Wild Cam: Dens crucial for polar bear survival 

On a morning in late March 2024, a female polar bear ambled across a frigid slope in Svalbard, Norway. One, two, then three cubs popped out of the snowy bank, seemingly out of nowhere, one sliding down the mountain and another climbing up its mother’s back.

“The cubs are playing, moving around, strengthening their muscles and exploring, as well as strengthening bonds between mother and cub,” said Louise Archer, Polar Bears International postdoctoral fellow at the University of Toronto Scarborough. “Sometimes we have a camera deployment that doesn’t survive the curiosity of a polar bear family,” she laughed.

While adorable, researchers are recording these videos for an important reason. Polar bear (Ursus maritimus) cubs are especially vulnerable during denning—linking this critical period to the survival of the species.

In the Arctic, pregnant polar bears dig dens in snowbanks where they stay throughout the winter, giving birth to tiny, vulnerable cubs. Over the next three to four months, the cubs drink the mother’s fat-rich milk and grow to about 20 times their initial birth weight within the den. “It’s like an extension of the womb,” Archer said.

Polar bears can build their dens on snowbanks on sea ice or on land, but as sea ice in the Arctic shrinks, polar bears are building their dens more often on land. Humans, too, are expanding their presence in the region, which can be a problem as polar bear mothers have been known to abandon their dens and cubs upon disturbance. On average, less than half of polar bear cubs make it past their first year.

Due to the disturbance problem, denning has been challenging to study for researchers as well. Researchers don’t know much the first few weeks when mothers first bring their cubs out of the snow and into the world. In a study published in the Journal of Wildlife Management, Archer and her colleagues analyzed nearly a decade of satellite tracking and camera trap data to get a better understanding of the first delicate moments that occur as polar bear families emerge from their dens in Svalbard, Norway.

The terrain in Svalbard is intense, Archer said. Among the fjords and mountains, pregnant females dig into the leeward slopes so snow builds over their dens throughout the winter. “When you’re looking at a closed den, all you see is a white blanket. It would be impossible to pick one out by eye alone,” she said. Using locations derived from satellite collars placed on the bears by the Norwegian Polar Institute, the researchers deployed cameras facing suspected dens.

Of the 13 deployed, nine camera traps captured footage of polar bear mothers and cubs emerging from their dens for the first time. On average, bears first broke out of their dens on March 9. After their first emergence, as seen starting at 2:15 on the above video, the bears stayed in their dens for an average of 12 days, although this period varied. “Each den had its own story,” Archer said. One bear family stayed only two days, while another stayed for 31. The length of time the mothers and cubs spent outside the den also varied. Sometimes they came out for less than a minute, sometimes for several hours.

Past research shows better survival when cubs hang around their dens longer after initial emergence. “The longer they spend around the den, the more time they have to acclimatize,” Archer said. It’s a trade-off, though. Each day that the cubs grow stronger and more capable within the safety of the den, the mother spends another day without eating. “She hasn’t eaten for five to eight months, so for her it’s really important to get back onto the ice and build back her fat reserves.” Understanding what the triggers are for leaving the den is important for understanding variation in polar bear behavior.

Archer was surprised to observe den-switching: two polar bear mothers built separate dens after their initial emergence. A less surprising finding was how dependent polar bear cubs are on their mothers—the young bears only emerged from their dens without their mothers 5% of the time.

Camera traps are expensive and impractical in such difficult terrain, especially when researchers are guessing where to place the cameras to capture an emergence. But Archer and her colleagues hope that their work will provide a new tool to polar bear researchers studying such delicate and inaccessible dens. “We were able to use the photos to link each behavior with a data point on the collar and build out a predictive model,” she said.

Overall, improved knowledge of how, when and why polar bears emerge from their dens can help researchers answer questions in the future about the survival of cubs in this vulnerable stage and how that might change over time due to factors like climate change and increased human disturbance. “Being able to study and monitor polar bears during this critical period will give the populations the best chance of survival into the future.”

This photo essay is part of an occasional series from The Wildlife Society featuring photos and video images of wildlife taken with camera traps and other equipment. Check out other entries in the series here. If you’re working on an interesting camera trap research project or one that has a series of good photos you’d like to share, email Olivia at omilloway@wildlife.org.