Derocher and Stirling (1994) noted that litter size varied with maternal age, increasing until age 14 years, after which it declined. Heavy hunting reduces numbers of prime-age and older polar bears of both sexes (Amstrup et al. 1986). If such changes occurred without density-dependent increases in reproductive performance for young animals, overharvesting could have the additional population-depressing effect of actually reducing reproduction at low population densities rather than increasing it. Polar bears in the Hudson Bay area were heavily harvested into the 1970s, but numbers there appear to have increased since then (Prevett and Kolenosky 1982; Derocher et al. 1997). Litter size, litter production rate, and other reproductive factors can be expected to change with population size relative to carrying capacity. It also changes in a response to hunting pressure and other population perturbations. Hence, comparisons among populations or within populations over time must take into account the status of the population relative to natural and anthropogenic features of the environment.

In most parts of the Arctic, female polar bears cannot complete a reproductive cycle more frequently than every 3 years. The interbirth interval is determined by the length of time cubs are attended by their mothers, which most commonly is 2.3 years (Stirling et al. 1976, 1980; Lentfer et al. 1980; Amstrup et al. 1986; Amstrup and Durner 1995) (Fig. 27.9). LØnØ (1970) concluded that in the Svalbard area, most cubs were weaned by about 17 months of age. Likewise, Ramsay and Stirling (1988) reported that during the 1970s and early 1980s, a significant proportion of female polar bears in the Hudson Bay region weaned their cubs at about 1.3 years of age. After weaning her cubs in the spring of their second year (at age 1.3 years), a female bear could breed again that same spring and achieve a 2-year reproductive interval.

The historically shorter reproductive interval of polar bears living in Hudson Bay (Stirling et al. 1977b) meant that they were more prolific than most other populations of polar bears. Captures of many hundreds of female polar bears and their young in Alaska, Canada, and Svalbard have suggested geographic differences in litter size, litter production, onset of maturity, and reproductive interval. For example, mean litter sizes of cubs and yearlings in Alaska were 1.63 and 1.49, respectively (Amstrup 1995). In Svalbard, these values were 1.81 and 1.32, respectively, whereas litter sizes of polar bears in Hudson Bay during the early 1980s were 1.9 and 1.7 for cubs and yearlings, respectively (Ramsay and Stirling 1988, Derocher and Stirling 1992). Annual litter production rates as high as 0.45 litters/female have been reported for polar bears in the Hudson Bay area (Derocher and Stirling 1992). Nearly half of the females in that population were annually producing a litter of cubs at that time. By comparison, only one fourth of the female polar bears in the Beaufort Sea produce a litter of cubs each year (a litter production rate of 0.25) (Amstrup 1995). That is, in Hudson Bay, each female had a litter nearly every other year, but in the Beaufort Sea, each female produced a litter only every fourth year. Because polar bears in Hudson Bay also produced larger litter sizes, these differences in litter production rates translated into a much higher overall reproductive rate there than in the Beaufort Sea. Female polar bears in the Beaufort Sea produced only 0.40 cub/year, whereas in the Hudson Bay area they produced up to 0.90 cub/year at the time those studies were conducted (Derocher and Stirling 1992). Reproductive rates in most other areas appear to be more similar to those in the Beaufort Sea than in Hudson Bay.