Arctic Science Explained: Polar Bears

July 14, 2022
By Liz Weinberg

By Katherine Schexneider

Welcome to Arctic Science Explained! Each month, IARPC Collaborations community member Katherine Schexneider breaks down a topic related to Arctic science. If you have a topic you’d like to see featured, please email web@iarpccollaborations.org. This month, Katherine takes on polar bears.

I saw my first polar bear a few weeks ago while on an expedition cruise in the Arctic. We were able to spend over an hour watching a mother and her teenage cub walk along the ice floes and nibble on the remains of a recent seal kill.

a polar bear on an ice floe

A young polar bear stands on an ice floe. Photo: Katherine Schexneider

During the cruise and then later from some enlightening talks with polar bear experts, I learned about polar bear natural history, behavior, and roles in the Arctic’s complex ecosystem, but my experience with this magnificent animal began with my awe. It was hard to believe this was real. Here they were in the flesh right in front of us. I felt a sense of childlike wonder, causing me to put my camera down and just watch them, transfixed. At the end of viewing, as the cub wandered over to our ship, looking and sniffing curiously, I felt something deeper: uneasiness about his future. What is his life going to be like? Will he be okay?

The ship’s captain engaged the reverse engines, and we slowly pulled away, everyone looking down at their cameras at the evidence of what just happened.

two polar bears, a mother and teenage cub, on ice floes near a recent seal kill

The mother and teenage cub stand near a recent seal kill. Photo: Katherine Schexneider

What is it about the current Arctic that’s fragile and now threatening to the apex predator?

Polar Bear Basics. An optimal life for a polar bear takes place mostly on the ice, where seals, especially the ringed seal with its ample body fat, live. Polar bears selectively feed on the blubber of their prey.

Their main hunting tactic is to lie in wait for a seal to emerge from a breathing hole and then seize upon it. The bear is an incredibly efficient predator, saving as much energy as possible for a short, dramatic attack. Mothers have quite a challenge teaching young cubs to hunt in this fashion, as the cubs are typically playful and noisy. This may explain the two-plus-years cubs spend with their mother before they are ready to live on their own.

a mother polar bear and two cubs on sea ice

A mother polar bear and her cubs stand on a snowy shore in northern Alaska. Photo: Hans-Jurgen Mager, via Unsplash

There are about 26,000 bears spread across the circumpolar Arctic in 19 discrete geographic subpopulations. Each subpopulation has its own habitats, behaviors, and threats. Across all the populations, though, sea ice plays a key role.

Polar bears spend much of their lives on the sea ice—mating, rearing young, and hunting—and the loss of sea ice habitat is the primary threat to their long-term persistence. Summer sea ice area has declined in all 19 subpopulations, making it difficult for polar bears to carry out their normal behaviors and find food. Additionally, some areas where polar bears live, like Western Hudson Bay, Canada, have a long history of being ice-free in summer. Here, the bears reside on land for part of the year, occasionally feeding on land-based food until the ice returns in the fall. Unfortunately, land-based foods (like caribou who have a high muscle-to-fat ratio) are often poor choices for a fat-dependent species like the polar bear. As a result, as the ice-free period lengthens, it becomes more difficult for bears to maintain a healthy body condition.

Reproduction presents its own set of challenges and is also impacted by diet and fat stores. During pregnancy and lactation, mothers hole up in a snow den and do not eat. Surprisingly, their newborns cubs are only the size of a soda can and spend their first two and a half months in the protection of the den, nursing and growing. Lactation is the most costly part of the reproductive cycle, requiring energy drawn from the mother’s fat stores. A mother who is too thin will rear cubs who are too small or too weak to survive, or she won’t be able to produce a litter at all. It’s here, in cub production and survival, that the danger to polar bear numbers is greatest, and this contrasts with the common wisdom that the adults will go hungry and die off. It’s not them we should worry about quite so much; it’s the cubs.

a mother polar bear stands over two small cubs on a snowfield

A mother polar bear guards her young cubs. Photo: Steven Amstrup/USGS

Research Techniques. Scientists use a number of techniques to count and monitor polar bears. These include labor-intensive aerial surveys that manually tally individuals over vast areas as the Arctic weather allows. The gold standard for counting (called “deriving abundance estimates”) is the mark-recapture method. Bears are tagged with a tattoo, chip, or ear tag so scientists can re-identify them later, typically annually in multi-year surveys. Even with highly technical tracking and analysis systems, though, population estimates are still estimates, not complete population counts, meaning that 26,000 could actually be off by several thousand.

Monitoring involves observation of behavior as well as direct contact. One U.S. Geological Survey researcher I spoke with employs a biopsy dart which obtains a small sample of fat tissue in a fairly atraumatic fashion. The sample provides a DNA fingerprint as well as data on diet and nutritional status, including any contaminants the bear may have ingested, such as polychlorinated biphenyls (PCBs) and mercury, among others.

a polar bear lying on a large pile of ice and snow

A polar bear rests on an iceberg in the Arctic. Photo: Annie Spratt, via Unsplash

The Greatest Threat. The status of sea ice and, linked to it, body condition, pose the most danger to this remarkable animal. In simple terms, adequate sea ice means ideal hunting grounds, which means access to those high-fat ringed seals, and a high-fat diet means peak body condition, which means healthy mothers, which means healthy cubs, which means the next generation has a shot at a good life. As we warm the planet and melt sea ice, the polar bears and the health of the broader Arctic ecosystem are increasingly at risk.

We tend to look at polar bears as sort of a bellwether of Arctic health, perhaps because they are the apex predator—large, strong, majestic. So, even though polar bears are just one of a thousand indicators of the state of the Arctic, they are the most recognizable. Let’s hope they stay that way.

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Katherine Schexneider is a retired US Navy physician who now does volunteer work in Arctic research and climate change.

Note: Views expressed in this article are the author’s and do not necessarily reflect the views of the IARPC community.

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