Adaptations of the reed frog Hyperolius viridiflavus (Amphibia, Anura, Hyperoliidae) to its arid environment. VII. 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 prevailing in their African savanna habitats during dry season (air temperature 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 dry plants without evaporative cooling. We developed a heat budget model to understand the mechanisms of how an anuran can achieve this unique tolerance, and which allows us to predict the anuran's core and surface temperature for a given set of environmental parameters, to within 4% of the measured values. The model makes it possible to quantify some of the adaptive mechanisms for survival in semiarid habitats by comparing 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 radiation of up to 0.65 under laboratory and even higher in the field under dry season conditions. 2) The half-cylindrical body shape of H. viridiflavus seems to be optimized for estivation compared to the hemispheroidal 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 exposed to direct solar radiation. 3) Another important contribution of body shape is a high body surface area to body mass ratio, as found in the 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, 1000-2500 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 temperature can be 43-45 degrees C, H. viridiflavus must have a very unusual tolerance to transient core temperatures of 43-45 degrees C. 5) If air temperature rises above this lethal limit, the estivating frog would die despite all its optimizations, but moving from an unsuited to a more favorable site during estivation can be extremely costly in terms of unavoidably high evaporative water loss. Therefore, H. viridiflavus must have developed behavioral strategies for reliably choosing estivation sites with air temperature staying on average within the vital range during the whole dry season.