Polar Bears In Depth
Reproduction
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FIGURE 27.9. The 3-year reproductive cycle typical of polar bears throughout most of their range. Exact timing of birth, shown here as 1 January, is not well known and may vary geographically from November to January. Within geographic regions, the timing of birth also may depend on the condition of the female on den entry. SOURCE: Modified from LØnØ (1970). Click image to enlarge.
In assessing reproductive intervals, it is critical to confirm weaning, as opposed to mortality of cubs. Many polar bear cubs die in their first year of life (Amstrup and Durner 1995). Those females can breed again in the year of the loss (if it occurs early enough in the spring) or the next year. The breeding frequency, by itself, might suggest a short reproductive interval when it is actually prolonged by poor cub survival. In addition to documenting that tagged females were no longer accompanied by yearling cubs in the spring, Ramsay and Stirling (1988) also captured many weaned yearlings in the autumn of their second year (approximately 1.8 years of age), confirming that many females in the Hudson Bay region actually did have a 2-year reproductive interval.
LØnØ (1970), Stirling et al. (1977b), and Ramsay and Stirling (1988) reported on populations that may have been well below carrying capacity due to unregulated hunting (Stirling et al. 1977b; Larsen 1986; Derocher and Stirling 1992, 1995a). Likewise, breeding intervals in the Hudson Bay area have increased, possibly in response to increased relative density of bears in the area (Derocher and Stirling 1992, 1995b). Annual litter production rate in the Hudson Bay region declined from 0.45 litter/female in the period from 19651979 to 0.35 during 1985 1990 (Derocher and Stirling 1992). A higher proportion produced cubs every 3 years in the latter period. The inverse of the litter production rate is the interbirth interval. That increased from 2.22 years in 1965 1979 to 2.86 years in 19851990. Simultaneously, cub mortality from spring to autumn was significantly higher in the latter period (Derocher and Stirling 1992). The proximate factor associated with all of these trends was the declining weight of adult females during this 25-year period (Derocher and Stirling 1992). Age of first successful reproductive effort increased, although pregnancy rates did not change noticeably. An increasing age of maturation may indicate that a population is approaching carrying capacity. Age of maturation in mammals is often associated with attainment of a threshold body mass (Sadleir 1969) which could be more difficult to attain as competition for resources increases.
A delay in reaching that threshold mass may signal densitydependent influences on the population. Such influences, however, also could result from environmental changes that reduce carrying capacity rather than from increases in polar bear numbers. The documented declines in body weights of females, declines in numbers of independent yearlings, and protracted reproductive intervals appear to be closely related to earlier deterioration of the sea-ice of Hudson Bay (Stirling et al. 1999). The sea-ice extent in the Arctic has been declining throughout the past two decades (Gloersen and Campbell 1991; Vinnikov et al. 1999). Declining Arctic sea-ice cover by itself is difficult to link with polar bear reproductive performance. The timing of melt of the sea-ice in Hudson Bay, however, is more easily connected. Polar bears there, especially pregnant females, depend heavily on the spring and early summer foraging for seals to carry them through the ice-free period (late summer to autumn). Pregnant females, unlike other polar bears in Hudson Bay, remain ashore in autumn when ice returns, and may be food deprived for up to 8 months. Those females must secure sufficient fat stores during the spring and summer to see them through that long period of food deprivation (Stirling 1977; Derocher and Stirling 1992). The mean date of sea-ice break-up in the late 1990s was >2 weeks earlier than it was in the 1970s and early 1980s (Stirling et al. 1999). Earlier break-up and the shortened foraging period accompanying it may mean a significant reduction in the fat stores female polar bears can accumulate before denning. This hypothesis is strengthened by the observation of a transient increase in condition of females coming ashore during the early 1990s when cooler than normal temperatures resulted in later break-ups (Stirling et al. 1999).
Evidence of the critical link between availability of seal prey and reproduction in polar bears is also available in more northerly parts of the range. Weights of females and their reproductive output in the Beaufort Sea decreased markedly in the mid-1970s following a decline in ringed and bearded seal populations (Stirling et al. 1976, 1977b; Kingsley 1979; DeMaster et al. 1980; Stirling et al. 1982; Amstrup et al. 1986). The strength and longevity of declines in reproductive parameters varied both geographically and temporally with the severity of ice conditions that reduced numbers and productivity of seals (Amstrup et al. 1986).
LØnØ (1970), Stirling et al. (1977b), and Ramsay and Stirling (1988) reported on populations that may have been well below carrying capacity due to unregulated hunting (Stirling et al. 1977b; Larsen 1986; Derocher and Stirling 1992, 1995a). Likewise, breeding intervals in the Hudson Bay area have increased, possibly in response to increased relative density of bears in the area (Derocher and Stirling 1992, 1995b). Annual litter production rate in the Hudson Bay region declined from 0.45 litter/female in the period from 19651979 to 0.35 during 1985 1990 (Derocher and Stirling 1992). A higher proportion produced cubs every 3 years in the latter period. The inverse of the litter production rate is the interbirth interval. That increased from 2.22 years in 1965 1979 to 2.86 years in 19851990. Simultaneously, cub mortality from spring to autumn was significantly higher in the latter period (Derocher and Stirling 1992). The proximate factor associated with all of these trends was the declining weight of adult females during this 25-year period (Derocher and Stirling 1992). Age of first successful reproductive effort increased, although pregnancy rates did not change noticeably. An increasing age of maturation may indicate that a population is approaching carrying capacity. Age of maturation in mammals is often associated with attainment of a threshold body mass (Sadleir 1969) which could be more difficult to attain as competition for resources increases.
A delay in reaching that threshold mass may signal densitydependent influences on the population. Such influences, however, also could result from environmental changes that reduce carrying capacity rather than from increases in polar bear numbers. The documented declines in body weights of females, declines in numbers of independent yearlings, and protracted reproductive intervals appear to be closely related to earlier deterioration of the sea-ice of Hudson Bay (Stirling et al. 1999). The sea-ice extent in the Arctic has been declining throughout the past two decades (Gloersen and Campbell 1991; Vinnikov et al. 1999). Declining Arctic sea-ice cover by itself is difficult to link with polar bear reproductive performance. The timing of melt of the sea-ice in Hudson Bay, however, is more easily connected. Polar bears there, especially pregnant females, depend heavily on the spring and early summer foraging for seals to carry them through the ice-free period (late summer to autumn). Pregnant females, unlike other polar bears in Hudson Bay, remain ashore in autumn when ice returns, and may be food deprived for up to 8 months. Those females must secure sufficient fat stores during the spring and summer to see them through that long period of food deprivation (Stirling 1977; Derocher and Stirling 1992). The mean date of sea-ice break-up in the late 1990s was >2 weeks earlier than it was in the 1970s and early 1980s (Stirling et al. 1999). Earlier break-up and the shortened foraging period accompanying it may mean a significant reduction in the fat stores female polar bears can accumulate before denning. This hypothesis is strengthened by the observation of a transient increase in condition of females coming ashore during the early 1990s when cooler than normal temperatures resulted in later break-ups (Stirling et al. 1999).
Evidence of the critical link between availability of seal prey and reproduction in polar bears is also available in more northerly parts of the range. Weights of females and their reproductive output in the Beaufort Sea decreased markedly in the mid-1970s following a decline in ringed and bearded seal populations (Stirling et al. 1976, 1977b; Kingsley 1979; DeMaster et al. 1980; Stirling et al. 1982; Amstrup et al. 1986). The strength and longevity of declines in reproductive parameters varied both geographically and temporally with the severity of ice conditions that reduced numbers and productivity of seals (Amstrup et al. 1986).
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