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Seals & other marine mammals that polar bears capture from the surface of the sea ice are the mainstay of the polar bear’s diet. Image: Daniel J. Cox/Natural Exposures.com

Eggs and berries are bad news for polar bears, a new study finds. The news may be even worse for climate-change deniers. 

As you may have seen, climate-change deniers frequently comment that even if sea ice melts and polar bears can no longer eat seals, they'll just start munching on goose eggs and berries. Well, according to a study published today in Frontiers in Ecology and the Environment, these land-based foods are neither plentiful nor nutritious enough to keep polar bears alive. 

“These are big, active animals,” says Steven Amstrup, chief scientist for Polar Bears International and an author of the new study. “Their caloric demands are much more than what they could get from these land-based foods.” He equates it to a few familiar human foods. “If you eat a head of lettuce you’re probably going to be just as full as if you eat a big ribeye steak, but the nutritional benefits to you are very different.”

Polar bears have, indeed, been observed eating terrestrial foods, especially in recent years as they spend more time on land due to melting sea ice. Previous studies have documented this, and plenty of newspapers and magazines (including Earth Touch News) have covered the story. But according to the new paper, this behaviour has never been observed on the scale that media reports would lead you to believe. Even amidst the largest polar bear population – which range from 900 to 2,000 animals – no more than 30 individual bears have been seen consuming bird eggs. 

If more polar bears did start eating eggs, the demand would far outstrip supply. “It’s pretty clear that there aren’t enough eggs to sustain the total population for any length of time,” Amstrup says. For example, if the approximately 900 polar bears that live in western Hudson Bay in Canada ate every goose egg available to them, it would only make up for one and a half days of sea-ice foraging for seals.

Even when egg-eating has been observed, the health of the polar bears in that area appears to have been suffering. “In the regions where terrestrial feeding by polar bears has been documented, polar bear body condition and survival rates have declined,” the study’s lead author, Karyn Rode of the U.S. Geological Survey, says in a press release.

The survival rate was especially poor for cubs, something Amstrup attributed to polar bears’ nature as a 'k-selected' species. “That’s a species that lives a long time, has a low reproductive rate and sacrifices the survival of their young in favour of survival of the adults when nutritional conditions are poor,” he says. “That’s what we’re seeing in Hudson Bay and the Beaufort Sea in Alaska.”

The loss of cubs in one generation can have a dramatic effect in later years. Another study published this month in the journal Ecological Applications – also co-authored by Amstrup – found that polar bears in the Beaufort Sea population have declined 40 percent over the past decade after a period when almost no cubs survived from 2004 through 2006.

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A female polar bear & her cub snack on kelp on the shores of Hudson Bay. Such foods may fill bellies, but don’t meet the bears’ nutritional needs. Image: Daniel J. Cox/Natural Exposures.com

Perhaps most importantly, the paper cites recent research suggesting that when polar bears evolved from brown bears hundreds of thousands of years ago, they adapted to an extremely high-fat, low-protein diet (ringed seals are at least 34 percent fat) in order to maintain their energy levels and insulating fat layers. Five of the major genetic differences between brown bears and polar bears involve metabolising lipids, which includes fats and fat-soluble vitamins. This means a land-based diet of proteins and carbohydrates may not suit the physical needs of polar bears.

Brown bears, on the other hand, can eat these land-based foods ... but that doesn’t mean they thrive. “Look at northern Alaska, where grizzly bears live on shore adjacent to where polar bears live offshore,” Amstrup says. “The grizzly bears are doing the best they can at making a living and yet they are among the smallest and most sparsely distributed of any brown bear population. The terrestrial environment there can only support a small number of brown bears. That’s sort of a giant experiment that Mother Nature has already done for us.” The situation there could become worse for both species if melting sea ice forces polar bears and grizzly bears to start using the same habitat and competing for resources, Rode cautions.

Amstrup says he hopes this new paper sets the record straight about the potential for polar bears to adapt to land-based foods. “We can’t really expect that environment to suddenly support whole populations of the largest bears in the world,” he warns. 

Will climate-change deniers accept the news? “I kind of doubt it,” Amstrup admits. “There are a number of people out there who just want to deny the reality of global warming. Because polar bears have been the sort of fuzzy face of global warming, if they can somehow cherry-pick information that makes it look like polar bears really aren’t in trouble. By proxy, they’re able to deny the existence and the threat of global warming.”

Amstrup also hopes that the news media will pay attention in their future coverage of polar bears. “The media has been really quick to jump on the observation that polar bears were eating things on land and jumping to the conclusion that they won’t need sea ice after all. What we have shown here is that these terrestrial foods just don’t have the ability to benefit polar bears.” 

Top header image: Alex Berger, Flickr