krill and other prey, oftentimes in coordinated attacks. It’s a form of pack hunting, which might seem strange for a species celebrated as gentle giants. But baleen whales are serious predators, not like grazing cows but more like wolves or lions, pursuing their quarry with strategy and efficacy. Don’t be fooled by their lack of teeth, or just because krill don’t bleat in terror.

Hours later the Ortelius prowled silently through Wilhelmina Bay, with a pair of massive spotlights leading the way in search of icebergs in our path. Outside on the bow, I watched heavy snowflakes drift through the cones of light as Ari unfolded the metal radio antenna that would lead us back to our tag. The tags we were using had to be re-collected to yield their data; we would have to find and physically pluck them out of the water, provided that they’d already been knocked off the whales’ backs. By design, they can last minutes, hours, or even days from the force of suction alone, before being scraped, being bumped, or falling off. Its buoyant neon housing would keep the whole device floating on the surface until we triangulated its position.

Over the course of his career, Ari has probably tagged more species of whales than anyone, in all circumstances and in every ocean. Beyond friendship, mutual colleagues, shared ambitions, and wry humor, we were working together in Antarctica to build a bridge between our disciplines—paleontology for me, ecology for him—because questions about how whales evolved to become masters of ocean ecosystems over fifty-odd million years need to be grounded in the facts of what whales do today. Bridging gaps between disciplines sometimes necessitates spending time side by side. All the better if it’s in the field.

The metal prongs that Ari assembled looked like an elaborate set of rabbit ears from an old television set. He plugged the antenna into a small receiver with a speaker, and after a few moments, we heard a series of intermittent beeps. “That gap between the beeps tells us that the whale is sleeping, rising up to the surface to breathe, and then sinking back down.” Ari smiled. “Just dozing, belly full of krill. Not a bad way to spend a Saturday night.” We would need to come back later and listen again for our tag until it floated freely, beeping uninterrupted.

Most large baleen whale species alive today belong to the rorqual family, which feed on krill and other small prey by lunging underwater. They comprise the more familiar members of the cetacean bestiary, including humpbacks, blue whales, fin whales, and minke whales. Rorquals are also the most massive species of vertebrates ever to have evolved on the planet—far heavier than the largest dinosaurs. Even the smallest rorquals, minke whales, can weigh ten tons as adults, about twice as much as an adult bull African elephant. Rorquals are easy to distinguish from any other baleen whale, such as a gray whale or a bowhead whale: look for the long, corrugated throat pouch that runs from their chin to their belly button. (And yes, whales have belly buttons, just like you and me.) The features that make rorquals so obviously different from other baleen whales also play a critical role in how they feed.

Across whole ocean basins, individual whales find their food using probability, heading for feeding grounds burned into memory from a lifetime of migration. Rorquals travel routes that span hemispheres over the seasons; an individual whale might migrate from the tropics in the winter in search of mates and to bear young, then to the poles during the summer to forage under constant sunlight. Baleen whales still retain olfactory lobes, unlike their toothed cousins, such as killer whales and dolphins, which have lost them. Baleen whales might smell some aspect of their prey at the water’s surface, and it is possible that this mechanism could refine their search once on the scene. Originally their sense of smell evolved for transmission through air, not water; we know little beyond the basics about this sense in whales. Somehow, whales manage to be in the right place at the right time to feed. And what’s clear from biologging is that once in the right place, baleen whales spot the prey patches from below, probably approaching them by sight. Lacking the echolocation of their toothed relatives, vision is likely the dominant sense for baleen whales at short range.

With prey in range, a rorqual accelerates, fluking at top speed, and begins the amazing process of a lunge. Surging from below, it opens its mouth only seconds before it arrives at a patch of krill or school of fish, which may be as big as or bigger than the entire whale. When it lowers its jaws, the rorqual exposes its mouth immediately to a rush of water that pushes its tongue backward, through the floor of its mouth, into its throat pouch. In mere seconds, the accordion-like grooves of its throat pop out like a parachute. After engulfing the prey-laden water, the whale slows, almost to a halt, pouch distended and looking bloated, nothing like its airfoil-shaped profile from moments prior. Over the next minute, it slowly expels water out of its mouth through a sieve of baleen, until its throat pouch returns to its original form, the prey swallowed. For their part, krill and fish deploy collective defensive behavior by dispersing to try to escape the oncoming maw of death. In the end, a successful whale takes a bite out of a much bigger, more diffuse and dynamic superorganism.

Lunge feeding has been described as one of the largest biomechanical events on the planet, and it’s not hard to imagine why when you consider that an adult blue whale engulfs a volume of water the size of a large living room in a matter of seconds. Tags on humpbacks in other parts of Antarctica show how they sometimes feed close to the seafloor in pairs, swimming alongside each other as they scrape the bottom with their protruding chins in mirrored unison. Tags have also shown us that rorquals are right- or left-handed, just like us, favoring either a dextral or sinistral direction when they roll their bodies to feed.

The more scientists tag whales, the more it’s apparent that there’s still much that we don’t know. It turns out that blue whales have a behavior where they spin 360 degrees underwater in a pirouette before they lunge, probably to line up their mouths precisely with a patch of krill. Other lightweight tags, launched with barbs that cling more deeply beneath the skin on the dorsal fin, have tracked the movement of Antarctic minke whales migrating over eight thousand miles of open ocean, from the Antarctic Peninsula to subtropical waters. These tags upload data directly to satellites whenever the whale surfaces, over the course of weeks to months, before eventually falling out. These tags are also especially useful for species that are rarely seen, such as beaked whales. Satellite-linked dive tags deployed on Cuvier’s beaked whales revealed, in a precise way, the astonishing extremes of their foraging dives for squid and fish—over 137.5 minutes of breath holding, 2,992 meters deep—data that set new dive records for a mammal. If the idea of holding your breath for over two hours doesn’t alarm you, imagine doing it while chasing your dinner to a depth of nearly two miles.

Tag data combined with tissue samples taken from biopsy darts tell us that these humpback whales feeding in the western Antarctic Peninsula are merely seasonal visitors for the austral summer. By early fall, they depart the icy bays, cross the great Circum-Antarctic Current that rings the seventh continent, and undertake various paths over thousands of miles to arrive at temperate latitudes. At Wilhelmina Bay the overwhelming majority of humpbacks return to the low latitudes of the Pacific coasts of Costa Rica and Panama to mate and give birth before returning to the Southern Ocean for the next austral summer to feed.

We eventually retrieved the tag, along with its data, and continued to Cuverville Island, on the other side of Wilhelmina Bay. As the Ortelius maneuvered out of the Gerlache and toward the island, I watched from the ship’s stern as we passed icebergs more massive than any I had yet seen. Their fragmented sides, a hundred feet high, were etched in luminous, milky blues and grays. They held light from sea to sky, glowing in unearthly ways, as if they could not have been formed on this planet. And of course they were mostly sheathed underwater, which was a bit of an ominous thought; the Ortelius kept a careful distance. But the incomprehensibility of something as overwhelming as an iceberg is belied by its transience: even the largest ones, platforms the size of cityscapes, will eventually shed their layers of ice, annealed over hundreds of thousands of years, and become part of the sea.