Hardly any other animal represents the devastating consequences of global warming as much as the polar bear – and in fact, longer ice-free phases in the Arctic make it difficult for the predators. This is shown by a complex study in the journal “Nature Communications”, for which researchers observed 20 polar bears in Canada very closely over several weeks. The team led by Anthony Pagano from the Alaska Science Center found that the polar bears also search for food on land, but are less successful and lose weight.

Advancing climate change is leading to a decline in sea ice in the Arctic. This is a problem for polar bears because they hunt seals on the ice from late spring to early summer, which give birth to their pups at this time. If the sea ice retreats, polar bears are forced to go on land. Due to global warming, the ice-free periods have lengthened significantly: from 1979 to 2015 by three weeks, so that polar bears now spend 130 days a year on land.

22 to 67 percent fewer young animals

Polar bears are adaptable and occasionally hunt prey on land. But if the ice-free period continues to lengthen, scientists fear that the survival of the animal species will be severely endangered. According to estimates, 22 to 67 percent fewer young animals could be born by 2050. Other researchers assume that a quarter of males will starve if the Arctic sea remains ice-free for 180 days.

Despite these assumptions, it is unclear whether polar bears could survive longer during the sea ice-free period by using less energy or exploring new food sources. To find out, Pagano’s team examined 20 polar bears in Canada’s Hudson Bay during the sea ice-free period.

The authors determined the animals’ daily energy consumption and changes in their body mass. Using GPS trackers equipped with a camera, the researchers were able to observe how the animals behaved, what they ate and how much they moved.

19 out of 20 polar bears lost weight

“We observed very different behaviors among the polar bears,” Pagano said, according to a news release from Washington State University. “Some bears simply lay down and used up a similar amount of energy as when hibernating. Others actively searched for food and fed on bird and caribou carcasses, seaweed and berries. Three animals even swam several kilometers across the sea in search of food to search.”

Depending on the activity, the researchers also found large differences in the animals’ daily energy consumption. Overall, 19 of the 20 polar bears lost weight: 0.4 to 1.7 kilograms per day and thus 8 to 36 kilograms within the three-week observation period. “Although some animals were able to find food, they ultimately used more energy searching for food than they could regain through eating,” explains Pagano.

Polar bears cannot be compared to grizzlies

Some experts had assumed that polar bears on the mainland would behave like their relatives, the grizzly bears: they either go into rest mode or look for food on land. But that is apparently not the case. “Polar bears are not white-furred grizzlies. They are very different from each other,” said co-author Charles Robbins, director of the Washington State University Bear Center. “Polar bears are larger and weigh significantly more. To maintain their weight, they eat the energy-rich fat of seals – and they catch them on the sea ice.”

Although polar bears can adapt their behavior to a great extent, the results show how much a longer period without sea ice increases the risk of the animals starving. “Because polar bears have to retreat to the mainland earlier, they also have less time to build up energy reserves that are essential for survival,” says Pagano. “We assume that more animals will starve in the future, especially younger polar bears and females with cubs.”

The nature conservation organization WWF pointed out that the development can also have an impact on people. “The more time polar bears spend on land, the greater the risk of interactions and conflicts with people in Arctic coastal communities,” the organization said. “As a large predator, polar bears pose a significant threat to human life and encounters can result in property damage, injury and loss of life to both humans and bears.”

According to Pagano’s experts, the polar bears in western Hudson Bay are probably more affected by the consequences of climate change than those in other regions in the Arctic.