Warming limits energetic investment in feeding and post-feeding metabolism in bumblebees

Environmental warming is generally expected to increase metabolic demand in ectotherms. However, facultatively endothermic insects such as bumblebees actively regulate body temperature and may instead reduce thermogenic investment under warm conditions to avoid overheating, potentially altering physiological performance and responses to climate change. We combined flow-through respirometry and infrared thermography to test how elevated ambient temperature (25 vs 35°C) affects feeding energetics and postprandial metabolism in the bumblebee Bees maintained substantially lower thoracic temperature excess at 35°C than bees at 25°C, both before and during feeding. Feeding metabolic rate was also substantially lower at 35°C and was strongly positively associated with thoracic temperature excess, indicating that variation in feeding energetics was primarily explained by thermoregulatory state rather than ambient temperature alone. Elevated temperature substantially reduced both the probability and energetic magnitude of SDA responses, including total SDA expenditure, early-phase postprandial metabolism, and peak metabolic amplitude. In contrast, SDA duration and time to peak response showed comparatively weak temperature dependence. This pattern suggests that elevated temperature constrained postprandial metabolic expenditure, while having comparatively little effect on their temporal organisation. Our results demonstrate that warming can suppress rather than elevate energetic expenditure in facultatively endothermic pollinators by limiting thermogenic investment and postprandial metabolic responses. These findings extend current understanding of insect thermal physiology beyond locomotion and flight and suggest that high ambient temperatures may constrain the energetic flexibility underpinning foraging performance in bumblebees under climate warming.