Adaptations of the reed frog Hyperolius viridiflavus (Amphibia, Anura, Hyperoliidae) to its arid environment : IV. Ecological significance of water economy with comments on thermoregulation and energy allocation.

Abstract

After breeding African savanna dwelling reed-frogs of the "superspecies" Hyperolius viridiflavus face a severe dry season. The frogs withstand the adverse abiotic conditions in exposed positions, clinging to dry vegetation. Only juveniles (300-700 mg) are able to adjust water economy and metabolism to a prolonged dry season. Wet season frogs attain low levels of evaporative water loss (EWL) within 6-8 days after incipient water shortage. This time course is mainly determined by the animal's ability to lower metabolism and activity level to the minimum demands of a dry season. Barriers against diffusion of water which most probably are built up by the stratum corneum and/or the overlying film of dried mucus seem not to be essentially modified during adjustment to dry season conditions. Changeover to dry season physiology is greatly accelerated through preconditioning frogs to water shortage. AdultHyperoliusare unable to reduce activity and metabolism as fast and effectively as juveniles. Most probably these are the main reasons for their very restricted survival capability under dry season conditions; the generally poor energy reserves after the breeding period may further shorten their survival time. At the critical thermal maximum (CTM) Hyperolius uses skin gland secretions for evaporative cooling. Acclimation effects and regulation of evaporative cooling within some 1/10° C help to employ limited water reserves very economically. Dry adapted, dehydrated frogs take up water, whenever available, via specialized skin areas. Rate of uptake is high and is mainly determined by the actual stage of dehydration. The onset of the water-balance-response is also affected by preconditioning. Survival time of small (<500 mg) estivating Hyperolius is limited by their water reserves.Due to their unfavourable surface to volume ratio they loserelatively more water by evaporation than larger conspecifics. Therefore, smaller specimens should allocate energy preferably to growth, until reducing EWL so far to survive the average periods between the rare precipitations. In larger frogs (>500 mg) the amount of stored energy determines maximal survival time. When a critical size is reached in postmetamorphic growth, a change in energy allocation from body growth to energy storage would improve prospects of survival and should therefore be expected. Species specific differences in regard to EWL and CTM indicate a strong correlation between physiological properties and ecological demands.