Foraging, Feeding, Energetics, and Environment: Interactions Between Physiology and Ecology of Gila Monsters

The ecology of Gila monsters (Heloderma suspectum) is largely related to the endurance of unpredictable resource availability, and to the tolerance of predictably harsh environmental conditions. Gila monsters use a novel suite of behavioral, physiological and morphological adaptations to overcome the vagaries of their desert habitat, and as a result, they survive by interacting with their environment in unique ways that would seem impossible for other species.This dissertation investigates the physiological and ecological consequences of the unique ways in which Gila monsters interact with their environment. Each chapter analyzes different aspects of how individuals acquire, process, store, and use essential food and water resources, while simultaneously managing the constraints and opportunities provided by the environment.Chapter one is a laboratory study that investigates the thermal preferences of Gila monsters, the effects of feeding on body temperature selection, and the ability of individuals to physiologically control heat exchange. These investigations set the stage for the ecological importance of body temperature regulation, and also provide a baseline by which to appropriately measure behavioral thermoregulation.Chapter two explores the importance of temperature regulation by Gila monsters in the field. The physical limitations imposed by the thermal environment are used as a framework to analyze trade-offs between foraging activity and thermoregulation. In order to forage, lizards must nearly always tolerate thermal conditions that are outside of their preferred range, but in doing so, they sacrifice their ability to thermoregulate and risk reduced performance.Chapter three examines the influences of body temperature regulation and activity on the use of energy and water. The magnitude of energy use is measured directly, and specific components of energy partitioning are modeled mathematically to yield seasonal and annual energy budgets for Gila monsters in southern Nevada. These analyses are then extended to a larger geographic scale, and the effects of both environmental variation (temperature) and individual variation (body mass) are used to simulate differences in resting energy use between four ecologically distinct populations of Gila monsters.Each of these chapters takes an eco-physiological approach to investigating the interactions between Gila monsters and their environment. While in some ways Gila monsters may seem maladapted to desert life, I show how they use specific behavioral and physiological strategies to overcome environmental limitations