Polar Bear Maternal Denning

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Pregnant polar bears enter maternity dens in October/November, give birth to cubs in December/January, and exit dens in March/April. Historically, most polar bears from the Southern Beaufort Sea (SBS) population constructed maternity dens on the sea ice.  Over the last three decades, as sea ice has become thinner and prone to fragmentation, there has been a landward shift in the distribution of dens. Based on data from radio-tagged adult female bears in the SBS, maternal denning now occurs at relatively high densities along the Arctic coastal plain of Alaska. The availability of denning habitat―mediated by landscape features that facilitate the formation of snow drifts―appears to increase in the eastern portion of the Alaska coastal plain. In the Chukchi Sea, polar bears historically denned mostly on land in both Russia and the Alaska. Recently, Chukchi Sea polar bears have shifted land-based denning northward on Islands in Russia and rarely on the Alaskan coast.  Identifying factors influencing the distribution and duration of dens will allow us to better monitor reproductive success and mitigate the potential for disturbance of denned bears from anthropogenic activities.

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A view from the inside of a polar bear den in the Chukchi Sea, Alaska

A view from the inside of a polar bear den in the Chukchi Sea, Alaska. (Public domain.)

Mapping terrestrial maternal denning habitat

Denning is one of the most vulnerable times in polar bear life history as the family group cannot simply walk away from a disturbance without jeopardizing survival of newly born cubs. Because future industrial activities could overlap habitats used by denning polar bears, identifying these habitats can inform the decisions of resource managers tasked to develop resources and protect polar bears. To help inform these efforts, we are investigating methods for improving the detection and mapping of maternal denning habitat. Recently, we have investigated the efficacy of using interferometric synthetic aperture radar (IfSAR) and light detection and ranging (LiDAR) digital terrain models (DTM) to identify putative denning habitat on portions of Alaska’s coastal plain. This work provides a foundation for developing improved models of terrestrial denning habitat for the entire coastal plain region.




Denning phenology and reproductive success

Female polar bears must accumulate sufficient energy reserves prior to entering dens to produce and nurse cubs before exiting dens in spring. Adequate time spent in a den is important to optimize cub development for withstanding harsh Arctic spring conditions. A recent examination of relationships between den phenology (i.e., timing of den entry and exit), reproductive success, and environmental factors showed that females observed with cubs emerged from dens later and remained in dens approximately two weeks longer than females that emerged from dens but were seen without cubs. Females occupying land-based dens, where estimated snowfall was greater, had higher reproductive success. During years with a greater area of autumn sea ice, reproductive success was higher at land-based versus sea-ice dens, suggesting continued decline in sea ice could negatively affect recruitment. Future work will delve into better understanding mechanistic relationships between denning phenology, environmental conditions, and reproductive success. Because females emerging later from dens had higher reproductive success, den duration could be a useful metric in population monitoring.

Maternal den catalog and database

We maintain a catalog and database on the approximate locations and methods of discovery of polar bear maternal dens found in the Beaufort Sea and neighboring regions since 1910. The catalog includes a description of data collection methods, biases associated with collection method, primary time periods, and spatial resolution of the approximate den location. Data (updated periodically) on past polar bear maternal den locations are provided to inform the public and to provide information for natural resource agencies and industry in planning activities to avoid or minimize interference with polar bear maternity dens.