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

Ontogenetic comparisons of standard metabolism in three species of crocodilians.

Due in part to their large size, aggressive temperament, and difficulty in handling, there are few physiological studies of adult crocodilians in the literature. As a result, studies comparing individuals across an ontogenetic series and comparisons among species are also lacking. We addressed this gap in knowledge by measuring standard metabolic rates (SMR) of three species of crocodilians (Crocodylus porosus, C. johnsoni, and Alligator mississippiensis), and included individuals that ranged from 0.22 to 114 kg. Allometric scaling of SMR with body mass was similar among the species, but C. porosus had significantly higher SMR than did C. johnsoni or A. mississippiensis. Differences in SMR among species are potentially related to behavioural differences in levels of aggression; C. porosus are the most aggressive of the crocodilians measured, and have rates of standard metabolism that are approximately 36% higher at the grand mean body size than those measured for C. johnsoni or A. mississippiensis, which are among the least aggressive crocodilians

Removing the rubbish: frogs eliminate foreign objects from the body cavity through the bladder

During the course of a telemetry study on three species of Australian frogs (Litoria caerulea, Litoria dahlii and Cyclorana australis), we found that many of the surgically implanted transmitters had migrated into the bladder. We subsequently implanted small beads into L. caerulea and they were expelled from the body in 10–23 days. Beads implanted into cane toads (Rhinella marina) to document the process were either expelled or were enveloped into the bladder. This appears to be a unique pathway for expulsion of foreign objects from the body, and suggests that caution should be employed in telemetry studies when interpreting the separation of some animals from their transmitters as a mortality event

Scaling of standard metabolism (O₂ consumption) for three species of crocodilians measured at 30°C.

<p>The highly aggressive <i>Crocodylus porosus</i> has significantly higher standard metabolism than the less aggressive <i>Crocodylus johnsoni</i> and <i>Alligator mississippiensis</i>. Inset figure shows mass-independent means (bars) and standard errors (error bars) of O₂ consumption. Species not connected by same letter are significantly different.</p