Polar Bears Reluctant to Abandon Dens, Even When Disturbed

When it comes to polar bear dens and disturbance, both the distance and the intensity of human activity appear to make a big difference in outcomes. That’s the conclusion of a new study my colleagues and I published today in the journal Arctic.

The research team, composed of scientists from Brigham Young University and Polar Bears International, analyzed the responses of denning bears to human activities on Alaska’s North Slope. This study represents the first summary analysis of responses by denning bears across a range of years and disturbances.

“The findings will be extremely useful to managers when setting guidelines for industrial activities in polar bear denning areas in Alaska and elsewhere,” said lead author Wesley Larson of Brigham Young University.

Years of field research

In addition to Larson, the team included Dr. Tom Smith of Brigham Young University and me. Between us, we have decades of experience with bears, (polar, brown, and black), denning polar bears, polar bear-human conflict, and Arctic research.

We based our study on visual observations made by biologists during the polar bear denning period in from den excavation through emergence, gathered from multiple sources over 42 years.

“Den emergence” refers to the period when a polar bear family breaks free of the snow den where the cubs were born, a phase when they start to venture out into the world for short intervals. During this period, which can last up to two weeks, the family spends most of the time sheltered in the den. They depart from the den site for good when the mom decides her cubs are strong enough to follow her to the sea ice, where she’ll replenish her fat reserves by hunting seals.

Our team found that, during this emergent phase, most human activities caused some level of reaction from the mother bears, with low-flying aircraft eliciting the strongest response. The responses, however, were lower than anticipated, and caused few den abandonments. Closed dens seemed to offer the greatest protection and most den abandonments recorded happened in November and prior to any cubs being born.

“We found that polar bear moms with cubs rarely leave their dens in response to disturbances,” said Larson. “We also found that, in places like the North Slope, the current one-mile buffer zone around known dens appears to be effective in shielding families from human and industrial activity.”

He noted, however, that all observations related to overt responses of bears—that is, what we could visually describe. A bear that ‘looks’ undisturbed may in fact be quite stressed. Determining this would require physiological measures, which we did not implement for this study. Also, direct responses to high intensity stimuli, like seismic testing, remain unknown due to lack of data.

Important catch

Our findings suggest that polar bears feel secure in their dens and are reluctant to abandon them, especially during the critical time after birth and before emergence. Sealed dens cannot be easily detected visually and are therefore hard to avoid or manage. A recent analysis of the oil and gas industry’s main den-detection method, aerial FLIR surveying, suggested they miss 55% of polar bear dens.

This could make bears in undetected dens vulnerable to harm from heavy equipment or high intensity disturbance because they have such a strong sense of security while sheltered inside, coupled with significant maternal investment. If a family was forced to abandon their den, which we noted on one occasion, it would do so at great risk to the small young cubs. Polar bear mothers create and stay in dens for a reason.

When the female has opened her den, the den, and sometimes the bears, are visible and disturbance can be managed. It’s the dens we can’t see that are of greater concern. Given the documented 40% decline of the adjacent Southern Beaufort Sea polar bear population, the success of every den in this region becomes more important to ensure Alaska has polar bears around for generations to come.

In addition to emphasizing the importance of the one-mile buffer zone to afford adequate protections, the study highlights the need for added research to develop more effective den detection in winter and to evaluate the impacts of high intensity stressors on denning polar bear families.

Reference: Human Interaction and Disturbance of Denning Polar Bears on Alaska’s North Slope, by Wesley Larson, Dr. Thomas S. Smith, and Geoffrey York, Arctic, Vol. 73 No. 2 (2020): June: 141-277.

This article originally appeared in The Rising.