Pervasive sublethal impacts of heat exposure revealed by telomere dynamics.

Predicting species responses to anthropogenic environmental change remains one of the most pressing and complex challenges in contemporary biology. While mass mortality events and localized extinctions have been linked to global warming, the more elusive and probably common, effects of sublethal heat are less well understood. In a cooperatively breeding bird from the Australian tropical savannah, we identified that hotter conditions accelerated telomere attrition through two critical life-history stages: juveniles transitioning to nutritional independence and adults transitioning into a reproductive phase. In both stages, there was no capacity for environmental or social conditions to mitigate heat-related telomere attrition, except in adults inhabiting high-density vegetation. These findings demonstrate that sublethal heat effects can have pervasive cryptic consequences for individual somatic state, thereby reducing fitness. More generally, our findings, although probably widespread and concerning for population persistence, also highlight the importance of preserving climate refugia in conservation strategies amid rising sublethal temperatures.