Anhomeostasis: Seasonal Water and Solute Relations in Two Populations of the Desert Tortoise (Gopherus agassizii) during Chronic Drought

Seasonal patterns of osmoregulation and water balance were compared in two populations of the desert tortoise (Gopherus [= Xerobates] agassizii) inhabiting portions of the Mojave Desert with different seasonal rainfall patterns. During an extreme drought year, tortoises lost as much as 40% of their initial body mass, and mean total body water volume decreased to below 60% of body mass. They stored wastes in, and apparently resorbed water from, their large urinary bladders. Osmolality of initially dilute bladder urine increased until it was isosmotic to blood plasma, after which osmolality of both fluids increased, eventually to some of the highest levels known for terrestrial reptiles. The increase in plasma osmolality comprised increases in plasma sodium, chloride, and especially urea concentrations. Bladder urine osmolality increased (owing primarily to soluble potassium) most rapidly during periods when tortoises ate annual plants; thus, their normal diet apparently was osmotically stressful. When rainfall events occurred, tortoises at both sites drank copiously, voided concentrated bladder urine, and stored dilute urine; body mass, total body water, and plasma and urine concentrations returned to hydrated levels. Drinking rainwater was as important for tortoises in the western Mojave, where summer rain is rare, as in the eastern Mojave, where it is predictable. Tortoises osmoregulate opportunistically, a tactic made possible by their capacity to tolerate temporary "anhomeostasis" and by extremely low rates of water loss (measured with isotopically labeled water). This opportunism resulted in wide, behaviorally based variation among individuals in all variables measured, as well as large differences (due to timing of rainfall) between populations and in temporal patterns between years within populations.