The secret lives of polar bear families

Despite being the largest land carnivore and a top Arctic predator that can weigh over 600 kg, polar bears start off surprisingly small. Blind, almost hairless, and weighing just 600g at birth, cubs are born in maternity dens under the snow. These snow caves keep newborns warm and safe for the first few months of their life, when they grow rapidly by nursing on their mother’s rich milk.

After three to four months in the den, cubs will have grown to about 20 times their birth weight and will be large enough and furry enough to follow their mothers out into the frigid Arctic spring.

In a study published in The Journal of Wildlife Management, we used remote cameras to study polar bear families as they emerged from their dens in Svalbard, Norway, gaining insight into the behaviour of mothers and cubs as they experience the world outside the den for the first time.

An elusive phenomenon

While they provide ideal conditions for developing cubs, maternal dens are difficult for researchers to study and monitor. Challenging weather, limited daylight and the remoteness of many den sites means opportunities for direct observation are few. Often, denning polar bears are identified using tracking devices worn by a bear — usually collars, but also ear or fur tags. These transmit location data via satellite, allowing researchers to track individuals and to study movement patterns.

As technology has developed, additional data can also be collected from these devices, including data on activity and temperature. An extended stationary period and low activity readings are the telltale signs of denning. Above-ambient temperatures also indicate a bear in a den; insulated by snow and warmed by the mother’s body heat, the interior of the den can be more than 20 °C warmer than the outside.

In Svalbard, polar bears build their dens on slopes of fjords and mountainous areas, where drifting snow means dens are often impossible to distinguish from the snow-covered surroundings.

Locating dens

We relied on GPS locations transmitted from satellite collars worn by females to locate 13 den sites. With the return of daylight to Svalbard in the spring, our team installed time-lapse cameras facing the entrance of each suspected den, capturing footage of polar bear families as they exited. To minimize any disturbance, the final approach was made on foot or by ski, and cameras were collected several months later — long after the polar bear families had departed for the sea ice.

After processing thousands of images, the camera gave us a detailed look at this cryptic component of polar bears’ life cycle. By linking images back to data from the collars, we were also able to develop a model of the various behaviours caught on camera, providing a new tool to remotely monitor denning bears more accurately.

A feat of endurance

Although critical to cubs, denning can be tough on a mother. Pregnant female polar bears usually enter a den in the fall, give birth in mid-winter, and remain in the den nursing their cubs until the family is ready to emerge in the spring. Although their offspring guzzle down high-energy milk, mother polar bears don’t feed at all during this time and rely on their fat reserves, losing up to 43 per cent of their body mass while in the den.

Despite this clear motivation to get back to hunting seals on the sea ice, polar bear families will often hang out at the den for days or weeks after emerging. On average, the families we monitored in Svalbard stayed at the den site for a further 12 days after first emerging.

During this time, mother and cubs frequently left the den to explore, sometimes staying outside for less than a minute, and in other cases emerging for hours at a time. Cubs rarely ventured outside without their mother and were seen alone in only five per cent of camera observations. In general, bears spent longer outside when temperatures were warmer and the more days had passed since they first emerged outside.

This post-emergence period may allow cubs time to acclimatize to the external environment, and to develop the skills and strength they’ll need to follow their mother across the sea ice for the next two-and-a-half years.

We also saw incredible variation in behaviour post-den emergence, with one family abandoning the den after only a couple of days, and another family remaining at the den for a full month after first appearing outside. Two females even decided to move their cubs to new dens after emerging.

Consequences of Arctic change

These kinds of insights lead to new questions: what drives decisions to stay or leave the den, what cues do families respond to? While we continue to build out our data set to better understand these behaviours, on average, we noted that polar bears abandoned their dens about a week earlier than previously recorded in the region. The Barents Sea is one of the fastest warming regions on the planet, and continued monitoring will make clear if this is an emerging trend in response to sea ice loss.

To get even more detailed information, we have also been testing custom designed camera systems that can capture behaviour continuously.

Climate warming has already resulted in declining polar bear health in parts of the Arctic that are experiencing rapid loss of sea ice. With continued warming jeopardizing the persistence of polar bears across much of their range, successful denning and reproduction is essential to give the next generation of polar bears a chance.

Read more: Polar bears may struggle to produce milk for their cubs as climate change melts sea ice

Time spent denning, the date of den exit and the amount of time bears remain at the den after emerging all contribute positively to the subsequent survival of cubs. Yet climate warming means the human footprint in the Arctic is expanding, risking encroachment on denning habitat and disturbing polar bear families.

Improved monitoring and a deeper understanding of denning behaviour will help to protect polar bears during this critical time.

Louise Archer receives funding from Polar Bears International. She is affiliated with University of Toronto Scarborough and Polar Bears International. This study was performed in collaboration with the Norwegian Polar Institute and San Diego Zoo Wildlife Alliance.