All mammals are endothermic and capable of maintaining stable body temperatures. However, because of the high energetic cost of defending high body temperatures, many mammals vary their body temperature in response to changes in ambient temperature to save energy. The breadth and limits of the thermoneutral zone, the range of environmental temperatures where minimal energy is required to thermoregulate, vary among species. Early studies of the thermoneutral zone focused predominantly on species found in the northern hemisphere and relatively few physiological studies have been conducted in the tropics, leading to biased generalizations of thermoregulation. Contrary to holarctic species, tropical mammals evolved within a narrow range of temperatures, potentially leading to alternative thermoregulatory phenotypes. By fluctuating body temperature with changing ambient temperatures, species can save energy in response to changing weather conditions. We hypothesize that by increasing their flexibility of body temperature regulation, tropical species can effectively expand their thermoneutral zone, resulting in greater thermolability and thus a greater range of inhabitable temperatures. To test this and other hypotheses on the evolution of mammalian thermoregulation, we are assembling a high-quality database containing thermoregulatory data from hundreds of mammals, which can be used to test predictions of different species’ reactions to climate change. The dataset contains species’ body temperature in relation to the upper and lower limits of the thermoneutral zone and metabolic rates at those limits. Our dataset quantifying the upper and lower limits to the thermoneutral zone in species across latitudes allows us to better understand the evolution of mammalian thermoregulation.