Dehydration effects on the water balance of a Neotropical anuran (Rhinella schneideri)

Due to their permeable skin, terrestrial amphibians are constantly challenged by the potential risk of dehydration. However, some of the physiological consequences associated to dehydration may affect aspects that are, themselves, relevant to the regulation of water balance. Accordingly, we examined the effects of graded levels of dehydration on the rates of evaporative water loss and water absorption through the skin in the terrestrial Neotropical toad, Rhinella schneideri. Concomitantly, we monitored the effects of dehydration on the mass of visceral organs, hematocrit and hemoglobin content, plasma osmolality, and plasma concentration of urea and ions. To promote dehydration, toads were placed individually in a wind tunnel toads lost water by average of 10 g.h-1. We found that dehydration caused an increase in the concentration of body fluids, as indicated by virtually all the parameters examined. There was a proportional change in the relative masses of visceral organs, except for the liver and kidneys, which exhibited a decrease in their relative masses greater than the whole-body level of dehydration. Changes, or the preservation, of relative organ masses during dehydration may be explained by organ-specific physiological adjustments in response to the functional stress introduced by the dehydration itself. As dehydration progressed, evaporative water loss diminished and water reabsorption increased. In both cases, the increase in body fluids concentration associated to the dehydration provided the osmotic driver for these changes in water flux. Dehydration, therefore, while posing a considerable challenge on the water balance regulation of anurans, paradoxically facilitates water conservation and absorption. Such effects may have important ecophysiological implications.