Polar Bears In Depth
Denning
Page 3 of 8
Denning on the Pack Ice. Although most maternal denning appears to occur on coastlines of mainlands and islands, Amstrup and Gardner (1994) discovered that 53% of the dens of polar bears radio-collared between 1981 and 1991 were on drifting pack ice. They also found that 4% were on land-fast ice adjacent to shore. Lentfer and Hensel (1980) recognized the occurrence of dens on pack ice, but suggested that it was limited to bears that could not make it to shore to den. Harington (1968) concluded that denning on ice was not preferred, and Messier et al. (1994) reported no maternal denning on pack ice, although some "shelter denning" on pack ice was observed. The discovery that half of the bears in the Beaufort Sea may den on drifting sea ice, therefore, was not expected.
Bears that den on pack ice potentially are subject to a number of disruptions that could not affect bears denning on land. First, the sea-ice changes throughout the year. It shifts, breaks up, and refreezes. Ice floes can turn over or have other floes rafted onto them. Therefore, a maternal den could be overturned, buried, or otherwise compromised any time in the denning cycle. Amstrup and Gardner (1994) reported observations of six polar bears in pack-ice dens that were swept past Point Barrow and southwest into the Chukchi Sea due to unusually unstable ice. Two of those females were observed after their dens had been destroyed by rafting action of sea-ice in mid-February. The females carried tiny cubs in their mouths, probably in desperate attempts to relocate to a new den site. When observed later that spring, however, neither of these bears was accompanied by young. Only one of six females swept into the Chukchi Sea that year had cubs when reobserved later. Hence, there are risks involved in denning on sea-ice.
On emergence of the female and her new cubs, the predictability of available resources may be limited even if a pack-ice den remains intact. Bears that den on pack ice may drift up to 1000 km during the winter (Amstrup and Gardner 1994). Despite observed and hypothetical risks, production of cubs from dens at sea was not significantly different than that from dens on land (Amstrup and Gardner 1994), and sea-ice denning has obviously been maintained as a successful reproductive strategy in the Beaufort Sea region.
Despite the absence of conclusive reports, sea-ice denning probably occurs at some level in other areas. When engaged in polar bear aerial surveys on the high pack ice northeast of Greenland and north of Svalbard, Larsen et al. (1983a) observed numerous tracks of females with cubs of the year near 84°N latitude. These animals were moving predominantly in a southeasterly direction, toward Svalbard. The distance of these sightings from land and the time of the year in which they were recorded suggested those cubs were born on the pack ice. Little significance was attached to those footprints at that time. However, that observation takes on a greater significance in light of the confirmed frequency of sea-ice denning in the Beaufort Sea (Amstrup and Gardner 1994; Amstrup et al. 2000) and the recent recognition of a possible polar basin stock of polar bears. Because other recent studies using satellite radiotelemetry have not revealed significant amounts of sea-ice denning, it seems reasonable to assume that its overall frequency is low. The linear coastline of central Arctic Russia may be more similar to the Beaufort Sea than other areas, and hence may be another area where sea-ice denning is common. Satellite data from that region, however, are too few to test that hypothesis.
In addition to questions about security of animals while in dens, the phenomenon of pack-ice denning also raises questions about navigation capabilities of polar bears. No other vertebrate is passively transported this far "in the blind." Thus, not only do polar bears range far and wide, they are able to determine where they are and return to previously used areas after long distances of passive transport. How polar bears accomplish this is unknown.
Bears that den on pack ice potentially are subject to a number of disruptions that could not affect bears denning on land. First, the sea-ice changes throughout the year. It shifts, breaks up, and refreezes. Ice floes can turn over or have other floes rafted onto them. Therefore, a maternal den could be overturned, buried, or otherwise compromised any time in the denning cycle. Amstrup and Gardner (1994) reported observations of six polar bears in pack-ice dens that were swept past Point Barrow and southwest into the Chukchi Sea due to unusually unstable ice. Two of those females were observed after their dens had been destroyed by rafting action of sea-ice in mid-February. The females carried tiny cubs in their mouths, probably in desperate attempts to relocate to a new den site. When observed later that spring, however, neither of these bears was accompanied by young. Only one of six females swept into the Chukchi Sea that year had cubs when reobserved later. Hence, there are risks involved in denning on sea-ice.
On emergence of the female and her new cubs, the predictability of available resources may be limited even if a pack-ice den remains intact. Bears that den on pack ice may drift up to 1000 km during the winter (Amstrup and Gardner 1994). Despite observed and hypothetical risks, production of cubs from dens at sea was not significantly different than that from dens on land (Amstrup and Gardner 1994), and sea-ice denning has obviously been maintained as a successful reproductive strategy in the Beaufort Sea region.
Despite the absence of conclusive reports, sea-ice denning probably occurs at some level in other areas. When engaged in polar bear aerial surveys on the high pack ice northeast of Greenland and north of Svalbard, Larsen et al. (1983a) observed numerous tracks of females with cubs of the year near 84°N latitude. These animals were moving predominantly in a southeasterly direction, toward Svalbard. The distance of these sightings from land and the time of the year in which they were recorded suggested those cubs were born on the pack ice. Little significance was attached to those footprints at that time. However, that observation takes on a greater significance in light of the confirmed frequency of sea-ice denning in the Beaufort Sea (Amstrup and Gardner 1994; Amstrup et al. 2000) and the recent recognition of a possible polar basin stock of polar bears. Because other recent studies using satellite radiotelemetry have not revealed significant amounts of sea-ice denning, it seems reasonable to assume that its overall frequency is low. The linear coastline of central Arctic Russia may be more similar to the Beaufort Sea than other areas, and hence may be another area where sea-ice denning is common. Satellite data from that region, however, are too few to test that hypothesis.
In addition to questions about security of animals while in dens, the phenomenon of pack-ice denning also raises questions about navigation capabilities of polar bears. No other vertebrate is passively transported this far "in the blind." Thus, not only do polar bears range far and wide, they are able to determine where they are and return to previously used areas after long distances of passive transport. How polar bears accomplish this is unknown.