REGULATION OF BODY-WATER BALANCE IN REEDFROGS (SUPERSPECIES HYPEROLIUS-VIRIDIFLAVUS AND HYPEROLIUS-MARMORATUS - AMPHIBIA, ANURA, HYPEROLIIDAE) LIVING IN UNPREDICTABLY VARYING SAVANNA ENVIRONMENTS

The regulation of body water balance was examined in the reedfrogs Hyp erolius marmoratus taeniatus and Hyperolius viridiflavus ommatostictus . Temperature and stage of post-metamorphic development significantly affected the rate of water uptake. Hydrated reedfrogs prevented hyperh ydration by voiding diluted urine when obtaining water. Within 48 hr a fter rehydration, body fluid osmolality remained at low levels, which may be supportive to counter excessive cutaneous water influx in hydra ted frogs. Once evaporative water loss exceeded 10-12% total body mass , reedfrogs became anuric. The rate of water uptake strongly increased with increasing body water deficit. Both the anuric response and the increased rate of water uptake are assumed to strongly enhance the eff icacy of using very briefly available water sources during dry-period conditions. Dry-adapted and estivating reedfrogs survived evaporative water losses between 40 and 55% total body mass. Bladder fluid stores contributed substantially to this desiccation tolerance. During a 16-d ay period of desiccation, H. v. ommatostictus could replace approximat ely 25% of evaporative water loss from the bladder fluid store. During desiccation, the level of free amino acids selectively increased in t he gastrocnemius muscle tissue, which may support cell volume regulati on and/or protect cellular structures from osmotic stresses. Even stro ngly dehydrated reedfrogs rehydrated quickly with no obvious osmoregul atory problem. Rehydration was associated with a higher than expected decrease of free amino acids in the gastrocnemius muscle tissue, a res ponse that may help to protect cells from bursting during fast rehydra tion. (C) 1997 Elsevier Science Inc.