What do polar bears eat and how is their food threatened?
The animals have adapted over tens of thousands of years to an environment that is growing increasingly fragile.
Mon, Nov 19, 2012 at 04:09 PM
A polar bear eats the remains of a seal. (Photo: Lindsay Robinson/Flickr)
It all starts with a whiff of the cold air. A ringed seal swims under the thick layer of ice that covers the Arctic Ocean, uses the sharp claws on its front flippers to carve a small breathing hole, and takes a breath. That hole, along with 10 or 15 others the seal carves nearby, will be its main source of oxygen for months on end.
But not far from that hole, a polar bear sniffs the air. Smelling the scent of seal on the wind, the bear approaches the ice hole and waits ... and waits ... and waits. Then, when the seal at last returns to the hole for another breath of air, the bear strikes. Dinner is served.
Seals like this form the basis of the polar bear's diet. Although these predators will eat just about anything — including walruses, bird eggs and even whales — they have adapted over a period of hundreds of thousands of years to rely on seals and sea ice for their survival. The evolutionary advantage has allowed polar bears to thrive. "The polar bear's diet, which is highly specialized to consume the fat-rich seals that they catch on the surface of the sea ice, has allowed them to become the largest of the bears," says Dr. Steven Amstrup, chief scientist for Polar Bears International.
But this evolutionary advantage has also put polar bears at risk. Global warming endangers the species, so much so that the animals have become one of the icons that conservationists use to illustrate the threat posed by climate change. As the Arctic sea ice melts, the polar bears lose their primary hunting ground — not to mention their most plentiful and nutritious prey. It remains a question whether they will be able to adapt to changing conditions and survive.
A species adapted for its environment
Polar bears spend so much time at sea that they are actually considered marine mammals. Their water-repellant fur and thick layers of fat keep them dry and warm during the harshest winter months. They use the periods of sea ice not only to hunt, but to pack enough fat on their bodies to last through the lean months when the ice recedes and the bears are forced to retreat back onto land, where other food sources remain scarce.
In addition to their fur and fat, their bodies tell us how polar bears have adapted to their harsh environment. "It's clear that the shape of the polar bear skull and their dentition is clearly adapted to a carnivorous diet," Amstrup says. "Polar bear teeth have drawn toward the almost catlike look, with well-developed carnassial or shearing surfaces on their pre-molars. The shearing actions of these teeth are more suited to cutting through blubber and hide." Other bears, such as brown bears, don't have this hide-slicing adaptation and have teeth and skulls more suited to eating vegetation, fish and land-based mammals.
Too much change
Unfortunately, the environment today may be changing too rapidly for the polar bear to adapt. "By the end of this century, temperatures are likely to rise above anything polar bears have seen any time in their past history," Amstrup says. "Polar bears' ancestors may have separated from those of brown bears as early as 4 million years ago, and we know they had achieved their sea-ice specialty by about 120,000 years ago. We can't expect them to suddenly, in the course of a few decades, make up for the degree of specialization they acquired during their evolutionary history."
Amstrup emphasizes that some polar bear populations might actually benefit, albeit temporarily, from warming temperatures, especially the bears that currently live in the coldest climates. As polar bears spend more time on land, there might also be short-term adaptations that allow them to survive for a few years without dining on their usual diet of ringed seals. "Some have suggested they may increase their feeding on goose eggs, for example," he says. "Goose eggs and the eggs of other ground-nesting birds are energy-rich and a few bears may temporarily make up for lost feeding opportunity on the ice by taking advantage of such food sources." But Amstrup says this is not a permanent solution: "If the whole bear population was trying to survive on eggs, soon there would be no geese left. You have to ask, is there some terrestrial food out there that could work in the long run, and the answer is no. We haven't seen any evidence that anything could provide long-term salvation."
Other evidence already shows that polar bears are unlikely to thrive once they become land-locked. "We have two major pieces of evidence that polar bears will not be sustained in anything like current distribution or numbers by land-based foods," Amstrup says. "First, in western Hudson Bay, studies have shown that polar bears lose about two pounds of body weight for each day they are stuck on land. If there was something out there that could forestall that weight loss, they would be consuming it.
"Second, in northern Alaska we have polar bears living on the sea ice offshore and grizzly bears living on land. The polar bears are the largest bears in the world. The neighboring grizzly bears, on the other hand, are the smallest of all brown bears and live at very low densities because the environment is poor from the standpoint of a bear. Why would we think that whole populations of the largest bears in the world could successfully be placed on a landscape that currently supports only small numbers of small bears?"
Polar bears are opportunistic predators. Given a chance, they'll eat just about anything. But with the world warming and sea ice disappearing along with the polar bear's food sources, the species' chances of survival are in question.
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