Temporal dynamics of polar bear (Ursus maritimus) pregnancy rates in western Hudson Bay: influence of mass, age and timing of first breeding

Reproduction is the most energetically costly undertaking for female mammals and for capital breeders. Understanding factors that influence individual body condition and reproductive success is essential to understanding population demography. We investigated long-term trends in pregnancy rates to assess the impacts of individual and environmental factors on polar bear reproduction. Pregnancy status was determined from serum progesterone levels in blood collected from free-ranging polar bears captured on shore in late summer to early autumn in western Hudson Bay, Canada. We analysed 541 blood samples for progesterone level from 441 individuals from 1991 to 2021 and compared to data from 1982 to 1990 (354 individuals from 476 occasions). We used a generalized linear model to investigate individual and environmental factors that could influence pregnancy rates. The percent of solitary females that were pregnant declined significantly over time and between time periods from 85% in 1982–90 to 73% in 1991–2021. Interannual variation in pregnancy was high, ranging from 46 to 100%. Pregnancy rates were influenced by mass and age, with higher pregnancy rates for heavier females and those >4 and <24 years old. The percentage of pregnant 4-year-old females declined from 82% in 1982–90 to 55% in 1991–2021. The mass of pregnant females declined over time and the lightest pregnant female known to have produced cubs weighed 196 kg in the autumn. We suggest further research is needed to understand mechanisms resulting in pregnancy rate variation, which may be related to previous reproductive status and recent litter loss.