Polar bear (Ursus maritimus) COSEWIC assessment and status report: chapter 5

5. Biology

5.1 Life Cycle and Reproduction

The reproductive capability of polar bears varies among subpopulations. Age at first reproduction may be as early as 4 years, with most subpopulations having females producing litters at relatively high rates by age 6 (Table 3). The latest age at first reproduction is near the northern extreme of the species’ range (Table 3) in Kane Basin (age 6) and Norwegian Bay (age 7).

Male polar bears likely become physiologically mature at 5–6 years of age. Fully formed spermatozoa appear only in low concentrations in testes of bears aged 2–4 years; concentrations asymptote at 5.8 years of age (Rosing-Asvid et al. 2002). Despite physiological maturity, younger males are not likely to reproduce because older males (if they are around) prevent them from doing so. Saunders (2005) recently demonstrated using paternity analysis that older adult male bears sire a disproportionate number of cubs compared to their representation in the population. It appears that most males do not enter the reproductive segment of the population until they are 8–10 years old (Ramsay and Stirling 1988; Derocher and Stirling 1998; Saunders 2005).

Table 3. Estimated means (and standard errors [SE] in parentheses) of post-den-emergence litter size and age-specific probabilities of litter production (LPR) for lone females or females with dispersing (2-year-old) cubs (because of the 3-year reproductive cycle of polar bears, females with cubs-of-the-year or yearlings are not available to mate and are not included in the LPR computation). Recruitment data have yet to be reported for remaining subpopulations.
Subpopulation
(primary data source)
Cub (age 0)
litter size
Age 4 LPR Age 5 LPR Age 6 LPR Age 7+ LPR
Baffin Bay (Taylor et al. 2005) 1.587 (0.073) 0.096 (0.120) 0.881 (0.398) 1.000 (0.167) 1.000 (0.167)
Gulf of Boothia (Taylor et al. 2008c) 1.648 (0.098) 0.000 (0) 0.194 (0.178) 0.467 (0.168) 0.965 (0.300)
Kane Basin (Taylor et al.2008a) 1.667 (0.083) 0.000 (0) 0.000 (0) 0.357 (0.731) 0.978 (0.085)
Lancaster Sound (Taylor et al. 2008b) 1.688 (0.012) 0.000 (0) 0.107 (0.050) 0.312 (0.210) 0.954 (0.083)
M’Clintock Channel (Taylor et al. 2006) 1.680 (0.147) 0.000 (0) 0.111 (0.101) 0.191 (0.289) 0.928 (0.334)
Northern Beaufort Sea (PBTC 2007) 1.756 (0.166) 0.118 (0.183) 0.283 (0.515) 0.883 (0.622) 0.883 (0.622)
Norwegian Bay (Taylor et al. 2008b) 1.714 (0.081) 0.000 (0) 0.000 (0) 0.000 (0) 0.689 (0.534)
Southern Beaufort Sea (Regehr et al. 2006)Table notea 1.750 (0.170) 0.000 (0) 0.470 (0.090) 0.470 (0.090) 0.470 (0.090)
Southern Hudson Bay (PBTC 2007)Table noteb 1.575 (0.116) 0.087 (0.202) 0.966 (0.821) 0.967 (0.022) 0.967 (0.022)
Viscount Melville (Taylor et al. 2002) 1.640 (0.125) 0.000 (0) 0.623 (0.414) 0.872 (0.712) 0.872 (0.712)
Western Hudson Bay (IUCN/SSC 2006 and PBTC 2007)Table notec 1.540 (0.110) 0.000 (0) 0.257 (0.442) 0.790 (0.180) 0.790 (0.180)

Females enter estrus in March, which lasts until June and peaks in late April and early May (Palmer et al. 1988; Amstrup 2003). Ovulation is induced by coitus (Wimsatt 1963; Ramsay and Dunbrack 1986), and implantation is delayed until October (Palmer et al. 1988). Pregnancy rates appear to vary markedly among subpopulations, with as few as 50% of adult females (>5 years) that are available to mate (i.e., having no cubs or cubs that are about to be weaned) producing cubs the following year (e.g., Kane Basin; Table 3) to as many as 100% (Baffin Bay; Table 3).

