Marine heatwaves suppress nutrient utilization and reproductive flexibility in humpback whales.

Marine heatwaves (MHWs) are intensifying with climate change, raising questions about how extreme thermal events alter the physiology of large migratory mammals. We reconstructed eight years of continuous metabolic histories from baleen of three female humpback whales using compound-specific amino acid isotope analyses integrated with bioenergetic models. These records reveal pronounced shifts in nutrient allocation and metabolic indicators during the 2014-2016 MHW, with model outputs predicting reduced field metabolic rates marked by suppressed protein turnover and diminished lipid use. In contrast, non-MHW periods showed more efficient nutrient routing that supported elevated metabolic demands. Pregnant females were predicted to exhibit constrained metabolic flexibility during MHWs, and isotopic patterns following the event suggest energetic limitations consistent with unsuccessful early pregnancies or pseudopregnancies. These results illustrate how climate-driven extremes and reproductive demands interact to shape metabolic strategies in humpback whales, underscoring vulnerabilities in capital breeding species as MHWs become more frequent.