Visitors to Churchill, Canada, sometimes hear the story of a polar bear in the 1950s that followed a terrified man across the ice. If the bear had been running its possible two-minute mile, the man would have hardly had time to become frightened. But polar bears hate to overheat, and besides, this one was probably more curious than anything else. Or maybe it couldn't resist a taste of something it detected with its phenomenal sense of smell, capable of locating seal breathing holes under three feet of snow: When it caught up to the man, it licked his hand.

On Alaska's northern coast, people are still spooked by a 1993 polar bear attack on a civilian mechanic at a military radar facility on the Beaufort Sea. The bear broke through a window and mauled his victim until the post's cook shot the bear. The mechanic has since sued the federal government for negligence, citing among other problems disfigurement and flashbacks of the attacking bear. "It tore my face off," he told the Anchorage Daily News.

Curious. Potentially deadly. Full of surprises. Those are among the qualities that impress all who have encountered the top predators of the Far North. "When we visit polar bears' environment, we have to recognize they're in charge," says biologist Steve Amstrup of the U.S. National Biological Survey. Still, as much as the creatures have proven that they can easily harm people, we are turning out to be more of a threat to the bears than they are to us. Their future is full of perils wrought by humankind. Not only that, scientists are finding that the creatures' reaction to people in the flesh is often more like that of the bear that supposedly licked his quarry and less like that of the ferocious beast that went after the mechanic.

Not so long ago, being shot was the most serious threat polar bears faced from people, who probably found Arctic conditions more challenging than the actual hunts. Writes Russian biologist Nikita Ovsyanikov in his new book Polar Bears, "You have only to sit and wait, and a polar bear will approach you."

He should know. Accompanied by a BBC cameraman, Ovsyanikov traveled in 1990 to Russia's Wrangel Island, where the two men were surprised to find their cabin surrounded by dozens of polar bears. The first night, so many curious bears came to the windows--peering in, sniffing and tapping--that the two men felt compelled to go outside to close the shutters, shooing away bears in the process.

To proceed with his research into the bears' behavior, the next day Ovsyanikov decided to walk among his subjects carrying only a big stick as a weapon, deliberately acting confident and aggressive. Luckily for him--and for the cameraman who followed the scientist out into the bear crowd--the strategy worked. Since then, Ovsyanikov has had more than 500 encounters with polar bears and lived to tell the tales. He observes, "From my experience, the polar bear is such a calm and curious creature that there can be nothing exciting about hunting it."

Still, by the mid-1970s, the bears were being overhunted by a combination of trophy and Native subsistence hunters. That development, combined with the oil industry's new interest in Arctic development, moved all five polar nations to ratify an agreement to protect polar bears and their ecosystems. In the United States, the bears are included in the Marine Mammal Protection Act, which allows only Alaska Natives to hunt them. The protection is working. Polar bear numbers--which total between 20,000 and 40,000--are increasing or stable in most regions.

There was a time when polar bears had only the challenges of their ecosystem to contend with--which for people is one of the most forbidding places on Earth. Says National Biological Survey biologist Amstrup, "From the scientific standpoint, the most difficult thing about studying bears is their habitat: a tortured jumble of constantly moving blocks of ice. Anytime you leave the north coast of Alaska, you're going away from salvation."

For the bears, of course, that world is home. The polar bear as we know it probably evolved between 100,000 and 250,000 years ago from brown bears. The new species developed curved, needle-sharp claws for clinging to ice and grabbing prey; and large, partially webbed paws for propulsion in the water. The bears, which can weigh more than 1,500 pounds, stay warm with a layer of blubber nearly 5 inches thick and lush fur. Each hair is a clear (not white), hollow prism that may conduct light to the bears' black skin for absorption. Though seals are its main prey, the polar bear is strong enough to hoist from the water a beluga whale several times its own weight. Pulling a 12-inch-thick seal through a 4-inch ice hole--often breaking most bones in the seal's body--is about as difficult for polar bears as taking a Kleenex from a box is for us.

Polar bears have reputations as solitary animals, and they do range far and wide alone. But on Wrangel Island and in Canada's Hudson Bay, groupings of the animals have proved to be playful and even downright sociable. Their most important interaction-- mating--takes place out on the ice, where males somehow track and locate females in late March to mid-July. Females delay implantation of their fertilized eggs until early fall. In late fall, they dig out and enter their dens; they give birth a month or two later--usually to two cubs. The denned mother often goes without food or water for as long as nine months.

Asked for the most exciting thing he has learned about polar bears, biologist Amstrup cites his confirmation that not only do bears den offshore in the Beaufort Sea, but half the population does so. "When a female comes out of the den in spring, she has to be able to find food," he says. "She goes into the den with that anticipation and then drifts perhaps several hundred kilometers. I just find that incredible. How can she predict what the resources are going to be upon emerging?"

As remarkable as that finding may be, one would think that Amstrup might have chosen instead a discovery he made one day while 50 miles offshore on the ice. Bent over a tranquilized female polar bear, he looked up to see a male bear only 50 feet away, silently running toward him. The scientist managed to shoot a pistol into the air and rush toward a helicopter along with the rest of his crew. The male halted and stood astride the sleeping female, glaring at the men. And that's when Amstrup discovered that a bear that seems to be charging a man can simply be in pursuit of a mate.

