Metabolic stability underpins thermal tolerance in benthic ecotypes of threespine stickleback (Gasterostues aculeatus)

Understanding how organisms adapt to divergent thermal conditions can inform efforts to preserve biodiversity in a warming world. Change in aerobic scope (AS)-total energy available to perform tasks outside of homeostasis-is an ecologically relevant predictor of performance across a range of temperatures. Mathematical models suggest that the shape of AS-temperature curves should evolve depending on environmental heterogeneity, but few studies have tested whether this occurs at the intraspecific level. We examined AS-temperature associations across two ecotypes of threespine stickleback fish (Gasterosteus aculeatus), including both short- and long-term heat treatments. We predicted that fish from shallow, thermally dynamic lakes would display greater AS stability at high temperatures than residents of deep, thermally stable lakes. We also tested whether heat and AS are correlated with variation in a fibrotic immune response after exposure to the cestode Schistocephalus solidus. We found few heat-associated differences in condition or immunity, but fibrosis decreased AS. Additionally, limnetic AS declined with heat and this shift was correlated with mortality. In contrast, benthic ecotypes displayed remarkable stability in AS and survival across temperatures. Connecting intraspecific variation in AS with local thermal environments is a promising avenue for estimating, and potentially fostering, population resilience to warming climates.