Pregnant females enter maternity dens in late October and the young, normally 1-2, are born between November and early January (Harington 1968; Derocher et al. 1992); however, according to Inuit traditional knowledge, the timing of parturition varies with latitude. Dens are generally excavated in snow (although dens in frozen earth and peat are common in the south [Clark et al. 1997]), and are then covered and closed by snowdrifts. They are frequently located on islands or land in close proximity to the coast and adjacent to areas with high seal densities in spring (Harington 1968; Brice-Bennett 1977, Stirling and Andriashek 1992; Messier et al. 1994; Kalxdorff 1997; Ferguson et al. 2000b; Van de Velde et al. 2003; Lewis et al. 2006), although in Ontario and Manitoba polar bears may den up to 120 km inland at traditional denning areas (Kolenosky and Prevett 1983; Ramsay and Stirling 1990; Lunn et al.2004; Richardson et al. 2005). Amstrup and Gardner (1994) observed that in the Beaufort Sea maternal dens on drifting pack ice were common, although this would be unusual for polar bears throughout much of the Canadian Arctic. All dens on sea ice observed by Messier et al. (1994) and Ferguson et al. (2000b) were classified as temporary shelter dens, rather than maternity dens. Fischbach et al. (2007) recently observed that in the Southern Beaufort Sea, the proportion of dens on the pack ice declined from 62% (1985–1994) to 37% (1998–2004) and that this change was related to changes and reductions in sea ice.

At birth, cubs weigh approximately 0.6 kg. They are nursed inside the den until sometime between the end of February and the middle of April, depending on latitude. By this time, cubs weigh 10–12 kg (Ramsay and Stirling 1988; Derocher and Stirling 1995a). As observed for brown bears (Ferguson and McLoughlin 2000), litter size varies little according to subpopulation (Table 3).

Lentfer et al. (1980) and Taylor et al. (1987) estimated an average interlitter interval of approximately 3.6 years. The exception is Western Hudson Bay where, in the early 1980s, up to 40% of females weaned their young at 1 year of age (Ramsay and Stirling 1988), although this proportion has declined since then (Derocher and Stirling 1995a).

Generation length in polar bears has been poorly studied, despite the variable being key to identifying categories of risk by bodies such as the IUCN/SSC and COSEWIC (i.e., likelihoods of decline over 3 generations). The IUCN/SSC Polar Bear Specialist Group (2006: p. 31) used 15 years as generation length: “calculated from the age of maturity (five years) plus half the length of the reproductive period in a complete life cycle (10 yrs; = 0.5 x 20 yrs).” COSEWIC identifies generation length as: “the average age of parents of a cohort (i.e. newborn individuals in the population).” Data on the average age of female polar bears with cubs-of-the-year in spring in a random sample of bears of all ages have seldom been reported. The paper of Regehr et al.(2006) allows us to compute this variable for bears of the Southern Beaufort Sea (from proportions presented in Table 3 of Regehr et al. [2006]). For the period 1967–1989, and conservatively assuming all bears in the age 20+ category were 25 years old, the mean age of females with newborns was 9.9 years. For 1990–2006 the average was 11.7 years. In Western Hudson Bay, the age-specific female mortality data of Regeher et al.(2007) suggest an average age of 12.7 years for females aged 5 years and older. This report will use 12 years as the generation time of polar bears.

Like other ursids, polar bears experience relatively high survival rates, and survival can generally be distinguished based on age or stage of life history. Generally, researchers assess survival rates separately for cubs-of-the-year (COYs), yearlings and subadults (ages 1–4), prime-age adults (ages 5–20), and senescent adults (ages 21+). Maximum age is often considered to be 30 years for bears in the wild, although lifespans longer than this are purported to be common in captivity. The general pattern is for COYs and yearlings to exhibit survival rates that are lower than subadults and prime adults, and senescent adults have lower survival rates than prime adults. Total survival rates (Table 4) are distinguished from natural survival rates (Table 5), which are computed by considering the fates of bears that die only of natural causes. Males often have lower total survival rates than females, due to purposeful sex-selectivity in the harvest and a greater propensity for males to become problem animals.

