Herbivory Imposes Constraints on Voluntary Hypothermia in Lizards

Animals that are unable to meet their metabolic needs should adopt compensatory behaviors to mitigate their energetic deficits. Voluntary hypothermia is one such energy-saving strategy. Physiological constraints associated with the digestion of plant tissues, however, may limit the thermoregulatory options of herbivorous ectotherms. In this laboratory study, selected body temperatures of Desert Iguanas (Dipsosaurus dorsalis), subjected to two different rations of a carnivorous and a herbivorous diet, were measured in a laboratory thermal gradient. Lizards fed a carnivorous diet and rationed to 50% of their daily energetic requirement selected significantly lower body temperatures than did lizards in the other diet/ration treatments. Lizards fed a herbivorous diet selected higher body temperatures than did the same lizards fed a carnivorous diet regardless of ration. Our study indicates that selected temperatures are influenced by both diet quality and quantity. These results support the hypothesis that herbivorous reptiles require elevated body temperatures to digest plant tissues. Our findings do not fully support the behavioral hypothermia hypothesis, which posits that diurnal lizards will seek cool temperatures at night to reduce their energy expenditure. Thermoregulatory strategies of Desert Iguanas and other herbivorous lizards may be influenced by the need to balance high body temperatures to digest plant material with the high metabolic costs of maintaining high temperatures.