But there are also differences.
Antarctica is a frozen continent in the Southern Hemisphere, surrounded by ocean. The Arctic is the northernmost ocean, encircled by land. Both regions boast whales and seals in their waters, but Antarctica and its environs have penguins, and the Arctic does not.
And the Arctic, unlike the Antarctic, has polar bears.
Polar bears are found only in the north, penguins only in the south; cartoons and Christmas cards notwithstanding, the paths of the two do not cross. Should a population of polar bears be picked up by a panhemispheric tornado and deposited on Antarctica, their stay would assuredly be brief; they would likely rampage through the resident seal and penguin populations during the summer but find themselves bereft of sustenance during the long, hostile Antarctic winter—when seals retreat to the sea and the coasts of sub-Antarctic islands and temperatures plunge to levels that would challenge even this hardiest of predators.
Defining the Antarctic is somewhat easier than delineating the Arctic. The conventionally acknowledged northern limit of the former is the Antarctic Convergence, where the cold waters of the Southern Ocean clash with warmer seas farther north and establish a genuine boundary between temperate and polar realms. In contrast, the outermost reaches of the Arctic are more land than sea, and accordingly there is no uninterrupted circumpolar current to provide similarly convenient demarcation.
Distance from the equator alone is an insufficient criterion for inclusion. For example, the Arctic Circle (the line at 66 degrees north of which the ancient Greeks were aware) excludes Iceland and large parts of Siberia; Berlin, which is plainly not an Arctic environment, is on the same latitude as the Siberian town of Irkutsk, which suffers winter temperatures as low as—40°F and therefore arguably is. Many authorities use as a boundary the 10°C isotherm, a term that neither trips off the tongue nor is intuitively simple to grasp. It refers to the line along which, during the warmest month of the year (usually July), the average temperature is no higher than 10°C (approximately 50°F ). That may seem somewhat arbitrary, but it does roughly correspond to the tree line, the point beyond which forest yields first to taiga (a region of scattered trees and scrub) and thence to tundra.
But there is another measure of greater relevance to our purposes: the southern limit of the winter pack ice. This encompasses all the waters that regularly freeze during winter, all the way south to James Bay in Canada, which is on the same latitude as England but is nonetheless, in terms of climate and ecology, Arctic in nature. It includes also Hudson Bay, Baffin Bay, Davis Strait, and Baffin, Banks, and Victoria islands in Canada; the coasts of Greenland; Svalbard and parts of the Barents Sea; Novaya Zemlya and the islands and coast of Siberia; the Kara, Laptev, and Chukchi seas off Russia; and the Bering Strait and Beaufort Sea off Alaska.
Not by coincidence, this is the range of the polar bear, Ursus maritimus, sea bear by scientific name and ice bear by nature. It is here, on the ice that covers bays and inlets, on the ice that is anchored to the coast, on the ice that drifts en masse offshore, on the ice at the Arctic's fringes, and even on the ice that encircles the North Pole, that the polar bear makes its home. It seems, to human eyes at least, desolate, a barren, hostile wasteland. Out of sight, however, below the feet of the prowling predator, the waters of the Arctic are rich in nutrients and life, some of which must occasionally come to the surface of water and ice to breathe and rest. It is in search of that life that the polar bear endlessly patrols its domain, a realm that it has made its own, to which it has, in a relatively short space of evolutionary time, become supremely well adapted.
To find a clue as to how that came to be, how, of all the bear species that live now or have existed in the past, just one should have ventured out onto the ice to hunt seals, we look to Alaska—not the icy coastal waters where polar bears roam today, but a very different environment indeed.
Of the many ways to explicate the scale of Alaska—the fact that it is twice the size of Texas, two and a half times as big as France, and almost six times as large as the United Kingdom; that it contains nineteen of the twenty tallest mountains in the United States, and the tallest in North America; the fact that its 35,000 miles of coastline, if somehow stretched into one long line, would encircle the equator with room to spare—the sheer diversity of its environments is one of the most impressive. The same state that in its northernmost reaches boasts tundra and sea ice is, in its southeastern region, lush and forested and regularly jammed with giant cruise ships.
