World Polar Bear Day: All Things Cubs With Polar Bears International's Geoff York


Rachael Funnell


Rachael Funnell

Digital Content Producer

Rachael is a writer and digital content producer at IFLScience with a Zoology degree from the University of Southampton, UK, and a nose for novelty animal stories.

Digital Content Producer

polar bear cubs den polar bears international

Polar bear dens are vitally important, but protecting them relies on knowing where they are. Image credit: Steven C Amstrup / Polar Bears International

Sunday February 27 marks International Polar Bear Day, where we take a moment to recognize and appreciate one of Earth’s most charismatic but threatened creatures. We caught up with Geoff York, Senior Director of Conservation for Polar Bears International, to talk all things baby bears as he reveals the details of polar bear den season.

Caution: adorable polar bear cubs ahead.


What do polar bear moms look for in a good den?

Across most of the Arctic, pregnant female polar bears dig their maternal dens in deep banks of snow, away from areas where other bears may be continuing to hunt and roam. They typically select geographic features that enhance drifting or hold snow longer: coastal tundra bluffs, riverbanks, lakeshores, and where available, mountains.

They also tend to select areas that are remote and largely undisturbed by activity, though there have been some notable exceptions of females selectively denning quite near human activity. Polar bears remain active throughout denning – periodically shaving ice from the den roof with her claws and physically moving the den upward to maintain a desired depth from the surface if snow continues to accumulate above her.

Geoff York studies polar bears at Polar Bears International. Image credit: Mike Lockhart / Polar Bears International

Why is it important we keep track of where these dens are?


Denning is one of the most critical times for polar bear reproduction. Cubs are born with light hair, blind, and fully dependent on both their mother and the safety of the den for early survival.

Suitable denning habitat is selectively chosen by polar bears in various regions, making these areas suitable for potential protection during the denning period. They are considered critical habitat areas for polar bears by definition.

However, it is difficult to protect what we cannot see, and polar bear dens are very hard to discern once occupied and covered with drifting snow. It is only through rare direct observation (human activity rarely overlaps with denning areas), satellite tracking, or use of technology like forward looking infrared (FLIR) to detect heat under snow.

polar bear cubs dens
Dens can easily be hidden under shifting snow. Image credit: Simon Gee / Polar Bears International

In areas of significant human activity like the northern slope of Alaska, protecting denning areas is even more critical. The Southern Beaufort Sea population that relies on safe denning habitat along the Alaskan Arctic coast has already declined by 40 percent in the last 30 years.


Successful reproduction is a key to stabilizing and ultimately reversing that decline. Active oil and gas exploration and development activities often rely on winter access for the use of tundra vehicles or the building of temporary ice roads out across the tundra. These activities could directly harm denning bears through disturbance resulting in early abandonment, or through direct trauma by heavy vehicles crushing dens they inadvertently drive across.

What advances have we made in polar bear den protection/detection, and what challenges do we still face?

Through the tracking of polar bears across several populations over two decades, we have learned a lot about where polar bears prefer to den, when they enter, and when they depart. Through nearly 20 years of visual den monitoring conducted by PBI and partners, we’ve refined den entry and exit times and learned about bear behavior at den sites, providing managers with better data regarding disturbance mitigation.

While initially quite promising, the use of FLIR to detect dens under the snow has proven challenging to implement in real time. Recent analysis on the use of FLIR as a management tool found that it may have missed 55 percent of the dens surveyed.


While some of this failure is related to improper protocol, a significant aspect is the narrow environmental window FLIR needs to operate successfully for den detection. FLIR requires dark, stable, cold weather over multiple days for the subtle heat from a den to become apparent on the surface.

polar bear dens cubs
Image credit: Steven C Amstrup / Polar Bears International

New snow, winds, drifting, and sunlight easily erase that signature. As the Arctic continues to warm at nearly three times the rate of other parts of our planet, those stable weather conditions are becoming ever more elusive.

PBI and partners are actively working to develop a more robust technology, synthetic aperture radar (SAR), to replace FLIR in active management scenarios. SAR is a surface penetrating radar originally developed for use by the military in detecting objects under the surface (buried military equipment, tunnels, explosive devices, etc.).

It is far less weather dependent, operates from an aircraft at higher altitudes, and has a larger view angle making it more efficient in scanning large areas of potential habitat. Initial work showed progress in Alaska, but research was delayed by a lack of known dens to test SAR.


Recent work by PBI and Brigham Young University utilized artificial dens dug and occupied by students in the mountains above Provo, Utah to refine the concept. We followed that with the collection of SAR imagery of known polar bears on the surface in Churchill, Manitoba last fall to give the engineers definitive target data.

As we speak, our team is deploying to Svalbard, Norway where they will attempt to detect several known dens within flight range of Longyearbyen.

polar bear dens cubs
 Image credit: Simon Gee / Polar Bears International

What happens in the den between birth and the cubs’ first appearance out in the world?

There is a lot we do not know about denning in the wild due in part to the remote locations bears typically use for denning, the dark and extreme cold of Arctic winter, and the fact that few have been willing to poke sensors or cameras into an occupied polar bear den. We do know that polar bear cubs grow quickly on the high fat content of their mother’s milk, going from about 0.68 kilograms (1.5 pounds) at birth to around 9 kilograms (20 pounds) at emergence in the spring.


We also know that female polar bears actively maintain the den throughout the winter, scraping off ice as it builds up on the ceiling and walls from respiration and sometimes physically moving the den while still below the snow surface. We do have some observational data from captive partners who have successfully raised cubs and today, most den areas in captive facilities have multiple cameras and are monitored 24/7. Mothers actively feed, groom, and keep their cubs warm during the denning period.

For more information about how you can support polar bears this International Polar Bear Day, click here.


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