Polar bear cub survival depends on this little-known den behavior

For the first time, researchers have used satellite collar data alongside specialized cameras to observe maternal denning in polar bears. This stage, when mothers give birth and remain in dens with their cubs, has long remained a mystery.

Studies have shown that the length of time mothers spend in dens impacts cub survival. However, scientists had little understanding of why they stay so long or what they do during that period. Until now, tracking devices had not been tested against direct observations of the bears.

Study lead author Louise Archer is a postdoctoral researcher at the University of Toronto Scarborough.

“The Arctic’s warming two to four times faster than the rest of the world, humans are expanding into areas that might be important for polar bear denning, and we know they’re sensitive to disturbance during this time,” said Archer.

Role of denning in polar bear survival

Denning begins when pregnant polar bears dig out snow dens to give birth. Cubs are born tiny and vulnerable, and the den shields them from the harsh Arctic cold as they nurse and grow rapidly.

During this time, mothers lose nearly half their body weight. Even after emerging, they do not immediately return to hunting. Instead, they stay near the den for several weeks, engaging in activities that seem more critical than eating.

Historically, researchers observed denning through binoculars, then remote cameras, and now satellite collars. Each method has its limitations. Satellite collars, which track location, activity, and ambient temperature, have become more common.

However, most collars only record data every few hours, making it difficult to capture short-term behaviors.

Tracking polar bears for six years

In a six-year study, Archer and her colleagues tracked 13 polar bears from the Barents Sea subpopulation. They placed cameras outside of nine dens in Svalbard, Norway, while also using satellite collars.

Comparing the two methods revealed differences of several days to over a week in estimating key denning stages.

Each time the experts collected collar data, they matched it with corresponding camera images to confirm bear activities.

The team then built three statistical models. These allow researchers to use collar data to predict not just what the bears are doing, but what they will likely do. The models forecast breakout times, den emergence patterns, and final polar bear departures.

One model also considers how temperature changes affect the behavior of mothers and cubs.

Temperature and den emergence patterns

“Collars do a good job at picking out these broader behaviors, like when the bears first come out of the den and when they depart. We found they corresponded pretty well to what we were seeing on camera,” said Archer. “But it was difficult to tease out the finer scale behaviors we saw on camera.”

Bears almost always emerged during daylight, with trips averaging 27 minutes. Some lasted under a minute, while others extended to nearly eight hours. Mothers brought their cubs outside about half the time, usually staying within 40 meters of the den.

Warmer temperatures increased the likelihood of polar bears leaving the den and prolonged their outdoor stays. The longer it had been since their first breakout, the more frequently they emerged.

Importance of denning for cubs

The data suggests that this period near the den helps cubs adapt to the outside world. Other research has found that a faster departure after breaking out reduces cub survival rates. If cubs do not spend enough time near the den, their chances of survival decrease.

“The Arctic is a really fast-changing area. We’ve got a lot of sea ice being lost, so seeing what polar bears are doing and how they’re responding to these changes gives us an insight into what we might expect in other parts of the Arctic down the line,” said Archer.

She recently completed a study linking a decline in polar bear populations to shrinking sea ice caused by climate change.

Future of polar bear research

This study involved researchers from Polar Bears International, San Diego Zoo Wildlife Alliance, and the Norwegian Polar Institute.

By improving tracking methods, scientists hope to gain a clearer picture of polar bear behavior in and around dens and the challenges they face.

Continued monitoring will provide valuable insights into how these animals adapt to their rapidly changing environment.

The study is published in the Journal of Wildlife Management.

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