Strategies of Water Conservation in Southern African Frogs
In arid or semi-arid areas of southern Africa, frogs are typically active nocturnally and seasonally. They may adopt a variety of strategies to conserve water during the day or during the unfavourable part of the year. Aquatic frogs (e.g. Xenopus, most species of Sana, Ptychadena and Phrynobatrachus) do not normally have a water conservation problem except when the water in or near which they are living dries up. Burrowing frogs (e.g. Bufo, Pyxicephalus, Tomoptema and Breviceps) are able to select microhabitats in the soil which reduce evaporative water loss. Burrowing may, in particular cases, be associated with the formation of a keratinized cocoon of stratum corneum which appears to offer a barrier to outward water movement. In arboreal frogs (Chiromantis xerampelina and Hyperolius spp.) the most interesting adaptations are found. Evidence is given in the case of HyperoUus nasutus for low evaporative water losses at the end of the dry season, when the frogs sit exposed to the sun and dry air, in flowers. During the rains when H. nasutus is in breeding aggregation the evaporative water losses are high. Chiromantis xerampelina is uricotelic and resistant to desiccation.
The three approaches which anurans have adopted in response to water conservation problems have their correlates in bladder size and function. hormonal regulation of water balance and the extent to which water loss can be tolerated. The potential that the southern African amphibian fauna offers for physiological research is emphasized.