Males reach sexual maturity at 3 years in Alaska, at age 4 in many parts of the Canadian Arctic, and at age 5 in the Beaufort Sea, but these younger males may not actually breed due to competition with prime males (>10 years old) (Stirling et al. 1976, 1977b, 1980, 1984 in Amstrup 2003); few males breed successfully until about 6 years old. Females generally are sexually mature in 3-6 years. Breeding occurs from late March to mid-May, with implantation delayed until autumn; gestation period is 195-265 days (DeMaster and Stirling 1981). Two cubs (sometimes 1 or 3) are born December-January (mid-November to mid-December along western Hudson Bay). Newborn cubs are totally helpless, among the least developed of placental mammal young, weighing 600-700 grams. Emergence from maternity dens occurs toward the end of March in Hudson and James bays, in April in Alaska. Young are weaned at approximately 24-28 months. Reproductive rate is low; adult females produce young every 3 years in most areas, in alternate years at lower Hudson Bay (Stirling 1991). Along western Hudson Bay, there was a high degree of reproductive failure; 33% of females classified as pregnant were not accompanied by cubs the following year; annual pregnancy rate of solitary females was 82-100%; pregnancy rates drop off for most females of 21 years or older, but some older females retain reproductive competency through life (Derocher et al. 1992). Generation time is about 15 years (see USFWS 2008).
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