Researchers get rare glimpse into polar bears' denning behaviour

A team of scientists led by the University of Toronto’s Louise Archer is shedding light on one of the most important yet poorly understood stages of polar bears’ lives – maternal denning, when polar bears give birth to and nurse their cubs.

Past research has established that polar bear cubs’ odds of survival are impacted by how long their mothers spend denning. However, the underlying reasons have long been a mystery, and data obtained from tracking devices hadn’t been tested against real-time observations of the animals – until now.

To fill this gap, Archer and colleagues compared observations from cameras – installed in 13 den sites in Svalbard, Norway over a period of six years – with data from satellite collars worn by denning bears. They found that the bears’ den breakouts were correlated to changes in collar activity and temperature, and that their behaviour after emerging from dens was influenced by factors such as the time of day and external temperature.

By linking photographic and collar-obtained data in this manner, the researchers were able to develop a predictive model for polar behaviour – which could prove vital in understanding how denning is impacted by climate change. The observations and models are described in a study published in the Journal of Wildlife Management.

“The Arctic is warming two to four times faster than the rest of the world,” says Archer, a Polar Bears International Postdoctoral Fellow in U of T Scarborough’s department of biological sciences. “Humans are expanding into areas that might be important for polar bear denning, and we know they're sensitive to disturbance during this time.

“We need healthy cubs to sustain populations. We're trying to develop tools to better monitor and understand their behaviour, so we can better protect them.” 

Denning begins with pregnant polar bears sealing themselves inside dens dug out under the snow. Polar bear cubs are born about as helpless as human babies, and the den protects them from the frigid weather as they grow to 20 times their size in just a few months of nursing.

Although mothers lose about half their body weight, they don’t immediately return to hunting upon breaking out of their dens. Rather, mothers and cubs hang around the den for a few more weeks.

After matching collar data and photos, the researchers created statistical models to predict when the bears break out of their dens, the times they emerge from the den, when they’ll leave the den for good and how external factors like temperature influence the behaviour of moms and cubs.

“Collars do a good job at picking out these broader behaviours, 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,” Archer says.

The work yielded a number of insights. For example, bears almost always emerged from dens in the daytime, and mothers had their cubs in tow about half the time – most often staying within 40 metres of their dens.

Bears were also more likely to be seen outside – and spend more time outside – in warmer temperatures.

These findings suggest the weeks around the den aim to help cubs acclimatize to the outside world – supporting past research that has shown faster departure after den breakouts meant cubs likely didn’t spend enough time outside the den and were less likely to survive.

Archer notes the research is yielding critical information at a time when the polar bears’ natural habitats are undergoing rapid change.

“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,” says Archer, who recently completed a study linking a decline in polar bear populations to shrinking sea ice caused by climate change. “That's why we're so invested in trying to build out this data set and continue monitoring bears in this region.”

The study was conducted with researchers from Polar Bears International, the San Diego Zoo Wildlife Alliance and the Norwegian Polar Institute.