Female Polar Bear Den Site Selection

In the fall, pregnant polar bear females make maternity dens in drifted snow, either on land or drifting pack ice, where they hibernate over the winter (Mauritzen et al. 2001). The tiny cubs are born during hibernation (from mid-November-January) and are nursed within the den for at least 2 months. Mothers and cubs emerge around March-April. Individual females appear to have preferences for den location: females that den on land and those that den on ice are known in most subpopulations studied, except western Hudson Bay and around Svalbard in the Barents Sea, where all females appear to den on land (Mauritzen et al. 2001; Ferguson et al. 2000; Fischbach et al. 2007; Lentfer 1975; Zeyl et al. 2010). "Long-term fidelity of denning areas and faithfulness to denning substrate (i.e. land vs. ice) has been observed" (Zeyl et al. 2010:1139).

Do females switch their den locations from one year to the next? If so, what is the level of loyalty ("fidelity") to either the area (the particular place on land) or the substrate (land vs. ice)? As all Svalbard area females appear to den on land, the issue addressed in this paper was whether mother's switched from one denning area to another within the Svalbard region, and whether related individuals (especially mothers and daughters) tended to den in the same area.

The authors used data collected from a technique called "mark-recapture," along with results from analysis of mitochondrial DNA (mtDNA, the kind inherited from mothers) to investigate the amount of loyalty that individual female polar bears had to five denning areas around Svalbard. They also used these data to address whether any loyalty to a specific area was transmitted from mothers to daughters to such an extent that it would result in genetic clustering of mtDNA haplotypes in different den areas.

The results of the study indicated that female polar bears have a moderate degree of loyalty to their chosen den areas (3/13 females switched areas from one year to the next) and that daughters had a tendency to den in the same areas as their mothers (only 2/8 did not). There was not, however, the kind of genetic clustering expected if such loyalty had been on-going over a long period of time. The authors note that the while the distance between successive mark-recapture locations for Svalbard females were similar to distances reported for land-denning females in western Hudson Bay (23.7 km vs. 34 km, respectively), these were much lower than documented for females in the Beaufort Seas (308 km), where a significant proportion of bears den on sea ice (Amstrup and Gardner 1994). Zeyl and colleagues also note that the level of denning site loyalty demonstrated by female polar bears on Svalbard is markedly lower than recorded for brown bears, their closest living relatives (who of course always den on land).

Zeyl and colleagues suggest that female polar bears may switch the precise location of their dens from one year to the next depending on prevailing snow or sea ice conditions. In a similar study to theirs, Fischbach et al. (2007) documented some females who switched between pack ice dens and land dens (i.e. a switch of substrate type). Zeyl et al. suggest that there is more flexibility than previously assumed in den site choice among polar bear females.

The demonstrated willingness of polar bear females to switch den locations from one year to the next, despite a general tendency towards den site loyalty, almost certain gives them the flexibility they need as a species to persist in the face of sea ice conditions that vary from year to year. The ability of polar bear females to shift from one den site location to another or one substrate to another, is just one aspect of the kind of plasticity needed to survive in this habitat.

Amstrup and Gardner (1994:8) stated more than 10 years ago that "Contrary to previous hypotheses (Stirling and Andriashek 1992), substantial polar bear denning occurs in the Beaufort Sea region of northern Alaska and adjacent Canada. Bears that den on pack ice are subject to risks not encountered by bears that den on land. Unstable, moving ice caused early abandonment of dens and, apparently, loss of cubs. However, the persistence of pack-ice denning indicated that those risks are not overwhelming."

As one Russian polar bear researcher commented years ago regarding bears in the eastern Russian Arctic, "our investigations on Wrangel Island have shown that the polar bear is a very plastic animal: it can rapidly change its way of life, spatial distribution and behavior according to new ecological conditions" Kochnev (2006:163). Zeyl and colleagues seem to agree (2010:1139), stating that "females are likely able to change denning locations if unsuitable ice conditions prevent them from reaching their preferred denning areas. We consider this plasticity an important attribute of polar bears when facing climate change."

Additional References
Amstrup, S.C. and Gardner, C. 1994. Polar bear maternity denning in the Beaufort Sea. The Journal of Wildlife Management 58: 1-10.

Ferguson, S. H., Taylor, M. K., and F. Messier 2000. Influence of sea ice dynamics on habitat selection by polar bears. Ecology 81: 761-772.

Fischbach, A. S., Amstrup, S. C., and D. C. Douglas 2007. Landward and eastward shift of Alaskan polar bear denning associated with recent sea ice changes. Polar Biology 30: 1395-1405.

Kochnev, A.A. 2006. Research on polar bear autumn aggregations on Chukotka, 1989-2004. In: Aars, J., Lunn, N.J. and Derocher, A.E. eds. Polar Bears: Proceedings of the 14th Working Meeting of the IUCN/SSC Polar Bear Specialist Group, 20-24 June 2005, Seattle, Washington, USA. Occasional Paper of the IUCN Species Survival Commission 32. Gland, Switzerland, and Cambridge, UK: IUCN. 157-165.

Lentfer, J. W. 1975. Polar bear denning on drifting sea ice. Journal of Mammalogy 56: 716-718.

Mauritzen, M., Derocher, A.E. and Wiig, Ø. 2001. Space-use strategies of female polar bears in a dynamic sea ice habitat. Canadian Journal of Zoology 79: 1704-1713.