Water, Energy, and Electrolyte Balance in Captive Namib Sand-Dune Lizards (Angolosaurus skoogi)

Abstract

Angolosaurus skoogi, were quantified in the laboratory. Both temporal and thermal acclimation of resting metabolic rate (measured by constant volume respirometry) were demonstrated. No significant difference could 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 rates showed a similar increase with temperature. It is predicted that, under field conditions, activity would result in about an eightfold increase above both resting metabolic and EWL rates. Animals can, thus, conserve a considerable amount of energy and water by remaining submerged. Energy, water, and electrolyte intake were quantified in two separate feeding experiments. Energy intake showed a fivefold and water intake a fourfold increase between 23 and 37 C. Digestive efficiency averaged 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 content of the natural diet. Digestive efficiency was independent of temperature, 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 temperatures are lower than sand temperatures (e.g., at night), animals can, thus, improve their gross energy assimilation. Compared to the diets of other desert reptiles, the natural diet of A. skoogi has a high water content and a relatively low electrolyte load. The presence of a functional salt gland and an efficient urate excretory pathway suggests that this species is capable of handling a far greater electrolyte load than that ingested with its natural diet. This may be adaptive in times of feeding stress.