Climate change, age acceleration, and the erosion of fitness in polar bears

Climate change is increasingly disrupting evolved life history strategies and decreasing population viability in wild species1. The magnitude and pace at which environments will change mean the persistence of wild populations will depend substantially on their ability to adapt genetically. However, we know very little about the capacity for evolutionary change in response to climate warming. We mapped the effects of climate change, beginning with the decline of cellular function through to the erosion of fitness and adaptive potential in an intensively studied polar bear (Ursus maritimus) population in western Hudson Bay, Canada. Using estimates of epigenetic age acceleration, an indicator of declining cellular function associated with exposure to stress, we found that polar bears aged approximately one year faster, on average, for each degree Celsius temperature increase they experienced. Declining cellular function should reduce fitness and counter adaptive evolution in rapidly changing environments. Individuals who reproduced early had higher lifetime reproductive success; however, this was before the onset of rapid warming. Fitness benefits associated with early reproduction declined with warming, and today, bears have similar lifetime reproductive success regardless of when they first reproduce. Finally, using a large pedigree, we found no evidence for genetic variation associated with reproductive success in this population-the population is not evolving in response to the changing environment. The physiological costs of climate change accumulate across lifetimes to degrade cellular function and, ultimately, adaptive capacity. These findings warn that adaptive responses to warming could be the exception rather than the rule. ### Competing Interest Statement The authors have declared no competing interest.