Climate change has the potential to alter entire ecological systems, including species’ range distributions. Both species of flying squirrel (Glaucomys volans and G. sabrinus) in North America have undergone dramatic range shifts northward over the past few decades, with rising temperatures being the suspected driver. The relationship between animals and their thermal environment has been gaining attention in the fields of ecology and physiology, and while other studies have focused on warming winter climates, we examine the rising summer temperatures as a driver of observed range shifts. We focused on the northern flying squirrel (Glaucomys sabrinus) to determine the effect of high temperatures on thermoregulation and energy use. We measured resting metabolic rate and subcutaneous body temperature of northern flying squirrels across a range of ambient temperatures using flow-through respirometry. We also measured free-ranging core body temperature of the species. The use of models that combine biophysical principles and climate data will also help to test predictions a way to test predictions of geographical patterns derived from biophysical mechanisms. This research will help us to understand the relationships between activity patterns and vulnerability to climate change, and will help to further our understanding of global patterns in mammalian thermoregulatory characteristics.