The Great White Bear (7 page)

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 camouflage, 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.)

But if modern polar bears are smaller than their predecessors, they are still the largest bear species in the world. The largest male weighed in Hudson Bay was a thirteen-year-old that tipped the scales at almost 1,450 pounds; the average weight for mature male bears in that population is approximately 1,100 pounds. Steven Amstrup observes that the heaviest bear he and his colleagues have weighed in the Beaufort Sea area of Alaska was roughly 1,350 pounds but that some tranquilized animals were so enormous that they could not be lifted up with a weighing tripod, or even by helicopter. Doug DeMaster and Ian Stirling have estimated that some weigh as much as 1,760 pounds. Females are significantly smaller, and their maximum weights rarely exceed 880 pounds even when they are at their most obese, when they have gorged following mating and prior to denning.

A mature male may measure over nine feet from the tip of his nose to his stubby tail, stand five and a half feet at the shoulder when on all fours, and reach thirteen feet into the air when standing on his hind legs. Polar bears are not only the largest bears in the world today, they are very nearly the largest that have ever existed; only their ancient ancestors and the extinct giant short-faced bear were larger.

Size, however, isn't everything. There is more to being a polar bear than being big.

***

It all begins with the head.

If you had never before seen a polar bear, never even knew that such a thing existed, but were nonetheless familiar with other things ursid, from a panda bear to a teddy bear, Yogi Bear to Gentle Ben, the moment you saw the face of a polar bear you would almost certainly know instinctively and immediately what type of animal it was. It is unmistakably...
bearlike,
with its rounded ears and furry muzzle. But compare the face of a polar bear to that of a grizzly and one difference is immediately apparent: the latter's seems flatter and wider, while the former's tapers into more of what might be called a Roman nose. The largest brown bear skulls are larger than the largest polar bear skulls; although they are in fact only marginally wider in relation to length than those of polar bears, they are also higher, creating more of a "dish-faced" impression than the more elongated visage of a polar bear. The more streamlined silhouette is accentuated by the neck, which in a polar bear is relatively long and slender, an adaptation that, in combination with the narrower skull, enables polar bears to succeed in hunting prey that would be beyond the reach of their evolutionary cousins.

"If a grizzly sticks its head in a seal hole," points out the San Diego Zoo's JoAnne Simerson, who has spent many years studying polar bears in captivity and the wild, "then it isn't going to be able to pull it back out."

And when a polar bear seizes a seal, the weapons it contains inside that elongated skull allow it to dispatch and dissect it with consummate efficiency.

Its forty-two teeth are significantly different in size, shape, and composition from those in a brown bear's jaw, and they are weighted far more heavily toward grabbing and holding prey and shearing meat. For example, the first premolars, the teeth immediately behind the pointed canines, are vestigial, effectively creating a gap between the canines and the molars that allows the former to penetrate deeply into seals and other prey without hindrance from adjacent cheek teeth.

The ears, too, are different from those of a brown bear. A grizzly's ears may not be its most prominent feature, but they are significantly larger than those of its nearest relative (while those of a sloth bear appear positively elephantine in comparison). The smaller the ears' surface area, the lower the amount of heat lost through them; and while losing heat is an occupational hazard of Arctic life, it is a danger to which a polar bear is tremendously well equipped to respond.

Any energy-conscious homeowner will agree that a sign of good attic insulation is a roof on which snow does not melt in winter. So it is with polar bears, the still-frozen snowflakes on their fur evidence that even as they maintain their core body temperature, which is almost identical to that of humans, the tips of their hairs may be as much as 75 degrees cooler.

By way of illustration of the effectiveness of polar bear insulation, Ian Stirling tells a tale of a fellow researcher who wondered if infrared photography could be used to detect polar bears on the ice, given that all warm bodies emit infrared radiation.

"To test the idea, he found a bear and took some pictures," Stirling wrote of his colleague. "The bear was so well insulated it gave off no detectable heat at all. But there was a spot on the infrared photo, just ahead of the bear's head ... made by its breath!"

So effective, in fact, is a polar bear at thermoregulation that even in temperatures that would threaten hypothermia in humans, its bigger concern is staying cool. This is the primary reason why its pace seems so unhurried: at temperatures between—4°F and—12°F, a bear's body temperature remains fairly constant when walking at about two and a half miles per hour; by the time it is moving at a mere four miles per hour, its body temperature may soar to 100°F. No wonder, then, that although adult bears are capable of bursts of high speed when launching surprise attacks on basking seals, those bursts are brief and, particularly if the seal escapes, followed by exhausted recuperation.

When the ambient temperature is too high for comfort, polar bears respond by doing as little as possible, seeking out shade or shelter and lying still, expending no unnecessary energy and waiting for conditions to cool. Even the arrival of lower temperatures and the season's first ice and snow may offer only marginal respite.

"Typically what you'll see them do is spread themselves on the ice, so their groin area can cool off," says JoAnne Simerson. "What I've seen them do, before there's snow and ice but the ponds are beginning to freeze, is actually punch through the thin layer of ice on the ponds and lie down and sit in the water."

And, when sitting in water doesn't help, they can immerse themselves in it.

Polar bears are excellent swimmers. They can—and frequently do—swim for hours at a time, although too prolonged an exertion can result in severe exhaustion and the need to recuperate for lengthy periods, and may be particularly dangerous for young cubs. The same long neck that enables a polar bear to plunge its head through ice holes and under water when hunting also allows it to keep its head above water for long stretches when swimming; and the fact that it is able to swim for long stretches owes a great deal to what are by far the largest feet in the bear world.

At up to twelve inches in diameter—almost twice as big as a brown bear's feet—a polar bear's front paws are striking in their immensity; in the photograph at which I am currently looking, which I took on the shores of Hudson Bay a few weeks before writing these words, a resting bear is looking at the camera and at me, its muzzle lying on a paw that seems almost as large as its head.

"They're like snowshoes," says Simerson; "it's all about weight distribution," a means of spreading the impact of a bear's bulk to make it easier for it to walk over ice. Indeed, when a bear is setting out uncertainly over ice that is particularly thin, it will often all but spread-eagle itself to distribute its weight as evenly as possible. But such large paws are also valuable for fatally swatting seals. And they make magnificent oars with which a bear can doggy-paddle through the water, while its rear legs curl up underneath its body and act like rudders.