On thin ice

ONE AUTUMN NIGHT, I fell asleep near a pair of hungry polar bears. I was out on the tundra in a mobile lodge on the shore of Hudson Bay, spending some time with the world's largest land predators. My bunk had a small window by it with a view onto a patch of snow illuminated by a large spotlight. After an unforgettable afternoon photographing bears at rest and play, I stretched out in my sleeping bag as they took turns sitting up on their haunches, peering back at me.

The largest member of the bear family, adult male polar bears (\o7Ursus maritimus\f7)\o7 \f7weigh between 770 and 1,500 pounds. The skin of a large male specimen could cover a small car. Incredibly, most polar bears are born weighing just over a pound.

They are the newest bear species, evolutionarily speaking. Polar bears are thought to have evolved into a distinct species between 100,000 and 200,000 years ago. Previously, seals were able to haul themselves onto the ice pack and nurse their young in peace -- until a brown bear \o7(Ursus arctos) \f7noticed and decided to investigate. The descendants of that bear came to depend on seals and developed into the white bears we know today.

Polar bears spend the winter and spring on the ice and are forced ashore in summer to await its return. Because there isn't much for them to do in summer, they spend a lot of time wrestling and watching the tourists and scientists who have come to watch them.

These days, however, it's becoming increasingly difficult to look them in the eye.

Life at the top of the Arctic food chain means polar bears' bodies concentrate many of the chemicals that waft up from our activities in the industrial south. For years, scientists have been tracking increasing levels of toxins (including DDT and PCBs) in polar bear flesh, organs and milk. And now, with polluted bodies, they are faced with an even bigger threat.

Polar bears rely on ice that forms fresh each year adjacent to land. They hunt seals at the ice's edge, at breathing holes in the ice and in snow-covered hollows on top of the ice where seals hide their pups. For polar bears, it's all about ice. Unfortunately, the ice is disappearing.