ADAPTATIONS OF THE REED FROG HYPEROLIUS-VIRIDIFLAVUS (AMPHIBIA, ANURA, HYPEROLIIDAE) TO ITS ARID ENVIRONMENT .7. THE HEAT-BUDGET OF HYPEROLIUS-VIRIDIFLAVUS-NITIDULUS AND THE EVOLUTION OF AN OPTIMIZED BODY SHAPE

Estivating reed frogs of the superspecies Hyperolius viridiflavus are extraordinarily resistant to the highly adverse climatic conditions pr evailing in their African savanna habitats during dry season (air temp erature up to 45 degrees C, solar radiation load up to 1000 W . m(-2), no water replenishment possible for up to 3 months). They are able to withstand such climatic stress at their exposed estivation sites on d ry plants without evaporative cooling. We developed a heat budget mode l to understand the mechanisms of how an anuran can achieve this uniqu e tolerance, and which allows us to predict the anuran's core and surf ace temperature for a given set of environmental parameters, to within 4% of the measured values. The model makes it possible to quantify so me of the adaptive mechanisms for survival in semiarid habitats by com paring H. viridiflavus with anurans (H. tuberilinguis and Rana pipiens ) of less stressful habitats. To minimize heat gain and maximize heat loss from the frog, the following points were important with regard to avoiding lethal heat stress during estivation: 1) solar heat load is reduced by an extraordinarily high skin reflectivity for solar radiati on of up to 0.65 under laboratory and even higher in the held under dr y season conditions. 2) The half-cylindrical body shape of H. viridifl avus seems to be optimized for estivation compared to the hemispheroid al shape usually found for anurans in moist habitats. A half-cylinder can be positioned relative to the sun so that large surface areas for conductive and convective heat loss are shielded by a small area expos ed to direct solar radiation. 3) Another important contribution of bod y shape is a high body surface area to body mass ratio, as found in th e estivating subadult H. viridiflavus (snout-vent lengths of 14-20 mm and body weights of 350-750 mg) compared to adult frogs (24-30 mm, 100 0-2500 mg) which have never been observed to survive a dry season. 4) These mechanisms strongly couple core temperature to air temperature. The time constant of the core temperature is 29 +/- 10 s. Since air te mperature can be 43-45 degrees C, H. viridiflavus must have a very unu sual tolerance to transient core temperatures of 43-45 degrees C. 5) I f air temperature rises above this lethal limit, the estivating frog w ould die despite all its optimizations, but moving from an unsuited to a more favorable site during estivation can be extremely costly in te rms of unavoidably high evaporative water loss. Therefore, H. viridifl avus must have developed behavioral strategies for reliably choosing e stivation sites with air temperature staying on average within the vit al range during the whole dry season.