Water and Electrolyte Metabolism in the Arenicolous Lizard Uma notata notata

Abstract

reaching a maximum during midday. Body temperatures of buried lizards (x = 34.3 C) were lower than in active lizards, but also were higher at midday than in the morning or afternoon. Eccritic body temperatures were lower during fall and winter than during spring and summer. Air temperatures 1 cm above the sand during times when lizards were active were also lower during fall and winter. Larger U. notata hibernated, while smaller individuals remained active throughout the year. U. notata were water independent when maintained in laboratory thermal gradients on a diet of meal worms. They possess a functional nasal salt gland with an ability to concentrate (x = 465 mEq C1-/L) and eliminate (x rate = 0.081 mg C1- g-1 day-1) chloride comparable to other terrestrial lizards. Cloacal water loss was low (x = 2.3 mg g-1 day-1) due probably, in part, to excretion of most nitrogenous wastes as urates or uric acid. Urinary chloride concentration was very low, but liquid urine was seldom voided. This may be due to resorption of water and salts from the kidneys or cloaca and excretion of the salts by the nasal gland. The relatively low rate of evaporative water loss (x = 0.88 mg g-1 hr-1) in dry air at 39.0 C compares favorably to rates for other lizards from arid habitats. Twenty-four hour rates of evaporative water loss (determined by direct weighing) for buried lizards at temperatures and humidities comparable to those in nature were lower than the rates for exposed lizards. A water budget calculated for representatives of U. notata showed that they remain in water balance even under conditions slightly more extreme than they normally encounter. It was concluded that evaporative loss in Uma, when it is exposed, is by far the most crucial factor in its water budget. Forty per cent of the total daily water loss occurs within the averaged 4.7 hr that the lizards are exposed. Uma saves considerable quantities of water by burying in the sand. Thus, these animals can exercise considerable behavioral control over their water budget by varying the length of time they are above or below the surface.