Table 4. Mean (SE in parentheses) of total (i.e., harvested) annual survival rates for age and sex classes of subpopulations of Canadian polar bears. No other subpopulations have reported total survival rates.
Subpopulation
(primary data source)
Males
Total Survival
0
Males
Total Survival
1
Males
Total Survival
2–4
Males
Total Survival
5–20
Males
Total Survival
>20
Females
Total Survival
0
Females
Total Survival
1
Females
Total Survival
2–4
Females
Total Survival
5–20
Females
Total Survival
>20
Baffin Bay
(Taylor et al. 2005)
0.538 (0.094) 0.879 (0.049) 0.879 (0.049) 0.923 (0.024) 0.874 (0.062) 0.600 (0.096) 0.901 (0.045) 0.901 (0.045) 0.940 (0.021) 0.913 (0.047)
Gulf of Boothia
(Taylor et al. 2008c)
0.817 (0.201) 0.875 (0.085) 0.875 (0.085) 0.935 (0.040) 0.935 (0.040) 0.817 (0.201) 0.875 (0.085) 0.875 (0.085) 0.935 (0.040) 0.935 (0.040)
Kane Basin
(Taylor et al. 2008a)
0.308 (0.172) 0.617 (0.180) 0.617 (0.180) 0.957 (0.046) 0.957 (0.046) 0.374 (0.180) 0.686 (0.157) 0.686 (0.157) 0.967 (0.043) 0.967 (0.043)
Lancaster Soundd
(Taylor et al. 2008b)
0.633 (0.123) 0.790 (0.073) 0.790 (0.073) 0.892 (0.030) 0.653 (0.085) 0.749 (0.105) 0.879 (0.050) 0.879 (0.050) 0.936 (0.019) 0.758 (0.054)
M’Clintock Channel
(Taylor et al. 2006a)
0.620 (0.15) 0.900 (0.04) 0.900 (0.04) 0.880 (0.04) 0.880 (0.04) 0.620 (0.15) 0.900 (0.04) 0.900 (0.04) 0.900 (0.04) 0.900 (0.04)
Northern Beaufort Sea
(Stirling et al. 2007)e
0.487 (0.173) 0.248 (0.124) 0.826 (0.073) 0.818 (0.071) 0.581 (0.104) 0.605 (0.170) 0.348 (0.147) 0.895 (0.046) 0.89 (0.044) 0.713 (0.079)
Norwegian Baya
(Taylor et al. 2008b)
0.633 (0.123) 0.790 (0.073) 0.790 (0.073) 0.892 (0.030) 0.653 (0.085) 0.749 (0.105) 0.879 (0.050) 0.879 (0.050) 0.936 (0.019) 0.758 (0.054)
Southern Beaufort Sea
(Regehr et al. 2006)
0.430 (0.110) 0.920 (0.040) 0.920 (0.040) 0.920 (0.040) 0.920 (0.040) 0.430 (0.110) 0.920 (0.040) 0.920 (0.040) 0.920 (0.040) 0.920 (0.040)
Southern Hudson Bay
(Obbard et al. 2007)f
0.492 (0.143) 0.485 (0.143) 0.812 (0.076) 0.811 (0.076) 0.293 (0.143) 0.645 (0.135) 0.640 (0.136) 0.893 (0.052) 0.892 (0.052) 0.444 (0.148)
Viscount Melville
(Taylor et al. 2002)
0.448 (0.216) 0.774 (0.081) 0.774 (0.081) 0.774 (0.081) 0.774 (0.081) 0.693 (0.183) 0.905 (0.026) 0.905 (0.026) 0.905 (0.026) 0.905 (0.026)
Western Hudson Bayg,h
(Regehr et al. 2007a)
0.620 (0.020) 0.620 (0.020) 0.810 (0.015)

