Читать книгу Spying on Whales: The Past, Present and Future of the World’s Largest Animals - Nick Pyenson, Ник Пайенсон - Страница 14

I think a lot about how whales die. That might sound like the ranting of a whale bone chaser gone full Ahab, but my preoccupation is not with the gore of decaying flesh or exploding body cavities (although those don’t really bother me). Instead, I’m fascinated by the details of the what, where, how, and why: what happens to their carcasses, their locations when they expire, how whales perish, and the reasons for their demise. You might think that these facts are easily uncovered in the scientific literature, or in the many accounts of whaling on the high seas. But they aren’t, not for all of the whales that have washed up on the world’s shores or been hauled up by whalers. So I parse these factors in my head instead—ocean currents, water depth and temperature, scavengers, time to burial, and even anatomical differences—that contribute to the many different ways that a whale carcass might become a fossil.

Figuring out what parts of the living world can be entombed in rock, and how we might find them, is a game of probabilities. Paleontologists tend to think about the life and death of organisms as a continuous thread from birth to death—and to museum drawer. We visualize this thread as a stream of information where a variety of biological and physical processes winnow away data at each step in a pathway of decay: a carcass scavenged to pieces, not buried intact; a skeleton, or parts thereof, coming to rest in an unpromising setting; the rocks containing the fossil accidentally destroyed. Even if a great specimen is uncovered, the fossils may lie silently in a museum, collected from the field but mislabeled or undiscovered in a drawer. The reality is that we lose information throughout this process; it’s an attrition of data from carcass to cabinet. Given the chances against any living thing becoming a fossil, it is a wonder that we know anything about life from the geologic past at all.

Thinking like a paleontologist makes you something of a connoisseur of dead things. My pursuit of dead whales has led to the rich record of strandings. Since antiquity, whale strandings have captivated the interest of everyone from Aristotle to casual spectators of exploding whale videos on YouTube. Strandings are a timeless motif—an immobilized, beach-cast leviathan, angrily tail-slapping against the surf. The image shocks us because we imagine whales to be fully a part of the aquatic realm. How would a whale end up landlocked in our world, a creature so large and strange suddenly so uniquely vulnerable?

Whale strandings happen in many different ways, for multiple reasons. Consequently, there is no single definition of a stranding—just an operational one, for the seemingly aberrant sight of a whale on a shoreline. For example, a stranding might consist of one whale, a mother-calf pair, several individuals from a single species, or several individuals from different species. Adding to the complexity is how they strand: whales may be dead upon arrival, alive but flailing on the shore, or already decayed to a raft of blubber, cartilage, and bones.

Beyond the how, there is still more complexity to the why: what causes a whale stranding? Senescence or disease may provide simple explanations in some cases, whereas the side effects of living near humans can be either plainly obvious (entanglement in fishing nets or ropes) or more difficult to plumb (toxin poisoning from marine algae). Certainly the sight of an entire pod of whales stranded, dozens in a row, begs some kind of explanation, although that is frequently elusive. True cause is often like that, in the natural world.

For naturalists working before the era of Yankee whaling in the midnineteenth century, strandings provided the only source of anatomical knowledge about whales. Despite all of the whales killed by Basque whalers for hundreds of years in Europe, there was essentially no documentation of what a whale looked like on the inside—fairly important evidence, when you consider that despite a few snout hairs, nipples, and nostrils for breathing air, whales otherwise largely look like fish on the outside. There are similarly limited firsthand descriptions of dissections on stranded whales from this era—though they must have been uniquely awful. Once word of a whale stranding arrived at the door of a rural doctor or an amateur naturalist in the eighteenth or early nineteenth century, the opportunity would have launched a whirlwind of ad hoc planning for several days of dreary, odorous dissection. The happenstance of a whale stranding dictated where the dissection occurred. The scale of the carcass would have dwarfed the tools at hand, the decay of flesh accelerated by nice weather or retarded by wet and cold conditions. The work was certainly not glamorous. And there were no modern winches or cranes, nor photographs to document the findings. Just ink, paper, and a strong stomach.

