WATER, ENERGY, AND ELECTROLYTE BALANCE IN CAPTIVE NAMIB SAND-DUNE LIZARDS (ANGOLOSAURUS-SKOOGI)

Components of water and energy balance of the Namib sand dune lizard, Angolosaurus skoogi, were quantified in the laboratory. Both temporal and thermal acclimation of resting metabolic rate (measured by constan t volume respirometry) were demonstrated. No significant difference co uld be detected in the metabolic or evaporative water loss (EWL) rates (measured gravimetrically) of animals resting above and below sand at 30 C. The ratio of active to resting (A/R) metabolic rates increased with temperature, ranging from 2.5 at 15 C to 3.8 at 40 C. EWL A/R rat es showed a similar increase with temperature. It is predicted that, u nder field conditions, activity would result in about an eightfold inc rease above both resting metabolic and EWL rates. Animals can, thus, c onserve a considerable amount of energy and water by remaining submerg ed. Energy, water, and electrolyte intake were quantified in two separ ate feeding experiments. Energy intake showed a fivefold and water int ake a fourfold increase between 23 and 37 C. Digestive efficiency aver aged 63% in animals fed their natural diet (the cucurbit Acanthosicyos horrida and the grass Stipagrostis hermannii) and 80% in animals fed a diet of carrots. The difference is attributed to the higher fiber co ntent of the natural diet. Digestive efficiency was independent of tem perature, but the gut passage time of food ranged from 5.6 days at 23 C to 2.4 days at 37 C. By remaining submerged when ambient temperature s are lower than sand temperatures (e.g., at night), animals can, thus , improve their gross energy assimilation. Compared to the diets of ot her desert reptiles, the natural diet of A. skoogi has a high water co ntent and a relatively low electrolyte load. The presence of a functio nal salt gland and an efficient urate excretory pathway suggests that this species is capable of handling a far greater electrolyte load tha n that ingested with its natural diet. This may be adaptive in times o f feeding stress.