0.720 (0.020)
0.900 (0.005) 0.750 (0.020)

0.650 (0.031)
0.700 (0.020) 0.700 (0.020) 0.860 (0.015)

0.780 (0.020)
0.930

(0.005)
0.810 (0.015)

0.720 (0.031)
Table 5. Mean (SE in parentheses) of natural (i.e., unharvested) annual survival rates for age and sex classes of subpopulations of Canadian polar bears. No other subpopulations have reported natural survival rates or rates that can be computed.
Subpopulation
(primary
data source)
Males
Natural Survival
0
Males
Natural Survival
1
Males
Natural Survival
2–4
Males
Natural Survival
5–20
Males
Natural Survival
>20
Females
Natural Survival
0
Females
Natural Survival
1
Females
Natural Survival
2–4
Females
Natural Survival
5–20
Females
Natural Survival
>20
Baffin Bay
(Taylor et al. 2005)
0.570 (0.094) 0.938 (0.045) 0.938 (0.045) 0.947 (0.022) 0.887 (0.060) 0.620 (0.095) 0.938 (0.042) 0.938 (0.042) 0.953 (0.020) 0.919 (0.050)
Gulf of Boothia
(Taylor et al. 2008c)
0.817 (0.201) 0.907 (0.084) 0.907 (0.084) 0.959 (0.039) 0.959 (0.039) 0.817 (0.201) 0.907 (0.084) 0.907 (0.084) 0.959 (0.039) 0.959 (0.039)
Kane Basin
(Taylor et al. 2008a)
0.345 (0.200) 0.663 (0.197) 0.663 (0.197) 0.997 (0.026) 0.997 (0.026) 0.410 (0.200) 0.756 (0.159) 0.756 (0.159) 0.997 (0.026) 0.997 (0.026)
Lancaster SoundTable notei
(Taylor et al. 2008b)
0.634 (0.123) 0.838 (0.075) 0.838 (0.075) 0.974 (0.030) 0.715 (0.095) 0.750 (0.104) 0.898 (0.005) 0.898 (0.005) 0.946 (0.018) 0.771 (0.054)
M’Clintock Channel
(Taylor et al. 2006a)
0.619 (0.151) 0.983 (0.034) 0.983 (0.034) 0.977 (0.033) 0.977 (0.033) 0.619 (0.151) 0.983 (0.034) 0.983 (0.034) 0.921 (0.046) 0.921 (0.046)
Northern Beaufort Sea
(Stirling et al. 2007)Table notej
0.489 (0.173) 0.928 (0.080) 0.906 (0.073) 0.940 (0.071) 0.859 (0.104) 0.607 (0.170) 0.931 (0.080) 0.956 (0.046) 0.929 (0.044) 0.730 (0.079)
Norwegian BayTable notei
(Taylor et al. 2008b)
0.634 (0.123) 0.838 (0.075) 0.838 (0.075) 0.974 (0.030) 0.715 (0.095) 0.750 (0.104) 0.898 (0.005) 0.898 (0.005) 0.946 (0.018) 0.771 (0.054)
Southern Beaufort SeaTable notek
(Regehr et al. 2006, 2007b)
0.430 (0.11) 0.930 (0.040) 0.930 (0.040) 0.930 (0.040) 0.930 (0.040) 0.430 (0.11) 0.930 (0.040) 0.930 (0.040) 0.930 (0.040) 0.930 (0.040)
Southern Hudson BayTable notel
(Obbard et al. 2007)
0.492 (0.143) 0.517 (0.143) 0.929 (0.076) 0.892 (0.076) 0.556 (0.143) 0.645 (0.135) 0.645 (0.136) 0.973 (0.052) 0.951 (0.052) 0.523 (0.148)
Viscount Melville
(Taylor et al. 2002)
0.448 (0.216) 0.924 (0.109) 0.924 (0.109) 0.924 (0.109) 0.924 (0.109) 0.693 (0.183) 0.957 (0.028) 0.957 (0.028) 0.957 (0.028) 0.957 (0.028)
Western Hudson BayTable notem
(Regehr et al. 2007a)
0.710 0.710 0.940
0.780
0.940 0.820
0.680
0.730 0.920 0.920
0.820
0.930 0.820
0.720

