Pregnancy rates and serum progesterone levels of polar bears in western Hudson Bay

Abstract

Blood samples were collected from 354 female polar bears (Ursus maritimus) on 476 occasions between 1982 and 1990. Serum progesterone concentrations were determined to evaluate reproductive status. Progesterone levels greater than 2.5 ng/mL were considered indicative of pregnancy in autumn. Of 85 females classified as pregnant that were seen the following year, 67% were with cubs and 33% were without cubs. Possible explanations for the large proportion of females with apparent reproductive failure include neonatal mortality, failure to implant, or abortion. The minimum weight of a confirmed pregnant female in autumn was 189 kg, but approximately 10% of the suspected pregnant females were below this and may have had insufficient fat stores to fast through the winter and produce cubs. Based on progesterone levels, implantation appeared to occur between mid-September and mid-October, likely followed by parturition from mid-November to mid-December. The annual pregnancy rate of solitary females, as determined by progesterone concentrations, varied between 82.4 and 100%, but the differences were not significant. Females bred for the first time between 3 and 5 years of age, at an average of 4.1 years. The age-specific pregnancy rates of 3- to 4-year-old females and females 21 years and older were lower than those of other age-classes. However, some older females appear to retain reproductive competency throughout their life.