A stranded whale affords a detailed look not merely at diagnostic traits—a ridge along the snout, a piebald underbelly, or a knuckled tailstock—but at its inner anatomy, musculature, and organ systems, which can’t be described from a boat. In the early nineteenth century, the first naturalists to roll up their sleeves and describe what they saw were enabled by an emerging infrastructure for scientific reporting—published scientific proceedings. By writing down, illustrating, and sharing what they saw, they created the basis for others to seek out their own comparisons. Even Aristotle knew that whales were mammals, but these first detailed dissections revealed facts of their inner world that were equally familiar and puzzling: they had a heart, lungs, a stomach, an intestine, and a reproductive tract just like a dairy cow or a tax collector. The early scholarship generated by careful anatomical work on whale strandings had serious impact on science, surpassed only by anatomists working under more stable laboratory conditions, with the benefit of refrigeration and power tools, generations later.

While today we know that, for instance, blue whales off Ireland, California, and South Africa all belong to a single species, naturalists in the eighteenth and nineteenth centuries did not. With incomplete (and sometimes incorrect) descriptions of other large whales, naturalists puzzled by variations in color or size would sometimes create a new scientific name based on a single stranding, or judge that a whale’s appearance far from another record merited its description as a new species. It took until the early twentieth century for Frederick William True, one of my predecessors at the Smithsonian, to unravel these issues for large baleen whales and demonstrate that blue whales, humpbacks, and fin whales, among several others, were the same species on both sides of the Atlantic—despite dozens of taxonomic names purporting otherwise. True spent years working with original name-bearing specimens (called type specimens) for these different species, doing what taxonomists largely regard as housecleaning—a time-consuming task involving chasing specimens that are archived around the world’s museums and figuring out their identity.

Even today there are some species of beaked whales known only from skulls washed up on a beach—yes, in the twenty-first century there are several ton-heavy species of mammals in our planet’s oceans whose scientific basis primarily relies on a single beach-cast skull. Beaked whales are among the deepest-diving whales, looking something like a bottlenose dolphin crossed with a submarine. In fact, we know very little about most species of beaked whales, which account for nearly a quarter of all whale species alive today—they simply live too far at sea, dive too deep, and are incredibly difficult to tag or photograph in life. Without museums to house the rare remains that do turn up, we would know far less about these enigmatic species.

Not every whale that dies washes ashore, of course. Whalers for several hundred years have known that some whales float after death while others sink. Dead sperm whales float because of the enormous oil chambers housed above their faces, as Yankee whalers knew well. Right whales earned their moniker because they were the right whales to hunt, and they float after death because of their massive blubber layer, a trait they share with bowhead whales, their close relatives of the Arctic. Other large baleen whales, such as blue or humpback whales, will sink after a prolonged time at the surface, although carcasses can refloat following enough decay, when the gases from decomposition make the carcass buoyant. It isn’t uncommon to see the large throat pouch of some of these whales balloon after death, like an emergency air bag that somehow failed to deploy properly in life.

Beyond these facts, known mostly to whalers and beachcombers, no one really knew much more until 1977, when a U.S. Navy submarine cruiser accidentally discovered a gray whale carcass on a seafloor more than four thousand feet deep, west of Catalina island, off the California coast. Of course, we already knew that some whale carcasses fall through the water column and reach the ocean floor, well beyond the depth that light penetrates—it’s just that no one had ever seen the result until then. The scientists who later worked up the growing number of these discoveries called them whalefalls.

At thousands of feet deep, the seafloor is not merely cold and bathed in black; its surface is mostly barren—until a carcass lands, ending its transit through the water column, an elision between two worlds that began with the whale’s last breath at the surface. Whatever flesh that has not already been picked away by sharks or pecked by seabirds provides immediate food for scavengers, such as deep-sea sharks, fishes, and crabs. (How, exactly, they find a fallen whale remains a mystery.) In very little time—researchers estimate weeks to months—these creatures will strip the carcass of its flesh, leaving only bone. On the deep seafloor, there’s little current to disturb the position of the bones, leaving the skeleton looking mostly how it looked as it fell through the water column: the jaws close to or in direct articulation with the skull, which itself is connected to the vertebral column, in a straight line, with arm and flipper bones off to each side, assuming these parts weren’t ripped off by scavengers at the water’s surface.