5.2 Predation

Polar bears have no natural predators. Intraspecific predation is, however, a potential limiting factor of population growth. The killing of cubs to bring females into estrus, or killing of cubs and adults for food, is not uncommon in Ursidae, including polar bears (Taylor et al. 1985; Derocher and Taylor 1994; Taylor 1994; Derocher and Wiig 1999; Dyck and Daley 2002). Intraspecific conflicts related to nutritional stress are expected to be higher as density (relative to the carrying capacity of the environment) increases. Hence, if climate change acts to reduce carrying capacity, we might expect increases in rates of intraspecific conflict where declines in population size lag behind changes in carrying capacity (e.g., Southern Beaufort Sea [Amstrup et al. 2006]). The potential for intraspecific predation to limit polar bears is discussed in more detail in Section 6.3.

5.3 Physiology

The most notable aspect of polar bear physiology, in the context of assigning status to the species, relates to the ability of polar bears to fast for long periods of time when forced on land during the ice-free season, without access to seals (50–60% of bears in Canada). While on land little food is available, and bears must rely on stored energy reserves until the sea ice forms again in late autumn (Ramsay and Hobson 1991; Derocher et al. 1993; Atkinson and Ramsay 1995). Pregnant females must also wait until young are born and old enough to be moved from the den before ending their fast; in doing so pregnant females may not eat for up to 8 months, while having to meet the energetic demands of gestation and lactation (Atkinson and Ramsay 1995). Adult polar bears lose approximately 1 kg of body mass per day during fasts (Derocher and Stirling 1995a; Polischuk et al. 2002), and pregnant females may lose as much as 43% of their body mass (Atkinson and Ramsay 1995). Because offspring body mass is closely tied to the amount of body fat carried by females (Atkinson and Ramsay 1995), reproductive success likely depends on how heavy females are when they begin, or more importantly end, periods of fasting.

As an apex predator in the arctic marine ecosystem, polar bears may be exposed to a number of environmental pollutants and contaminants that have the potential to affect survival and reproduction (Amstrup 2003). Important environmental contaminants and their potential limiting effects on polar bears are discussed in Section 6.3.

5.4 Home Ranges, Movements, and Dispersal

Polar bears travel over exceedingly large areas relative to other terrestrial mammals (Ferguson et al. 1999), and the only practical means by which to track their movements is via remote satellite telemetry (see Messier et al. 2001). Radios are generally fitted using collars only on adult females given practical difficulties in securely attaching transmitters to males (necks of males are often of wider circumference than their heads); hence, movement patterns of male polar bears are not well known. Female polar bears possess large annual home ranges, varying from 940 km² to 540,700 km² (x = 125,500 km², SD = 113,795, n = 93; Ferguson et al. 1999). Home ranges of polar bears vary with several factors, including local presence of attractants such as polynyas (Ferguson et al. 1999; Messier et al.2001). The ratio of land to sea within a given home range and seasonal variation in ice cover have been shown to explain up to 66% of the variation in home range size (Ferguson et al.1999). Bears using land during the ice-free season have larger home ranges than those with year-round access to ice, as do bears that possess home ranges with greater seasonal variation in type of ice cover (Ferguson et al. 1999).

Observations of movement patterns within home ranges reinforce the importance of sea ice to the ecology of polar bears. As expected from the size of home ranges, rates of movement are very high when compared to other terrestrial mammals, with most published, mean estimates of travel speeds on sea ice falling within the range of 0.5–2.1 km/h (Larsen et al. 1983; Durner and Amstrup 1995; Born et al. 1997; Amstrup et al. 2000; Ferguson et al. 2001). The highest activity is from May through June and July, depending on conditions of sea ice and coinciding with availability of newborn seal pups (Pasitschniak-Arts and Messier 1999; Amstrup 2003). Mauritzen et al. (2003) showed that movement rates of polar bears increased with decreasing thickness of sea ice. In the High Arctic, activity is lowest during winter, perhaps due to inclement weather, limited accessibility to seals, and energy conservation during the coldest months (Messier et al. 1992, 1994).

