Water is essential to life because its unique properties facilitate a vast number of biochemical processes that enable all organisms to exist. Maintenance of proper water concentration in the body is thus a vital activity. Colonization of terrestrial habitats placed organisms in desiccating environments that emphasized problems of water balance. The procurement and economical use of water has since been a central organizing theme in biology and has shaped the morphology, physiology, behavior, and spatial distribution of organisms. In hot, and environments like North American deserts, organisms are faced with especially challenging abiotic conditions characterized by variable and often extreme temperatures, intense solar radiation, low and patchy energy availability, and most notably, extremely limited spatially and temporally unpredictable water availability. Under such conditions, maintenance of homeostasis is difficult since abiotic constraints limit organisms' abilities to concurrently regulate various physiological processes (e.g., water, energy, and thermal balance). To meet simultaneous demands, organisms must make morphological, physiological, or behavioral adjustments that benefit one process at the expense of another.

This dissertation includes results of a series of in vivo laboratory experiments designed to identify specific morphological and physiological characteristics associated with water balance in a long-lived desert lizard, the Gila Monster (Heloderma suspectum), a two-year descriptive field study describing the utility of these characteristics, and a two-year water supplementation experiment used to elucidate the effects of water intake, independent of food intake, on surface activity patterns and body condition in nature. Gila Monsters store diluted urine in the bladder and can absorb water from the urine over 1-3 months to moderate dehydration during drought. Following food and water deprivation, stereotyped binge drinking behavior enables Gila Monsters to rapidly rehydrate and replenish urinary bladder contents. Despite this physiological reservoir, seasonal drought in the Sonoran Desert causes significant dehydration and reduced body condition, hydric condition, and surface activity in Gila Monsters. Water supplementation of individual lizards results in significantly better hydric condition, greater surface activity, and increased stored energy, indicating that Gila Monsters use evolutionary adaptations including behavioral adjustments and physiological tolerance to overcome severe hydric constraints in the Sonoran Desert.