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Hyperolius viridiflavus possesses one complete layer of iridophores in the stratum spongiosum of its skin at about 8 days after metamorphosis. The high reflectance of this thin layer is almost certainly the result of multilayer interference reflection. In order to reflect a mean of about 35% of the incident radiation across a spectrum of 300-2900 nm only 30 layers of well-arranged crystals are required, resulting in a layer 10.5 microns thick. These theoretical values are in good agreement with the actual mean diameter of single iridophores (15.0 +/- 3.0 microns), the number of stacked platelets (40-100) and the measured reflectance of one complete layer of these cells (32.2 +/- 2.3%). Iridescence colours typical of multilayer interference reflectors were seen after severe dehydration. The skin colour turned from white (0-10% weight loss) through a copper-like iridescence (10-25% weight loss) to green iridescence (25-42%). In dry season state, H. viridiflavus needs a much higher reflectance to cope with the problems of high solar radiation load during long periods with severe dehydration stress. Dry-adapted skin contains about 4-6 layers of iridophores. The measured reflectance (up to 60% across the solar spectrum) of this thick layer (over 60 microns) is not in keeping with the results obtained by applying the multilayer interference theory. Light, scattered independently of wavelength from disordered crystals, superimposes on the multilayer-induced spectral reflectance. The initial parallel shift of the multilayer curves with increasing thickness and the almost constant ("white") reflectance of layers exceeding 60 microns clearly point to a changing physical basis with increasing layer thickness.

Hyperolius viridiflavus nitidulus inhabits parts of the seasonally very hot and dry West African savanna. During the long lasting dry season, the small frog is sitting unhidden on mostly dry plants and has to deal with high solar radiation load (SRL), evaporative water loss (EWL) and small energy reserves. It seems to be very badly equipped to survive such harsh climatic conditions (unfavorable surface to volume ratio, very limited capacity to store energy and water). Therefore, it must have developed extraordinary efficient mechanisms to solve the mentioned problems. Some of these mechanisms are to be looked for within the skin of the animal (e.g. protection against fast desiccation, deleterious effects of UV radiation and overheating). The morphology of the wet season skin is, in most aspects, that of a "normal" anuran skin. It differs in the organization of the processes of the melanophores and in the arrangement of the chromatophores in the stratum spongiosum, forming no "Dermal Chromatophore Unit". During the adaptation to dry season conditions the number of iridophores in dorsal and ventral skin is increased 4-6 times compared to wet season skin. This increase is accompanied by a very conspicuous change of the wet season color pattern. Now, at air temperatures below 35° C the color becomes brownish white or grey and changes to a brilliant white at air temperatures near and over 40° C. Thus, in dry season state the frog retains its ability for rapid color change. In wet season state the platelets of the iridophores are irregularly distributed. In dry season state many platelets become arranged almost parallel to the surface. These purine crystals probably act as quarter-wave-length interference reflectors, reducing SRL by reflecting a considerable amount of the radiated energy input.EWL is as low as that of much larger xeric reptilians. The impermeability of the skin seems to be the result of several mechanisms (ground substance, iridophores, lipids, mucus) supplementing each other.The light red skin at the pelvic region and inner sides of the limbs is specialized for rapid uptake of water allowing the frog to replenish the unavoidable EWL by using single drops of dew or rain, available for only very short periods.

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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.