But once seafloor scavengers swim and scurry away, the story isn’t over. Scientists aboard deep-sea submersibles have set out in search of whalefall skeletons and even experimentally sunk whale carcasses to predetermined locations to learn more. With enough replications and time, they found that whalefalls undergo successive phases, not unlike a forest ecosystem that changes in composition and size as it matures over decades.

Once defleshed, whalefalls undergo a second phase of colonization by snails, clams, and polychaete worms—some feeding off cartilage and the surfaces of the bone, others burrowing into the apron of sediment around the skeleton, enriched by the organic material leaching off the whale’s blubber and oil. The snails, clams, and polychaetes take months to a few years to consume all that they can, and afterward a third phase begins, which can last decades or more (no one knows because whale-falls have been studied for only forty years). This presumably final climax stage involves two sets of bacteria living in or on whale bones: anaerobic bacteria that use sulfate in the seawater to digest oil locked in the whale’s bones; and then sulfur-loving bacteria that use the sulfide by-product of the anerobic bacteria to generate energy by combining it with dissolved oxygen. Sulfophilic bacteria support a variety of true whalefall specialists at this stage, including some mussels, clams, and tube worms that have the bacteria living symbiotically within them, giving them the opportunity to generate their own energy in a world devoid of sunlight. At these depths, whale carcasses give a second life to an otherwise barren, abyssal world.

While the precise duration of these skeletons on the seafloor remains unknown, the upper bounds of some estimates suggest that a single whale carcass can provide up to one hundred years of sustenance. So little is known about the breadth and variation in whalefalls that new discoveries are being made all the time: one is an organism called Osedax—literally, Latin for “bone devouring”—a species of deep-sea worm whose entire life cycle depends on whalefall skeletons. Appearing as pinkish filaments only a few millimeters long covering the surfaces of bone, Osedax does not have a mouth or a gut, just wavy tendrils called palps facing outward. Instead of harboring symbiotic bacteria that use a sulfur-based pathway for decomposing bone lipids, its symbionts are a type of bacteria that mobilize proteins directly from the bone itself by dissolving it, using a tangled mat of bacteria-filled roots burrowed into the bones.

Not all whalefall colonizers are specialists; some are generalists that also make appearances on hot vents and methane cold seeps deep on the ocean floor. The range of the temperatures and environmental settings across these deep-sea habitats has led some scientists to argue that whalefalls, over millions of years, have served as evolutionary stepping-stones for invertebrates living in one habitat to leap to another. This idea remains hotly debated, with little known about all of the species that feast on fallen whales, or how often and where these skeletons are likely to appear on the seafloor.

A whale’s size would seem to play an important role for a unique ecosystem that is fundamentally tied to its carcass. After all, a larger dead body should provide more opportunities for whalefall specialists. It turns out that size doesn’t make too much of a difference where whalefalls are concerned, and the reason why we know has to do with the fossil record. As a graduate student, I had the good fortune to come across a fossil whale skull that had been collected a few decades prior, from rocks exposed on Año Nuevo Island, off the coast of central California. These rocks represent extremely deep-sea sediments about fifteen million to eleven million years old, and I didn’t think much about the fossil’s context until I was cleaning the skull in the fossil preparation lab at Berkeley’s paleontology museum and came across tiny clamshells nestled in crevices of the skull. They clustered together, almost lifelike, and I decided to pry one off for a closer look, after first documenting their arrangement. A mollusk specialist confirmed a possibility that had entered my mind: they were chemosymbiotic clams belonging to a family that specialized in whalefalls. In short, the fossil I had been preparing belonged to a fossil whalefall.

Fossil whales with whalefall mollusks affixed to them had been found before and, while unusual, they weren’t earthshaking discoveries on their own. But what was different was that this skull belonged to a whale that would have been barely eleven feet long in life. Tiny baleen whales—much smaller than those today—were common during the Miocene, but the remarkable aspect of this find was that their small body size did not prevent the whale-fall from reaching the peak phase of sulfur-loving invertebrates. In other words, size doesn’t really determine the community that colonizes a whalefall ecosystem. If not, then what does? That’s still not clear, although it may be something about the lipids locked in the bones that controls which species can colonize the carcass, along with the stages of whalefall succession.