Denning Phenology Mediates Sea-Ice Loss Impacts on Early Reproductive Success in Polar Bears

Climate change is disrupting myriad biological processes, including the ability of species to synchronize life-history events with optimal environmental conditions, with potential fitness impacts. Changes in migratory and breeding phenology and their consequences on fitness have been well studied in birds and some mammals. By contrast, changes in the phenology of overwinter sheltering and their influence on fitness have rarely been investigated. Pregnant female polar bears (Ursus maritimus) spend the winter fasting in a maternity den where they give birth and nurse their cubs until emergence in early spring. To sustain their fast, females rely on energy stores built primarily through consumption of ice-associated seals. Here, we assess the impact of sea-ice loss driven by climate change on early reproductive success acting through disrupted denning phenology and other unspecified factors. To do so, we develop a Bayesian hierarchical model combining capture-recapture and path modeling. We apply it to 38 years of capture-recapture data along with biologging data on denning status and phenology, while accounting for imperfect detection and uncertainty in individual denning schedules. We found that low sea-ice availability led to earlier den emergence, which in turn reduced early litter survival and possibly litter size. By contrast, there was little evidence that sea-ice availability affected early reproductive success through pathways independent of denning phenology (e.g., litter mass). Considering both direct and indirect effects, low sea-ice availability reduced early litter survival but not the size of surviving litters. Given the strong link between sea-ice conditions and body condition in female polar bears, we conclude that sea-ice loss exerts an energetic constraint on denning females, forcing them to depart from dens early, which jeopardizes their cubs, ultimately reducing maternal fitness. Similar impacts of climate change may occur in other species where phenology is determined by body condition.