Critical thermal limits often determine species distributions for diverse ectotherms and have become a useful tool for understanding past and predicting future range shifts in response to changing climates. Despite recently documented population declines and range shifts of bumblebees (genus Bombus), the few measurements of thermal tolerance available for the group have relied on disparate measurement approaches. We describe a novel stereotypical behavior expressed by bumblebee individuals

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... g entry into chill coma. This behavioral indicator of minimum critical temperature (CT min ) occurred at ambient temperatures of 3-5°C (approximately 7-9°C core temperatures) and was accompanied by a pronounced CO 2 pulse, indicative of loss of spiracle function. Maximum critical temperature (CT max ) was indicated by the onset of muscular spasms prior to entering an unresponsive state and occurred at ambient temperatures of approximately 52-55°C (42-44°C core temperatures). Measurements of CT min and CT max were largely unaffected by acclimation, age or feeding status, but faster ramping rates significantly increased CT max and decreased CT min . This high-throughput approach allows rapid measurement of critical thermal limits for large numbers of individuals, facilitating large-scale comparisons among bumblebee populations and speciesa key step in determining current and future effects of climate on these critical pollinators.