There are no polar bears in southeastern Alaska, but there are brown, or grizzly, bears. There are many of them, in fact, particularly on three large islands—Admiralty, Baranof, and Chichagof, known colloquially as the ABC Islands—that form the northernmost extent of a 300-mile-long chain of mountainous island peaks called the Alexander Archipelago. There are three times as many brown bears as humans on Admiralty Island, 1,600 or so over an area of approximately one million acres, the highest density of brown bears in the world.
The grizzly bears of the ABC Islands are predominantly brown, with outer hairs that are often tipped with white or silver, giving them the "grizzled" appearance from which their nickname derives. They possess the large hump of muscle over their shoulders that distinguishes brown bears from other bear species. They eat roots and berries, fish, and small mammals. The males stake out territories and defend them fiercely from rivals. They look and act, in other words, like any other grizzlies.
But inside, they hold a secret.
The DNA of brown bears on Alexander Archipelago is different from that of brown bears anywhere else in the world. So distinct, in fact, is the DNA of the Alexander Archipelago bears from that of the rest of the world's grizzlies that other brown bears aren't even their closest relatives.
The Alexander Archipelago brown bears are most closely related to polar bears.
But we are getting ahead of ourselves.
We should begin at the beginning.
Polar bears are carnivores, and not only in that they eat meat. Indeed, they are the only truly carnivorous bear, as the other seven extant species are omnivorous or, in the case of the giant panda, effectively vegetarian. But they, and their bear brethren, are carnivores also in terms of their place in the great mammalian order of things.
All bear species alive today are members of the family Ursidae, in the order Carnivora, an order that includes the bulk of predatory mammals. (By way of comparison, human beings, along with bonobos, chimpanzees, gorillas, and orangutans, are generally—although not universally—grouped together in the family Hominidae within the order Primata, the latter of which also contains gibbons, monkeys, marmosets, and lemurs.)
The cat in your lap is a carnivore (in both the dietary and taxonomic senses), as are its larger relatives in the plains of Africa, the canyons of North America, and the jungles of South America and Asia. The family dog that sleeps at the foot of your bed, the raccoon that tips over the trash can at night, the skunk in the woods: all are members of the order Carnivora and thus relatives—second cousins, if you will—of the polar bear. Some, however, are more closely related than others. Generally speaking, cats and hyenas constitute one branch of the Carnivora family tree, and the remaining species—martens, minks, mongooses, weasels, wolverines, badgers, dogs, otters, seals, sea lions, and walruses—sit alongside bears on the other branch.
The progenitors of the Carnivora took their bow about 55 million years ago, around 65 million years after the appearance of the earliest true mammals and roughly 10 million years after a well-placed asteroid or two ushered the dinosaurs off the stage and allowed our mammalian ancestors—which, to that point, had mostly peered nervously at the proceedings around them from beneath bushes—to emerge blinking into the open.
Those progenitors, the miacids, were civetlike creatures, with long bodies and tails, that were likely mostly arboreal and probably fed primarily on insects and insect-eating animals like shrews. The Carnivora evolved from the miacids somewhat over 40
million years ago, and once they became established, they rapidly diversified. Within 5 to 7 million years, the dog, cat, weasel, and mongoose families had developed; and a little more than 10 million years after that—about 22 million years before the present—came the first bear.
Dubbed Ursavus elemensis and sometimes called the "dawn bear," it is known to us only from fragments of teeth and jaws. That is enough for paleontologists to determine it was about the size of a small terrier, but as to its appearance and habits we can only speculate. We can also only infer from an incomplete fossil record how it spawned other species and which species, in turn, evolved from those. We do know that by about 10 million years ago, Ursavus had disappeared, presumably as a consequence of climatic changes, the subtropical Europe in which it had evolved having become drier and the forests for which it was adapted having given way to steppes, plains, and desert. But where Ursavus fell by the wayside, others—evolving from either the dawn bear or contemporaries—emerged, spreading from Europe into Asia and thence the Americas. Some of the species that branched off from these lines have survived to the present day, while others, such as the cave bears of Europe and North America, and the giant, long-legged, short-faced bear—which at 2,200 pounds would have been approximately twice the size of most male polar bears—have long departed.
