Maternity Denning at the Base of Icebergs

Researchers recently discovered a unique polar bear maternity denning habitat in northeast Greenland: snowdrifts at the base of icebergs frozen into landfast ice.

Unlike other North American bears, most polar bears do not go into dens for the winter but continue to hunt seals from the sea ice. Pregnant adult female polar bears are the exception. They enter maternity dens by late October or early November and give birth to cubs by late December to early January. At birth, the cubs are too small to survive winter conditions, so the maternity den functions as an external womb. Within the safety of the den, cubs can be nursed and sheltered for three to four months until they are large enough to follow their mothers onto the sea ice to hunt seals. Consequently, reliable locations of suitable snowdrifts for maternity denning, and the overwinter stability of the individual den sites chosen by pregnant females, are critical to the survival of cubs. 

Den location basics

In most areas around the circumpolar Arctic, females dig maternity dens into snow banks on secure land areas adjacent to the sea, often showing great fidelity to denning locations. In particular, they choose sites that recur annually in snowdrifts. These drifts are formed by the wind blowing snow over the leeward slopes of coastal hillsides, valleys, barrier islands, and other landscape irregularities.

The snowdrifts selected for maternity dens are usually compacted and hardened on the surface by winds that continue through the winter. Observations of adult females sampling snowdrifts for denning suitability in the fall indicate they are quite selective about the ones they choose. However, other than sufficient depth, it is not obvious what specific factors may influence a female’s selection of a particular snowdrift among many in a local area that might appear suitable to a human. Overall, polar bears across the Arctic show a preference for denning on land rather than sea ice. The overwinter stability of land-based snowdrifts is likely a significant factor, though a limited amount of maternity denning has also been documented in old, stable multiyear ice located well offshore from the less-stable annual ice near shore. 

Unusual findings 

During research on polar bears in northeast Greenland, conducted out of Danmarkshavn in April 2018 (Figure 1), a completely new aspect of polar bear denning ecology was discovered, almost by accident. Overwinter maternity dens dug by three adult female polar bears were found in snowdrifts formed by wind around the base of large icebergs (freshwater glacier ice) grounded on the sea floor and/or frozen into both annual and multiyear fast ice. This fascinating behavior had never before been reported in the scientific literature. In the following year, two more similar dens were confirmed, and a third suspected one, further north near Station Nord (Figure 1). In total, five polar bear maternity dens were confirmed, and a probable sixth one reported, approximately one to 10 kilometers offshore dug into snowdrifts around the stranded icebergs. Figure 2 illustrates the locations of four maternity dens in snowdrifts by grounded icebergs.

This type of potential maternity denning habitat is limited in distribution and is only possible in heavily glaciated regions of the Arctic where calving of marine-terminating glaciers form icebergs large enough to drift away, become grounded offshore, and remain in place for months or years. Although northeast Greenland has long been known to be an important maternity denning area for polar bears, with reports of dens on land as well as sightings and captures of females with young cubs in the spring, the use of snowdrifts around grounded icebergs for maternity denning had not been scientifically documented before. 

Why icebergs?

Of course, one of the first questions that comes to mind when finding maternity dens in a new habitat for the first time is whether there might be something different ecologically about them. For example, one major ecological difference in the terrestrial habitat in northeast Greenland is that it is a polar desert, meaning there is little snowfall. Consequently, there is much less snow for forming suitable drifts for denning. However, when we compared the basic habitat needs of pregnant polar bears, including predictable and stable overwinter habitat, the discovery was less surprising but equally fascinating. 

Overall, it appears that the driving factors determining the long-term fidelity of adult females to maternity denning habitat by grounded icebergs are much the same as those that characterize terrestrial maternity denning habitat elsewhere in the Arctic. These include: 1) annual predictability of abundant suitable snowdrifts for denning; 2) abundant habitat; 3) stable snowdrifts because of very cold conditions; and 4) proximity to rich feeding grounds in spring, in this case the biologically productive Northeast Water Polynya (an area of open water surrounded by ice), where seals are abundant and accessible for hungry females with cubs shortly breaking out of their dens in the spring.  

Thus, for the moment, the combination of features described above creates an excellent maternity denning habitat for pregnant females in the autumn, for much the same reasons that they dig dens on land in other parts of the Arctic. However, the continued decline of sea ice throughout the Arctic as a consequence of continued climate warming may eventually have a negative influence on the stability of grounded icebergs as sites for maternity denning if break-up expands into new areas or occurs earlier. Similarly, it is uncertain what the effects of increased human activity might be in nearby offshore areas because of their greater accessibility as the sea ice declines in future decades. These will be important issues to monitor in future years.

Dr. Kristin Laidre is a research scientist at the University of Washington and a biologist with the Greenland Institute of Natural Resources. Dr. Ian Stirling is an adjunct professor with the Department of Biological Sciences, University of Alberta, and a research scientist emeritus with Environment and Climate Change Canada.