Polars Bears Threatened by Melting Ice

"On sea ice everything is pretty much shades of white, and maybe a little bit of blue," he adds. "There's not a lot happening out there. So from a polar bear's perspective, anything that's big and dark and moving around out there could actually be something to eat."

He hasn't had much of a problem during his many trips to the ice, partly because his expeditions move primarily by helicopter, so there's lots of noise to notify bears of his presence. When they do show up, he says, it's mainly because they're curious. Once they realize it's just a bunch of scientists, he says, they seem to lose interest.

The best way to avoid them, of course, is to stay off the ice, but that's an option that isn't open to the bears themselves.

Keen Nose for Seals

Their primary food consists of ringed and bearded seals, and sea ice is essential to the hunt. The seals give birth only on the ice, so the bears have to be on the ice to capture them. The seals prefer ridges of jagged ice that look like low mountains that are formed when two sheets of ice crush together.

Derocher says during the winter the mother seal hollows out an area in a ridge, with a hole in the ice below giving her access to the water, and then covers her den with snow.

"The bears walk through these areas, searching for seals," he says. Polar bears have an amazing sense of smell — some experts say they can smell a seal 20 miles away — and that apparently guides them to the hiding place.

But Derocher says he's still amazed at how accurate they can be.

"Once they get close enough, they rear up on their hind legs and crash through the snow layer and try to pin the seal inside the lair," he says. "It's pretty fast and dramatic, and they don't always get it quite right. They may have to do a heck of a lot of digging to find the seal under the snow.

"But sometimes they get it with amazing accuracy and pop the seal right out and head off for lunch."

Stealthy Stalker

Polar bears can also stalk a seal, moving stealthily across the ice until close enough to grab it. And sometimes they just wait over a seal's breathing hole in the ice, so immobile that they look like they're asleep, and when the seal pokes its head up for a breath of fresh air, it's lunchtime for the bear.

The seals prefer the shallow waters close to shore, because that's where they find the rich nutrients they need for their own survival. But as the ice pack moves farther out to sea, that puts the bears farther away from shore, and thus farther away from the seals.

In the years ahead, the seals may learn to give birth on shore, although they've never been known to do that before, and that would put them even farther away from the bears.

So unless the bears can figure out a new lifestyle, survival is probably going to get much more difficult in the years ahead.

Lee Dye’s column appears weekly on ABCNEWS.com. A former science writer for the Los Angeles Times, he now lives in Juneau, Alaska.

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