Today, eight species of bear—giant panda, sun, spectacled, sloth, Asiatic black, North American black, brown, and polar—are spread among four continents (Africa, Australia, and Antarctica being bearless). Each of them, in its own way, meets the more obvious visible criteria for a bear: a furry, essentially tailless, and somewhat sturdy body; wide paws with prominent claws; and a relatively large head with rounded ears and a muzzle less pointed than that of wolves. But within that broad generalization there are many differences, in size, shape, appearance, habitat, diet, and behavior.
Bears are generally caricatured as very large animals, and some of them truly are. But while the polar bear, the largest surviving bear species in the world, weighs in at up to 1,500 pounds or sometimes even more, the smallest, the sun bear—a denizen of the rainforests of Southeast Asia—is no bigger than a medium-sized dog and is in fact known in Thailand as a "dog bear" for that very reason. (Its more common name derives from a crescent-shaped patch on its chest.)
Bears' coloring ranges from the white appearance of the polar bear, through the black and white of the panda, to the darker pelage of the spectacled, sun, sloth, and appropriately named black bears. Their habitats extend from the Arctic Ocean surrounding the North Pole to the rainforests of Asia and South America and the river valleys, mountain forests, and meadows of North America and Europe.
Their feeding habits are similarly varied. The sun bear spends evenings scurrying along the rainforest floor in search of food and its days nesting in the branches of trees; across the Pacific, much the same is true of the Andean spectacled bear, the only extant bear species in South America and, in the view of many scientists, the closest living relative of the extinct giant short-faced bear. Feasting on anything from fruits to insects to small animals, both species are opportunistic feeders, as are brown and black bears, which will eat fish and carrion but also consume plants and berries. Polar bears, in contrast, are truly carnivorous, their diet dominated by seals; the giant panda is famously all but reliant on thirty species of bamboo; and the shaggy-haired sloth bear of the Indian subcontinent, Nepal, and Sri Lanka is almost exclusively insectivorous, consuming in particular vast amounts of ants and termites, using an extraordinarily powerful force of suction to remove them from their hills and mounds.
The sloth bear acquired its name after the first skins reached Europe in the early eighteenth century. Its long claws and snout and shaggy mane led to the assumption that it was, in fact, a bearlike sloth, and it was initially so categorized. Even when the mistake was corrected and the species was properly classified as a bear, its common name stuck. It is not the only such example of bear-related confusion and uncertainty.
Although popularly referred to as bears, koalas, for example, are in fact marsupials that happen to boast certain physical traits commonly considered to be bearlike. Nor is the red panda, which shares with its larger namesake distinctive facial markings and a fondness for bamboo, a bear; rather, it appears to be more closely related to the raccoon.
For some time, there was considerable debate as to whether the great, or giant, panda was itself truly a bear. Its size suggests that it is—and indeed it was immediately identified as such by Père Armand David, the French priest and naturalist who was the first European to spy a panda pelt, and subsequently an actual panda, in 1869. But a panda's head and jaw are shaped more like a raccoon's, and whereas most bears are omnivorous with a tendency toward the carnivorous or insectivorous, pandas are almost exclusively vegetarian. Taxonomists debated for years whether the giant and red pandas were closely related, whether they should be grouped with raccoons or merited their own family. Ultimately, genetic studies demonstrated what anatomy alone could not: giant pandas are indeed bears—are, in fact, the oldest of the bear species currently on Earth, having split off from the main ursid line about 10 million years ago.
Because of the giant panda's unique qualities and early divergence, taxonomists classify it in a subfamily, Ailuropodinae, of which it is the sole member. The spectacled bear also occupies its own subfamily, the Tremarctinae. The six remaining species have a more closely shared ancestry, having all diverged from the same branch of the family tree.
