The diet of mammalian herbivores poses a number of challenges unique to having an herbivorous diet, and at the same time thermoregulating. I will present mechanisms by which thermoregulation is influenced by herbivorous diets, especially through the action of plant secondary compounds. Using the Australian marsupial possum-eucalypt study system, I will provide evidence for temperature dependent toxicity in wild folivores consuming plants rich in plant secondary compounds. I will also show that plant secondary compounds can uncouple mitochondrial oxidative phosphorylation interfering with energy metabolism and leading to excess body heat production. Furthermore, since a mammal’s ability to thermoregulate depends on sensing the thermal environment and responding appropriately, the binding of plant secondary metabolites to thermoreceptors can disrupt this process. Balancing of nutrients, and consuming a high fiber diet also interacts with thermoregulation in mammals consuming plants. I will discuss these phenomena in mammalian herbivores in general and how the interaction between plant based diets and thermoregulation means nutritional ecology and patterns of herbivory will likely change in a dynamic way as our climate changes in the future.