The Great White Bear (3 page)

They need to. Later in the summer, the sea ice breaks up, the seal pups disperse into the water, and the smorgasbord is over. In some parts of the polar bear's range, for example, north of Russia's Wrangel Island, the feast may persist a while longer, the females continuing to hunt on the drifting pack ice even as it begins to break apart and drift south, until eventually the floes grind against the shore and the bears come onto land. Even then the respite may be brief, the bears needing to shelter only a few weeks before returning to the hunt when the sea freezes anew.

At the other extreme, in Canada's Hudson Bay, the sea ice melts entirely by the end of July, forcing all the bears ashore. Because the ice does not form again until early November, they have no opportunity to hunt more seals before giving birth. As a consequence, a pregnant female in Hudson Bay may go eight months without any nourishment at all, almost certainly the longest period of food deprivation of any mammal on Earth.

Under such circumstances, the obvious solution might seem to be for the eggs to implant, and the cubs to be born, as soon as the bears are forced to leave the ice. But however much (were they able to dwell on such matters) the pregnant females might wish it could be so, they must wait.

They must wait to give birth so that the cubs are weaned in spring and not the depths of winter. And they must wait until the weather once more cools and the falling snow forms drifts large enough for them to fashion dens.

The precise timing at which they do so varies according to geographical location and the vagaries of each year's weather conditions, but its approximate schedule remains relatively constant, following the path of winter's onset from north to south. In the Canadian Arctic, bears enter their dens on average by mid-September; in Alaska and northern parts of Svalbard (an archipelago north of Norway), they generally do so by mid-October; in more southerly parts of Svalbard, it is not until late October or early November.

Not only pregnant females take shelter in dens, although they alone do so for the duration of the winter. Many other bears also seek refuge to avoid storms and extreme cold or heat, or when hunting is poor, particularly in the period after the sea ice breaks up and before it re-forms. It is a phenomenon seen less often in the High Arctic, where sea ice is generally available year-round, but observed with particular frequency at the southernmost reaches of the species' range, in Hudson Bay.

There, when the sea ice melts in summer, bears come ashore en masse and hole up in earthen dens, where the temperature is cooler and bothersome insects are less likely to intrude. Most of these dens were excavated in eons past by ancestral bears; over time they have grown, and continue to do so, as a result of the bears' body heat gently melting the permafrost beneath the surface.

Following the onset of fall and the return of the sea ice, males, females already with cubs, and females for which maternity is only a memory return to the hunt. Pregnant females that have also taken advantage of the topography, however, tarry a while, waiting until the drifting snow has covered their shelter to sufficient depth that they can tunnel into the drift and dig out a new den for the winter.

At its most fundamental, construction of a maternity den requires drifted snow in which to dig a hole, and still-drifting snow that will cover up the hole and the bear that has curled up inside it. The first consideration for any pregnant female, says Thomas Smith of Brigham Young University (perhaps the foremost authority on denning behavior), is security: selecting a spot that minimizes the likelihood of exposure to or interactions with other bears. Cannibalism has been recorded in all three bear species in North America—Smith and colleagues have recorded it occasionally in polar bears—and a nursing female with cubs is potentially vulnerable to any kind of attack. Accordingly, whether they den on pack ice or on land, females invariably seek to avoid areas where males are actively hunting. But in addition to that one fundamental concern, there are subtleties and nuances that make some locales more preferable denning sites than others.

Richard Harington, a polar bear biologist formerly with the Canadian Wildlife Service, found that in the Canadian Arctic, the vast majority of maternal dens face south. Prevailing winds from the north deposit greater amounts of drift, and, as with human housing, south-facing real estate is valuable in the polar bear world for its extra exposure to warming solar radiation. On Wrangel Island, in contrast, the distribution of dens follows no such discernible pattern; highly variable winds cause snow to build up on all sides of the island, making most areas suitable for denning.

