Climate change and loss of sea ice putting polar bears at risk of starvation, collar cameras show

What is a day in the life of a polar bear like? Researchers recently found out after strapping collar cameras to several bears.

Scientists have long hypothesized what would happen to polar bear populations in the Arctic as sea ice continues to melt at unprecedented rates. But now they have visual evidence of just how hard it will be for polar bears to find sustenance as the sea ice disappears.

Polar bears rely on sea ice to hunt their preferred prey -- seals that are stock full of fat and blubber that provide polar bears the energy they need to survive the elements of the Arctic.

As the sea ice experiences mass melting, the polar bears have no other choice but to gravitate toward land. And researchers are now witnessing how the species is adapting to land-dwelling after fastening cameras equipped with GPS trackers and cameras onto 20 bears living in the western Hudson Bay in Manitoba, Canada, Anthony Pagano, research wildlife biologist at the U.S. Geological Survey's Alaska Science Center, told ABC News. The bears were studied during a three-week ice-free period from 2019 to 2022, Pagano said.

The researchers monitored the daily energy expenditure, changes in body mass, diet, behavior and movement, finding that polar bears could be at risk of extinction during Arctic ice-free periods when they are forced to find food on land -- despite their ability to adapt their diets, hunting and foraging behaviors, a paper, published Tuesday in Nature, found.

The collar monitors indicated that individual polar bears chose different strategies to reduce energy loss during the ice-free periods, including fasting, reducing movement and resorting to eating berries and birds. The strategies were chosen independent of age, sex, reproductive stage or initial fat levels, according to the paper.

"The bears seem to be doing different things, regardless of their body condition," Pagano said. "There is no real clear explanation for why a particular bear might be more active or less active."

Some of the bears were "highly sedentary" and rested and fasted for the three-week monitoring period, Pagano said. However, most of the bears were "highly active" and moved about the landscape, feeding on a number of terrestrial foods, including bird carcasses, caribou carcasses, grass, seaweed and berries, he added.

"Regardless of which behavior strategy the bears were using, they all lost weight at similar rates, which highlighted that those bears were more active," Pagano said.. "Basically, they were compensating for their higher activity levels through the terrestrial foods they were eating, but they weren't getting any net benefit from that."

On average, the bears lost about 1 kilogram, or about 2.2 pounds, per day, Pagano said.

The findings suggest that polar bears obtain little benefit from foraging on land when it comes to prolonging the predicted time to starvation, as 19 of the 20 bears studied lost mass during that time, the researchers said.

If sea ice were available, the bears would typically gain weight because they have access to their preferred prey, Pagano said. Polar bears have perfected an efficient strategy called "still hunting" in which they rest near a breathing hole a seal will scratch open in the ice and wait there until a seal comes up to breathe -- when they pounce, he added.

At first glance, polar bears may not seem too different from their terrestrial cousins, like brown and black bears, John Whiteman, chief research scientist for Polar Bears International and assistant professor of biology at Old Dominion University in Norfolk, Virginia, told ABC News. But from an evolutionary standpoint, polar bears have become "backed into a corner" by becoming specialized on the marine mammal prey, he said.

The food available on land is much less energy-dense, and the bears exert much more energy to catch terrestrial prey, Pagano said.

"Those terrestrial food sources these polar bears are taking in basically does them no good," Whiteman said.

In recent years, sea ice has begun to break up sooner in the springtime and summertime and does not re-form until much later in the fall. The availability of sea ice is about a month shorter than it was about 30 years ago, according to recent research.

"With climate change and sea ice loss, the fear that is slowly being realized is that ultimately polar bear populations will shrink in size and then in some places they will start to disappear entirely," Whiteman said.

This particular population of bears is spending about three weeks longer on land than they were in the 1980s, Pagano said.

"With forecasted declines in sea ice, the longer bears are on land, the more weight they're going to be losing, and they're at greater risk of starvation in the future," Pagano said.

While most of the modeling so far indicates that many of the polar bear populations are at risk of future climate warming, bears at the southern extent of their range are at greatest risk, Pagano said.

Younger bears are likely at the greatest risk of starvation, particularly subadults and dependent cubs, as they are not able to accumulate as much stored energy relative to adult bears that are larger and much more skilled at hunting, Pagano said.

Not all of the video footage captured on the collar cams contained bad news, Pagano said. Personalities were on full display as researchers documented adult males play fighting with each other, both on land and on water.

"We were pretty blown away by the video footage," Pagano said. "There was a real variety of behaviors that the bears were exhibiting."

Outfitting the bears with the collars is a feat in itself. Typically, female bears are collared, because the devices typically won't fit around a male's neck, Whiteman said.

From a helicopter, the bears are typically sedated using a dart once the researchers determine they are in a safe enough space. Once the bear is immobalized, the collars are then fastened and are programmed to detach at the end of the study period, Whiteman said.

"Fortunately, polar bears are such large animals that you can put a fairly robust collar on them with a solid battery to power something like a video camera and be pretty confident that it's not influencing their behavior," he said.

Drastically lowering greenhouse gas emissions, the main culprit behind global warming, is the key to ensuring the survival of the species, the experts said.

"It's really a matter of mitigating CO2 emissions," Pagano said.