Reproductive trends of captive polar bears in North American zoos: a historical analysis

Despite the fact that nearly all captive polar bears ( Ursus maritimus ) are recommended for breeding, very few cubs are born each year and the rate of neonatal mortality is high. Animal caretakers tend to rely on anecdotal reports regarding reproductive events, such as timing of parturition, litter size and cub survival. To objectively document trends in polar bear reproduction during their tenure in North American zoos, this analysis utilised 99 years of records in the Polar Bear Studbook to characterise patterns in reproduction and cub survival. Factors evaluated included latitude, year of birth, parental demographics (age, origin (captive-born or wild-caught) and litter size), sex, survival, litter size and litter order. Between 1912 and 2010, 697 individuals (456 litters) were born at latitudes ranging from 25.90 to 52.94 oN. The average number of litters produced per year was 4.60 ± 0.51 with a range of zero to 18. The polar bear birth season lasted 106 days with mean and median birth dates of 29 November. Litter size was unaffected by any of the variables analysed: 52.7% of litters were singletons, 44.9% were twins and 2.4% were triplets. Older sires produced a higher proportion of male offspring than younger sires (P < 0.05). More than half of all individuals died prior to 30 days of age and 30.4% reached adulthood (four years). Cubs of captive-born parents lived longer than those of wild-caught parents (P < 0.05). Individuals born in litters of multiples were more likely to die as neonates than those born as singletons (P < 0.01) and individuals born to multiparous dams lived longer than those born to primiparous dams (P < 0.02). This study represents the largest analysis of captive polar bear reproduction conducted to date and may serve as a reference for individuals involved in the management and care of captive polar bears.