Indeed, Wrangel Island's sheltered, mountainous landscape provides ideal denning habitat. Other locations where drifts are easily formed—where there is an abundance of earth banks or hillsides, where there are valleys or mountains—boast a higher concentration of dens. Elsewhere, however, suitable spots are harder to find.

Along the North Slope of Alaska, the terrain is fiat; snow is blown by unforgiving winds across the tundra, and bears must make use of drifts wherever they can find them. In many cases, dens are hollowed out of drifts as little as four feet high, nestled along coastal and river banks.

Most dens are near the coast. In the Canadian Arctic, Harington found that 61 percent of dens were within five miles, and 81 percent within eight miles, of the shore, and a 1985 study found that most dens on Svalbard were no farther inland than two miles or so. The pattern is broken only in Hudson Bay, where the coastal plain is boggy and flat, and where females must sometimes trudge over sixty miles to find suitable terrain.

The dens' proximity to shore provides easier access to the seals the desperately hungry mother will need to replenish herself when she and the cubs emerge in the spring. But some bears take this a step further and actually make their dens on the ice itself: either land-fast ice, which is attached to the shore, or even the drifting pack. A 1994 study of Alaska bears fitted with radio collars found that over half the dens those bears made were on sea ice—a high degree of ice denning that is not found anywhere else. It might be a consequence of the fiat terrain of northern Alaska making suitable sites on land relatively scarce, but Steven Amstrup of the United States Geological Survey (USGS), who has likely spent more time studying the polar bears of Alaska than anyone else, believes that the sea ice in the southern Beaufort provides a more solid foundation than is the case elsewhere.

"Historically, the Beaufort Gyre circulated multiyear sea ice through the Beaufort Sea," he says. "That's ice that circulated through the Arctic year after year and kept getting thicker and thicker, so the sea ice in the southern Beaufort Sea had some of the most stable sea ice anywhere in the Arctic. It probably provided a very stable platform for polar bears to den on."

Researchers elsewhere have concluded that denning on sea ice is infrequent and often limited to bears that could not make it ashore before the ice broke up. Understandably so, for making dens on the pack poses risks not experienced by those that choose to den on land. Ice floes shift, break up, re-form; they can turn over or raft under other floes with which they collide. The authors of the aforementioned 1994
study, among them Amstrup, saw six polar bears in pack-ice dens that were swept past Point Barrow, the northern tip of Alaska, and southwest into the Chukchi Sea because of unusually unstable ice conditions. The dens of two of the females had been destroyed when ice floes had collided and rafted onto each other; in their mouths the females carried tiny cubs. The researchers saw the mothers again later that spring; of the cubs, however, there was no sign.

A pregnant bear knows instinctively and precisely what to look for in a denning site. She walks from snowbank to snowbank, testing each for consistency and depth, poking and prodding until she finds one that meets her criteria. She may travel many miles and take several days until she is satisfied with a location. Then, when she has found a spot she likes, she digs.

She will first excavate an entrance tunnel, which is normally about six feet long but so narrow—rarely more than two feet in diameter—that it is a wonder a fully grown, pregnant polar bear with a winter's worth of fat reserves could possibly squeeze through. Then she will angle slightly upward, so that the warm air does not escape through the passageway, and create the main chamber in which she and her cubs will spend the next several months. Some females, for unknown reasons, become creative, adding an antechamber or even two—although in many cases the most extensive additions are made after mother and cubs have emerged from the den but before they have opted to leave it behind completely. Thomas Smith has found a few dens with "interconnecting tunnels up to thirty-five feet in length, with as many as four entrances." There may or may not be a small ventilation shaft.
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The main chamber will itself measure approximately six and a half feet by five feet, and about three feet high: little room for the young family to do anything except remain curled up together.

This will be the mother's home for the next four months. When the chamber is completed and the entrance shaft has been resealed with drifting snow, she lies down and prepares to give birth.

Throughout the polar bear's range, there are approximately fifteen areas where dens are most concentrated, where geographical convenience and topographical advantages combine to create ideal conditions—specifically, a plenitude of sheltered areas with drifted snow—for den formation. Between them, the Russian Arctic islands of Wrangel and Herald host between 350 and 500 pregnant polar bears each year, approximately 85 percent of the breeding females in the Chukchi Sea population. In places, twenty dens may be squeezed into an area of a little more than a square mile.