But for the scientist, such insights are less impressive than findings that polar bears could soon be under siege, in several different ways. Take, for example, Alaska's two groups--the Beaufort Sea population of about 1,800 off the northern coast and the other, number unknown, off the west coast in the Chukchi Sea.

The Chukchi Sea bears are shared with Russia, where most den. Ovsyanikov is concerned about a big increase in poaching of the bears since the advent of Russia's free-market economy, and about the effects of oil from recent spills flowing from Siberian rivers into the Arctic Ocean. Then there is the prospect of oil drilling, with its attendant disturbance and risks. U.S. and Russian developers have been eyeing the Chukchi Sea, including part of the Wrangel Island Nature Reserve. "If an oil spill happens in the vicinity of Wrangel Island, it would be absolutely a disaster," says Ovsyanikov. "And not just for polar bears--for many other species also."

U.S. environmentalists feel the same way about potential oil development on the coastal plain of the Arctic National Wildlife Refuge (ANWR). A rich and fragile ecosystem with a summer explosion of life, the plain is the main denning area for 45 percent of pregnant polar bears in Alaska, almost all of which are from the Beaufort Sea. Most of the rest of pregnant Beaufort Sea bears den on the ice--which means polar bears far prefer the refuge's coastal plain over all other possible land denning sites in Alaska.

"Denning bears need snowdrifts," explains retired biologist Jack Lentfer, who worked for both the state and federal governments as one of the first to study Alaska's polar bears. "On a flat area where wind blows, you're not going to get much buildup of snow, whereas if you have a streambed with a bank, drifting snow will accumulate." That's also what happens out on the jumbled ice pack. Along the refuge's coastal plain, the mountains are closer to the ocean than elsewhere along the coast, with steeper slopes. And rivers and streams form deeper ravines--ideal snowdrift topography.

Biologists have long assumed that denning bears are sensitive to disturbance. But one recent study of denning bears in and near ANWR, by National Biological Survey biologist Amstrup, indicates bears are more tolerant of noises such as helicopters and seismic surveys than experts had feared. "It turns out that bears in dens probably can't hear very much of what's going on outside, because they're insulated by the snow," he says. If oil development occurs in ANWR, he concludes, "the potential for disturbance of denning polar bears will increase." But, he adds in his scientific paper, "rigorous adherence to flexible management strategies, including spatial and temporal restrictions of developments, could prevent the potential for many disruptions of dens from being realized." In other words, if the oil industry were very careful, the bears might be okay.

That is a mighty big "if," say biologists Lentfer and Ovsyanikov. "It looks good on paper," says Lentfer, "but the critical times for oil development are right when bears are going into and coming out of dens. Work with heavy equipment must take place when the ground is frozen. There's a built-in conflict right there."

Says Ovsyanikov, "We can't conclude that animal stress is a simple matter of whether the bears are still there or are still alive." Of Amstrup's 12 study subjects, Ovsyanikov points out, at least two were clearly disturbed. "Also," he adds, "his conclusion is based on very few occasions." On Wrangel Island, Ovsyanikov has seen females abandon cubs at the mere sound of a distant snowmobile. "I am absolutely sure that when they are disturbed, most leave immediately," he says.

Resilient or not, for now polar bears on the Arctic Refuge's coastal plain are protected unless Congress and the President act to open it up. Not only that, by a quirk of nature, Alaska's bears are also less exposed than some other populations to another big threat: toxic chemicals. Scientists theorize that volatile chemicals like polychlorinated biphenyls (PCBs) hop around the globe, picked up by wind and water, especially in warm temperatures. But when the chemicals reach colder climes, they stay put. Whether or not that model holds up, polar bears are accumulating toxics. In the fat of polar bears in Norway's Svalbard Archipelago--where air and water currents from the United States, Europe and Russia all converge--PCBs average 5 to 10 times higher than in polar bears in Alaska and Canada.

That threat could become dwarfed--or made moot--by potential global warming, which could literally melt the ice out from underneath the bears. For polar bears, all ice is not created equal. That's why researchers in Alaska are now gathering data on the role ice conditions play in interactions between the bears and their main prey, ringed seals. "If global warming is really occuring, one of the first places we're going to see it is in the polar region," says the National Biological Survey's Amstrup. "And we will see it in this interaction."

The bears do most of their seal hunting between land and about 60 miles offshore. In that zone, the water under the ice is relatively warm, shallow and full of life. That's where ice breaks apart, seals surface and bears hunt. The bears also pull seals from their breathing holes, which the seals maintain in the crust near the edges of the ice pack.

Researchers have one real-life indication of how global warming could affect the bears: In Canada's Hudson Bay, polar bears spend summers on land, where they wait in a fasting state for water to freeze. Over the last 15 years, females have been increasingly leaner at the start of summer, and their reproductive rates and cubs' survival rates have declined. Scientists think the cause may be a lengthening period each summer of open water--which for the bears would mean less feeding time. If warming brought more ice-free time each year, the bears could be in serious trouble.

They would not be alone. Like pollution swirling to the Far North from sources thousands of miles away and many of the bears' other threats, warming could imperil the entire ecosystem. Observes Russian biologist Osyanikov, "In conserving large predators, we test our capability to save nature; in conserving the polar bear, we test our capability to save the Arctic."

Lisa Drew is a senior editor of this magazine.