Before Ursavus departed this Earth for good, it begat Protursus simpsoni, which in turn led to Ursus minimus. The first true ursid, Ursus minimus was the approximate size of today's sun bear, although over the course of a few million years it apparently grew in size before giving rise to the yet larger Ursus etruscus, which was roughly equal in stature to the American black bear. Etruscus would in turn lead directly to the now-extinct cave bear of Europe, but it would also prove to be the granddaddy of six of the extant eight bear species: one branch led to the sun bear, one to the sloth bear, another to the Asiatic and then American black bears.* Then, between a million and a million and a half years ago, the brown bear made its maiden appearance, first in Europe, then Asia, and ultimately North America.
Finally, probably around 200,000 years ago, at much the same time as Homo sapiens was emerging in Africa, a new bear species took the stage. Clues to when, where, and how that came about are found in the brown bears of the Alexander Archipelago.
Studies have shown that the mitochondrial DNA of bears tends to change by about 6 percent every million years or so. The contention, repeated above, that brown bears split off from black bears around a million and a half years ago is supported by the observation that their mitochondrial DNA differs from that of black bears by between 7 and 9 percent. Polar bear DNA diverges from that of most brown bears by around 2.6 percent, which on its own might lead to the conclusion that polar bears evolved from brown bears a little under a half-million years ago. But the divergence between polar bear DNA and that of the Alexander Archipelago grizzlies (which themselves, according to the genetic clues, separated from the brown bear lineage between 550,000 and 750,000 years in the past) is a mere 1 percent, suggesting that their appearance is much more recent.
Any scenario to explain the sequence of events leading to the divergence of the Alexander Archipelago bears and the emergence of polar bears is inevitably and necessarily speculative. But thanks to the genetic evidence, such speculation is at least informed and allows us to paint a picture that looks something like this:
Half a million or so years ago, a stock of coastal brown bears in or near the Arctic became isolated, perhaps by surging glaciers or advancing sea ice. Sometime during the ensuing millennia, they began venturing out onto that ice, probably initially preying on young seals that had never had cause to fear predators, and perhaps then graduating to adult ringed seals at their breathing holes. Over time, natural selection presumably favored those bears with lighter coats for camouflag
e, with teeth better adapted for tearing at meat, and with other physical adaptations such as larger feet for easier swimming between ice floes.
It all happened in the blink of an evolutionary eye. When the last Ice Age began, there were no polar bears. By the time the most recent glacial period ended, polar bears as we know them had become established. The other descendants of the original, isolated brown bear stock that gave rise to them presumably either were reabsorbed into the broader grizzly gene pool or died out. Except, that is, for those on the Alexander Archipelago.
When ice advanced southward into what are now more temperate latitudes, the islands of the archipelago pierced the glacial shroud, acting as what biologists call a refugium, a safe haven where wildlife populations were able to endure. Those populations were isolated by the surrounding ice and then, when the ice retreated, by the passages of water between the islands and the mainland. Which is how it came to be that the archipelago boasts, for example, its own subspecies of dusky shrew and northern flying squirrel, even an Alexander Archipelago wolf—and, of course, the ABC Islands' brown bears, which today contain, deep within their cells, echoes of a distant past and the birth of a new species.
There is not an abundance of polar bear fossils—animals that live on sea ice tend to die on sea ice, and animals that die on sea ice tend to sink to the bottom of the ocean when the ice melts—but those that do exist suggest that early polar bears were larger even than the ones that wander the Arctic today. They were, in the words of one researcher, "gigantic."*
(Given the greater size of those early bears, a 1971 study that concluded average skull size was larger in polar bears in the Chukchi Sea between Siberia and Alaska was cited as evidence that this must have been where the species originated. But although the origination location may be correct, the notion that there is a gradient in skull size almost certainly is not. The study was based on skulls housed in museums around the world, but the sources of those skulls were not uniform; it is believed that many of those from the Chukchi Sea, for example, may have been donated by trophy hunters, who were of course adept at, and focused on, hunting larger bears.)
The Great White Bear Page 5