Polar Bears In Depth
Survival
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The very low reproductive rate of polar bears means that there must be a high rate of survival to maintain population levels. In fact, polar bears "defer" reproduction in favor of survival when foraging conditions are difficult (Derocher et al. 1992). A complete reproductive effort is energetically expensive for polar bears. So, when energetically stressed, female polar bears will forgo reproduction rather than increase risks by incurring the energetic costs of the reproductive process. The reproductive cycle lends itself to convenient early termination if that is appropriate (Ramsay and Dunbrack 1986; Derocher and Stirling 1992). Many radio-collared female polar bears in the Beaufort Sea region entered dens and then abandoned them early without cubs (Amstrup and Gardner 1994). Others lost cubs shortly after emerging from their den and bred again that same spring. Bears leaving dens early may have resorbed their fetuses or may have experienced a pseudopregnancy (Derocher et al. 1992). In any event, they did not complete a full reproductive cycle.
Breeding takes place in the early spring, long before the female can assess whether she will secure sufficient resources to bring her pregnancy to fruition. After fertilization, if she has been able to secure sufficient reserves, birth and rearing can follow pregnancy with some reasonable probability of success. Polar bears, however, also are equipped to abandon a reproductive effort if reserves are insufficient. Because implantation is delayed many months (Wimsatt 1963), and because neonates are so undeveloped (Blix and Lentfer 1979; Ramsay and Dunbrack 1986), early stages of reproduction are relatively inexpensive. Termination of the reproductive process, through abortion or resorbtion of the fetus or failure to nurse after birth, costs a female relatively little (Derocher et al. 1992). The biggest maternal investment begins with postpartum lactation (Ramsay and Dunbrack 1986). Even after emergence from the den, however, it is not unusual for females in poor condition to lose their cubs (Amstrup and Gardner 1994; Amstrup and Durner 1995). Polar bears that terminate a pregnancy and leave their dens early or lose their cubs in early spring usually breed again, preparing them for an opportunity to successfully rear cubs the following year if conditions improve.
In the Hudson Bay region during the 1980s, the survival rate of more than 200 cubs from spring through the ice-free period of autumn was 44% (Derocher and Stirling 1996). Although less mortality was thought to occur after the ice returned in autumn, first-year survival clearly was lower than the 48% reported by Larsen (1985). The body mass of cubs was a significant determinant of survival during early life that included the ice-free period of food deprivation. The mass of cubs, of course, is at least partly dependent on the mass of their mother. Survival of Hudson Bay cubs (N > 400) from their first to their second autumn was 35% (Derocher and Stirling 1996). Annual survival of yearlings ranged from 43% to 53%. The survival estimates Derocher and Stirling (1996) calculated for cubs >1 year old were derived from bears that were actually captured.
Breeding takes place in the early spring, long before the female can assess whether she will secure sufficient resources to bring her pregnancy to fruition. After fertilization, if she has been able to secure sufficient reserves, birth and rearing can follow pregnancy with some reasonable probability of success. Polar bears, however, also are equipped to abandon a reproductive effort if reserves are insufficient. Because implantation is delayed many months (Wimsatt 1963), and because neonates are so undeveloped (Blix and Lentfer 1979; Ramsay and Dunbrack 1986), early stages of reproduction are relatively inexpensive. Termination of the reproductive process, through abortion or resorbtion of the fetus or failure to nurse after birth, costs a female relatively little (Derocher et al. 1992). The biggest maternal investment begins with postpartum lactation (Ramsay and Dunbrack 1986). Even after emergence from the den, however, it is not unusual for females in poor condition to lose their cubs (Amstrup and Gardner 1994; Amstrup and Durner 1995). Polar bears that terminate a pregnancy and leave their dens early or lose their cubs in early spring usually breed again, preparing them for an opportunity to successfully rear cubs the following year if conditions improve.
In the Hudson Bay region during the 1980s, the survival rate of more than 200 cubs from spring through the ice-free period of autumn was 44% (Derocher and Stirling 1996). Although less mortality was thought to occur after the ice returned in autumn, first-year survival clearly was lower than the 48% reported by Larsen (1985). The body mass of cubs was a significant determinant of survival during early life that included the ice-free period of food deprivation. The mass of cubs, of course, is at least partly dependent on the mass of their mother. Survival of Hudson Bay cubs (N > 400) from their first to their second autumn was 35% (Derocher and Stirling 1996). Annual survival of yearlings ranged from 43% to 53%. The survival estimates Derocher and Stirling (1996) calculated for cubs >1 year old were derived from bears that were actually captured.