Fat stores are the key to the polar bear’s survival. Longer ice free periods make it difficult for polar bears to build up ample fat reserves to see them through the lean months until the ice returns.

© Craig Taylor

7/25/2018 3:23:50 PM

Polar Bear Basics

By Dr. Andrew E. Derocher

Ask anyone what Arctic species they’d like to see and polar bears top the list. Why? Perhaps because polar bears are the world’s “largest terrestrial carnivore”? Except they aren’t.

All three words are wrong. First, polar bears rely almost exclusively on marine ecosystems, so they aren’t terrestrial. Second, in the marine environment, polar bears lose out to killer whales for size. Lastly, there’s the carnivore issue. Polar bears make their living from seal blubber and not so much from meat. The seal’s thick fat layer makes the polar bear possible. Perhaps the term lipivore (fat eater) is more appropriate because the carni in carnivore refers to meat.

Built for a life on the sea ice

Polar bears evolved from a brown bear ancestor and adapted to their sea ice environment. When the separation occurred is blurred by ancient hybridization events in Ireland and Alaska. Regardless, the differences between the two species are profound. Beyond the obvious difference in color, the polar bear’s fur, claws, skull, teeth, physiology, and behavior have all evolved to deal with an intensely seasonal marine Arctic environment.

For polar bears, life is a feast and famine existence. Spring is their time of plenty because naïve seal pups are abundant. Further, mother seals trying to nurse their young on the sea ice also fall prey to polar bears. Adding to the spring bounty, male seals have their attention diverted by mating. This all adds up to a window of feeding dictated by the ecology of the seals and the dynamics of sea ice.

How many seals does a polar bear eat in a year? It’s a question scientists can’t easily answer. Consider that a newborn ringed seal weighs about the same as a human baby at birth, but an adult bearded seal weighs 3-4 times that of an adult human, and the peril of estimating the number becomes clear. A few adult bearded seals per year would suffice for most polar bears—given that they can eat about 20 percent of their weight in a single meal and deposit over 90 percent of that into their own fat stores—but most bears make their living from ringed seals of various ages. 

For a polar bear, fat is fit

Fat stores are the key to the polar bear’s survival. In the past, the summer sea ice would endure long enough to ensure polar bears had sufficient time to store enough fat to see them through the lean months until the ice returned. (When the sea ice melts, seals aren’t constrained to breathing holes and come up anywhere they like for a breath, making it very difficult for polar bears to hunt them.)

The ice-free season is increasing across the Arctic, although at different rates across the 19 polar bear populations. How quickly the ice is disappearing in each area is directly related to the threat a population faces. Despite claims that terrestrial foods may augment a polar bear’s diet, the energy in such foods pales in comparison to seal blubber. Any polar bear will happily supplement its diet with berries, grass, goose eggs, or any carrion it may find but they can’t make a living from such resources. Besides, in many parts of the Arctic, the terrestrial foods are exploited by grizzly bears. Arctic grizzlies are a fraction of the size of polar bears and their whole life history is the opposite. Grizzly bears hide from winter and hibernate while polar bears embrace the cold as a time of plenty. Polar bears lounge away the summer waiting for the Arctic Ocean to freeze so they can move about its surface to hunt.

It's up to us

The Arctic is a tough environment but an incredibly rich one from a polar bear’s perspective. Polar bears have evolved to live on the sea ice. In fact, some bears are born on the ice and never step onto land! However, such a life trajectory is quickly changing and more and more polar bears are spending more time on land.

The fate of polar bears is intimately tied to the fate of sea ice. During cold periods in the history of Earth, polar bears shifted south. We know they made it to Ireland, Sweden, and the Alaska Panhandle during the last ice age—but they're not there now.

How polar bears deal with diminishing ice is well understood from scientific studies. Humanity has choices to make and they include a future with or without polar bears. It’s our call.

Dr. Andrew E. Derocher is professor of biological sciences at the University of Alberta and a long-time scientific advisor to Polar Bears International. He has studied polar bears for 35 years.

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