Polar Bears In Depth
Denning
Page 7 of 8
Human Influences on Denning. The time spent in maternal dens is the only period in their life cycle during which polar bears are unable to simply move away from a potential disturbance. Premature exposure of altricial neonates to the outside arctic environment can be fatal (Amstrup and Gardner 1994). Therefore, disturbance of denning bears could result in reproductive failures. The only quantitative data on sensitivity of denning polar bears to human disturbances are from the Beaufort Sea region. Amstrup (1993) reported considerable tolerance of human activities near dens. Subsequent observations (S. C. Amstrup, unpublished data) corroborate those early records. Polar bears seem secure in their dens, and appear very tolerant of aerial and ground traffic very near maternal dens in winter and spring. These observations corroborate the results of Blix and Lentfer (1992), who observed that only seismic testing <100 m from a den and a helicopter taking off at a distance of 3 m produced noises inside the dens that were notably above background levels. They also concluded that a polar bear in its den is unlikely to feel vibrations unless the source is very close. Preliminary analyses of more recent work (MacGillivray et al. 2002) confirm that sound penetration into dens is greatly ameliorated. They also suggested, however, that helicopters and some ground vehicles may be detectable in dens at much greater distances than suggested by Blix and Lentfer (1992).
Observations of grizzly bears also suggested substantial tolerance of such activities. McLellan and Shackleton (1989) found that grizzly bears in summer were not displaced from the immediate vicinity of seismic testing supported by helicopters. Reynolds et al. (1986) reported some movements and possible increased heart rates when denned grizzly bears were exposed to seismic testing activities. However, they also observed that similar movements and heart rate patterns sometimes occurred in absence of human activities, and they concluded that "effects on the bears were probably minimal" (Reynolds et al. 1986 :174). Although the observations appear compelling, the sample sizes reported by Amstrup (1993) are small. The tests of Blix and Lentfer (1992) also were relatively limited in scope, and the degree to which information from grizzly bears applies to polar bears is uncertain. We know polar bear behaviors are highly variable among individuals. This variability means that additional data will be necessary to quantify the possible effects human activities may have on denning female polar bears. Fortunately, prudent spatial and temporal management of human activities using the best available information, can prevent most potentially disruptive activities from overlapping with polar bear denning.
Delayed implantation and birth of altricial young mean that early in pregnancy, parental investment is low. Female polar bears have less to lose by leaving a den in the fall than they do by leaving after parturition. Perhaps that explains why polar bears appear to be more willing to abandon dens in fall than later in the denning period (Amstrup 1993; S. C. Amstrup, unpublished data). Belikov (1976) also reported that polar bears were more easily displaced from their dens in the fall. Five Alaskan polar bears thought to abandon dens because of human interference in the autumn were successful in redenning elsewhere (Amstrup 1993, S. C. Amstrup, unpublished data). Likewise, three bears disturbed from dens near Hudson Bay relocated to other den sites (Ramsay and Stirling 1986a). The relative resilience of denning bears to disruptions in spring and their plasticity regarding den selection in autumn have significant management ramifications.
Observations of grizzly bears also suggested substantial tolerance of such activities. McLellan and Shackleton (1989) found that grizzly bears in summer were not displaced from the immediate vicinity of seismic testing supported by helicopters. Reynolds et al. (1986) reported some movements and possible increased heart rates when denned grizzly bears were exposed to seismic testing activities. However, they also observed that similar movements and heart rate patterns sometimes occurred in absence of human activities, and they concluded that "effects on the bears were probably minimal" (Reynolds et al. 1986 :174). Although the observations appear compelling, the sample sizes reported by Amstrup (1993) are small. The tests of Blix and Lentfer (1992) also were relatively limited in scope, and the degree to which information from grizzly bears applies to polar bears is uncertain. We know polar bear behaviors are highly variable among individuals. This variability means that additional data will be necessary to quantify the possible effects human activities may have on denning female polar bears. Fortunately, prudent spatial and temporal management of human activities using the best available information, can prevent most potentially disruptive activities from overlapping with polar bear denning.
Delayed implantation and birth of altricial young mean that early in pregnancy, parental investment is low. Female polar bears have less to lose by leaving a den in the fall than they do by leaving after parturition. Perhaps that explains why polar bears appear to be more willing to abandon dens in fall than later in the denning period (Amstrup 1993; S. C. Amstrup, unpublished data). Belikov (1976) also reported that polar bears were more easily displaced from their dens in the fall. Five Alaskan polar bears thought to abandon dens because of human interference in the autumn were successful in redenning elsewhere (Amstrup 1993, S. C. Amstrup, unpublished data). Likewise, three bears disturbed from dens near Hudson Bay relocated to other den sites (Ramsay and Stirling 1986a). The relative resilience of denning bears to disruptions in spring and their plasticity regarding den selection in autumn have significant management ramifications.