The nutritional decisions of herbivores are inextricably tied to the thermal environment they experience. This pertains not just to the total energy eaten to meet requirements, but also to the macronutrients from which they choose to get that energy, and to the plant secondary metabolites (PSMs) that complicate the decision. In this talk I will discuss and present results showing that the relationship between thermal physiology and nutritional ecology runs deeper than simply adjusting food intake to meet changes in metabolic rate. In particular I will focus on whether temperature-dependent toxicity applies to marsupial folivores ingesting diets with PSMs, resulting in different intake patterns at different temperatures, and the physiological basis for these differences. Following this I demonstrate how using the geometric framework of nutrition we can reveal influences of ambient temperature on the mixing of macronutrients to minimise heat generation. I then investigate whether PSMs found in Eucalyptus leaves cause mitochondrial uncoupling leading to greater metabolic heat production and hence exacerbate the heat dissipation problem at warm temperatures. Taken together ambient temperature can have considerable impact on nutrition, and warming ambient temperatures due to climate change are likely to result in new challenges for marsupial folivores, as well as other herbivore species ingesting PSM rich diets.