Adaptations of the reed frogHyperolius viridiflavus (Amphibia, Anura, Hyperoliidae) to its arid environment

Summary

Of all amphibians living in arid habitats, reed frogs (belonging to the super speciesHyperolius viridiflavus) are the most peculiar. Froglets are able to tolerate dry periods of up to 35 days or longer immediately after metamorphosis, in climatically exposed positions. They face similar problems to estivating juveniles, i.e. endurance of long periods of high temperature and low RH with rather limited energy and water reserves. In addition, they must have had to develop mechanisms to prevent poisoning by nitrogenous wastes that rapidly accumulate during dry periods as a metabolic consequence of maintaining a non-torpid state.

During dry periods, plasma osmolarity ofH. v. taeniatus froglets strongly increased, mainly through urea accumulation. Urea accumulation was also observed during metamorphic climax.

During postmetamorphic growth, chromatophores develop with the density and morphology typical of the adult pigmentary pattern. The dermal iridophore layer, which is still incomplete at this time, is fully developed within 4–8 days after metamorphosis, irrespective of maintenance conditions. These iridophores mainly contain the purines guanine and hypoxanthine. The ability of these purines to reflect light provides an excellent basis for the role of iridophores in temperature regulation. In individuals experiencing dehydration stress, the initial rate of purine synthesis is doubled in comparison to specimens continuously maintained under wet season conditions. This increase in synthesis rate leads to a rapid increase in the thickness of the iridophore layer, thereby effectively reducing radiation absorption. Thus, the danger of overheating is diminished during periods of water shortage when evaporative cooling must be avoided. After the development of an iridophore layer of sufficient thickness for effective radiation reflectance, synthesis of iridophore pigments does not cease. Rather, this pathway is further used during the remaining dry season for solving osmotic problems caused by accumulation of nitrogenous wastes. During prolonged water deprivation, in spite of reduced metabolic rates, purine pigments are produced at the same rate as in wet season conditions. This leads to a higher relative proportion of nitrogen end products being stored in skin pigments under dry season conditions. At the end of an experimental dry season lasting 35 days, up to 38% of the accrued nitrogen is stored in the form of osmotically inactive purines in the skin. Thus the osmotic problems caused by evaporative water loss and urea production are greatly reduced.