Physiological ecology of desert tortoises in southern Nevada

Seasonal changes in water balance, energetics, food consumption, daily behavior, diet, osmoregulation and body mass were measured using doubly labeled water, radiotelemeters, and field behavior observations of tortoises (Gopherus agassizii) through a year (1976-1977) in Rock Valley, Nevada. Aboveground activity extended over the warm seasons (between March and November), and tortoises hibernated in burrows through winter. In spring, tortoises were active for about 3 h every fourth day. Some drank rainwater and they all gained weight while eating succulent annual plants, which provided excess water and potassium that were stored in the urinary bladder, but energy intake was less than required to meet energy expenses via respiration, and body solids declined. As food plants dried in late spring, tortoises eventually achieved positive energy balance while eating grasses, but body masses declined due to negative water balance. Bladder urine became isosmotic with blood plasma, and both became more concentrated. As the summer drought continued, feeding diminished and tortoises spent progressively more time estivating in burrows. Thundershowers in July triggered emergence from estivation, and tortoises drank much rainwater, voided concentrated urine, accumulated dilute urine, restored normal plasma osmotic concentration, resumed feeding on the still-dry grasses and forbs, and accumulated much surplus energy. However, body masses declined during this time due to negative water balance, and osmotic concentrations in bladder urine increased. More rain in September again relieved this osmotic stress as tortoises drank, urinated, and stored dilute urine. Energy balance remained strongly positive until tortoises began feeding on the succulent new sprouts of annual plants that germinated in late September, and the tortoises returned to a spring-like physiological situation through mid-November when hibernation began. Tortoises apparently relinquish maintenance of internal homeostasis on a daily basis during most of the year, while tolerating large imbalances in their water, energy and salt budgets. This ability enables them to exploit resources that are only available periodically, while balancing their water and salt budgets on an annual basis and showing an energetic profit. The environmental properties of rainfall (beneficial to tortoises) and dietary potassium content (deleterious) appear to have major effects on the health of these impressive reptiles.