Reproduction in the Female Brown Bear

Reproduction in the female brown bear (Ursus arctos) was studied by gross examination of 96 reproductive tracts, including 16 from known-age animals, obtained from Kodiak Island and the Alaska Peninsula during 1959-1965. Additional data were obtained incidental to livetrapping 165 bears and from aerial surveys made within the same period. Gross features of reproductive tracts indicated females attained sexual maturity when 3 to 6 years of age and commonly during the fourth year of life. The breeding season extended from May through mid-July as evidenced by condition of genitals, follicle maturation, and observations of actual breedings. The gestation period was estimated to be approximately 245 days. Corpora lutea counts disclosed a mean of 2.10 ovulations. Placental scar counts disclosed a 2.53 mean conception rate. Litter size averaged 2.23 cubs. Young bears are weaned prior to or during the second winter of denning. Lactation probably inhibited reproduction during first and second summers following parturition. The hypothesis is advanced that brown bears normally breed at 3-year intervals as opposed to the common belief that young are produced on alternate years.