In August 1990, biologist Nikita Ovsyanikov arrived on Wrangel to begin a detailed field study of the islands' bears. He set up base at a point called Cape Blossom, where bears congregated in search of food, in the process passing within yards of his cabin. The bears at Cape Blossom are mostly males; during Ovsyanikov's study period they proved inquisitive, seemingly fearless, and apparently unconcerned by the researcher's presence.

The bears on Herald—home to such an abundance of maternity dens that the scientist dubbed it the region's polar bear nursery—were different, as indeed was the island itself. During our visit on board the
Arctic Sunrise,
our grizzled bosun sized up the otherworldly vista that thrust straight up from the ice into a low layer of clouds and declared it to be "like something out of Arthur Conan Doyle's
The Lost World.
" Ovsyanikov, too, noted Herald's "severity" on his first visit, observing that the "tall, jagged, almost vertical cliffs plunged straight down into the cold, dark green water with no beach to break the fall." It did not have anywhere obviously comparable to Cape Blossom, no areas for the congregation of seals or walruses and thus of polar bears; its attraction was purely the plethora of mountainous but sheltered nooks and crannies on its otherwise inhospitable surface. Wrangel may have been the kingdom of the ice bear, opined Ovsyanikov, but Herald was the castle of the snow queen.

Accordingly, the only bears on the island at the time of the researcher's visit were pregnant females preparing to den, and their behavior, Ovsyanikov observed, was markedly different from that of the bears on Wrangel. Whereas the bears at Cape Blossom would frequently approach the scientist's cabin, those on Herald studiously avoided it, even when he placed a box of thawing reindeer meat outside. These bears, he surmised, were clearly not only very full after a period of gorging but were extremely cautious and sensitive.

Just how cautious and sensitive became clear following an unintentional encounter that proved upsetting to both bear and human, when Ovsyanikov came unexpectedly upon a bear that was digging a den. He did not notice her, he wrote, until he had drawn level with the hollow she was creating; he saw the bear, which had her head down as she dug, before she saw him. He immediately dropped to the ground to avoid detection. But it was too late. The crunching of snow underfoot had given him away.

The bear, he wrote, lifted her head, looked at him with surprise and fear, and "hissed so loudly that I thought she was going to turn herself inside out." She stared at the scientist for several seconds before climbing up a steep slope away from him, still hissing, clearly struggling to move her distended body.

Ovsyanikov, a conscientious researcher, was deeply distressed that, however unwittingly, he had caused the bear to abandon the site she had selected, forcing her to search for a new suitable location. His presence had altered the animal's behavior, a violation of the field biologist's prime directive; as a result, the pregnant female—which, with resources at a premium, had little margin for error—was forced to expend extra energy she could ill afford to waste.

Even so, for a bear to desert a den during construction is neither unprecedented nor unusual. Researchers have found that abandonment of dens in favor of more agreeable sites can happen with some frequency during the late autumn months when females are looking for somewhere to bed down for the winter.

When the dens have been dug and the cubs born, however, it is a different story.

Ian Stirling, who has spent many decades studying the polar bear population in Hudson Bay, has noted that once pregnant females have become established in their dens, they seem determined to remain there, to the extent that they appear almost passive in the face of disturbance.

"I think their main interest is in detaching from the bad weather outside," offered Richard Harington, who in the 1960s conducted the pioneering study of polar bear denning behavior. "They want to be as secure as possible. They go into what is called a carnivore lethargy: their temperature drops, but it's nothing like hibernation. You can disturb them fairly easily. They can get up and start walking around. I've taken bits of the roof out over dens, and you can see the female inside. At first you don't hear anything but heavy breathing, and then when you poke through the roof you sometimes hear a bit of a growl. When you open it up you can see the female walking round inside, and sometimes the cubs against the far wall."