EXCHANGES OF WATER IN THE FROG

Life on the Edge: A Comparative Study of Ecophysiological Adaptations of Frogs to Tropical Semiarid Environments

A key goal of ecology and evolution is to understand the relative contributions of environment and history in determining the geographic distribution of organisms. For the Neotropical lowlands, where temperatures are similar across landscapes, we hypothesize that water balance may be a critical but understudied factor in determining the distribution of species. Amphibians are especially sensitive to variation in precipitation due to their permeable skin. Here we focused on lowland frogs of northwestern South America and investigated variation among 17 species in potentially important ecologically relevant performance variables related to water balance, testing for possible adaptations to semiarid conditions within species. We studied frogs from coastal xeric, savannah, and wet forest biomes under common laboratory conditions and quantified rates of evaporative water loss, rates of water uptake, and variation in water-searching behavior and performance. We found significant differences in all three performance variables among species even after accounting for shared evolutionary history. A phylogenetic ANCOVA showed that categorizing species by ecological habit (terrestrial vs. arboreal) explained much of the ecoperformance trait variation among species. Secondarily, environment explained additional variation; for example, coastal xeric species showed reduced rates of water loss, and terrestrial savannah amphibians showed lower rates of water uptake. Conspecific frog populations from different biomes exhibited similar performance. We compare our results with previous studies and conclude that ecological habit is the principle factor that predicts ecophysiological trait variation and the possible geographic distribution of lowland Neotropical frogs.

Volume regulation in salt-acclimated toad (Bufo viridis): the role of urea and the urinary bladder

Body water (weight) was studied in the euryhaline toad Bufo viridis during high salt (500 mOsm NaCl) acclimation. Plasma osmolality was greatly increased upon salt acclimation mainly by urea, and was always hyperosmotic to the ambient solution. Water content was regulated quite efficiently in slowly acclimated undisturbed toads. Repeatedly catheterized toads behaved like osmometers when transferred to hyperosmotic solutions. Total urea loss was greatly reduced in salt acclimated toads, suggesting urine was not voided under these conditions. It is concluded that urea accumulation, inhibition of the urine voiding response and the urine in the bladder are the principal factors involved in volume regulation under conditions of salt acclimation.

Vascular supply to the ventral pelvic region of anurans as related to water balance

The ventral pelvic integument of anurans in hypervascular as compared to other skin regions. Recent studies indicate that this area is primarily responsible for water uptake and rehydration. Nine species representing four genera of terrestrial and aquatic Anura were dissected for comparison of vascularity in the pelvic area. Major differences in the ventral pelvic integumental vessels were found between aquatic and terrestrial genera. These variations are described, and names are suggested for previously unnamed vessels. Changes in vascularity are discussed and related to the literature on Anura pertinent to rehydration rates, tolerance of desiccation, resistance to desiccation, and habitat. Data from this study and previous studies suggest a correlation among the parameters discussed and habitat. The suggestion is made that when species are classified as xeric, mesic, or aquatic on the basis of the habitat in which they live, their responses to rehydration, tolerance to desiccation, and vasculatity should be considered, as these characteristics more accurately represent the micro-habitat.

From frog skin to sheep rumen: a survey of transport of salts and water across multicellular structures

All higher animals, whether they live in water or on dry land, are faced with the necessity of regulating rather closely their intake and excretion of salts and water in order to maintain the constancy of their internal ionic environment. The kidney is in general the most important organ of the body as far as the excretion of sodium, potassium, chloride and water is concerned, but there are other tissues which also play a part in controlling the ionic balance between the internal and external environments, such as the intestinal mucosa, the skin and urinary bladder in amphibia, the gill epithelium in fishes, the salt gland in marine birds, and the epithelium of the rumen in ruminants. In addition to excretory and absorptive organs of this type, there are others which are secretory and whose function involves the production of fluids differing in ionic composition from the blood plasma. Examples include the glands which secrete saliva and sweat, the oxyntic acid-producing cells of the gastric mucosa, and the epithelium of the stria vascularis which generates the potassium-rich endolymph of the mammalian cochlea. The purpose of this article is to consider briefly what is known about the active transport of salts and water across some typical multicellular secretory tissues, and to attempt in the process to discern what properties they have in common and in what respects they are specialized.