Movements of pregnant females cease after they enter maternity dens in late autumn (Section 5.1), but non-pregnant females and males will also use snow shelters for 0.5–4 months of the winter (Harington 1968) and fast in a manner that is physiologically similar to torpor during periods of food shortages (Watts and Hansen 1987). However, use of shelter dens varies with conditions of sea ice and latitude and is more common in the High Arctic (Ferguson et al. 2000b). In the southern Arctic, where sea ice melts, bears may be forced to spend up to several months on land while waiting for freeze-up. This phenomenon is most marked at the southern range of the polar bear in Canada, especially Hudson Bay and James Bay (Stirling et al. 1977; Derocher and Stirling 1990), eastern Baffin Island (Stirling et al.1980; Ferguson et al. 1997; Taylor et al.2005), and Davis Strait (M.K. Taylor, Department of Environment, Government of Nunavut, pers. obs.).Once forced on shore for summer, movements are considerably less than on sea ice and bears spend most of their time resting or, if female and pregnant, investigating areas of potential den sites (Ferguson et al. 1997, 1998; Lunn et al. 2004).

Dispersal in polar bears is poorly understood largely because subadult bears have rarely been tracked using radio-collars. Subadults, though marked when captured, are not usually collared as these bears can quickly outgrow fitted collars. Dispersal events have, however, been recorded using genetic analyses (Crompton 2004; Saunders 2005). Results from bears in the Gulf of Boothia and M’Clintock Channel (Saunders 2005), and Western Hudson Bay, Southern Hudson Bay, Foxe Basin, and Davis Strait (Crompton 2004) suggest that dispersing bears can and do traverse identified subpopulation boundaries. Dispersal across subpopulation boundaries--initially identified based on movements of marked and radio-collared adults (Taylor and Lee 1995; Bethke et al.1996)--may in part explain the lack of sharp genetic differences among subpopulations (Tables 1 and Table2).

5.5 Interspecific Interactions

Polar bears are obligate predators of ice-dependent seals, especially the ringed seal. Coevolution between ringed seals and polar bears and the potential for distributional changes in the occurrence of ringed seals (and other ice-dependent phocids) on polar bear distribution are discussed in Sections 3, section4, and section6.

5.6 Behavioural Adaptations

In addition to being physiologically adapted to environmental stochasticity and surviving long periods without food, polar bears exhibit behavioural adaptations that allow them to survive in extreme or variable environments. Participants of recent ATK studies in Gjoa Haven, Cambridge Bay and Taloyoak (Atatahak and Banci 2001; Keith et al. 2005) communicated that polar bears readily adapt their movements to environmental conditions and availability of prey species, but can be sensitive to human activity. However, polar bears are known to use non-natural sources of food (e.g., garbage) and may habituate to the presence of humans, even in the presence of disruptive activities (e.g., hazing) if food rewards can still be obtained. The curiosity of polar bears makes them particularly vulnerable to human-caused mortality in defence of life or property. Polar bears are also attracted to and may consume foreign substances (e.g., petroleum products or ethylene glycol [antifreeze]) that can be harmful or cause death (Stirling 1988b; Amstrup et al. 1989; Derocher and Stirling 1991). Inuit observations of polar bears eating plastic bags and engine oil apparently increased through the 1990s (McDonald et al. 1997), and Inuit observers of polar bears in the Baffin Bay area report an expansion in the types of foods eaten by bears in recent years (Dowsley 2005), including eggs of sea birds and Inuit meat caches. As described in Section 4.1, the diet of polar bears can extend to several species of mammals and birds, Inuit meat caches; and vegetation including berries; however, polar bears are best characterized as an obligate predator of seals using sea ice as a hunting platform.

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