Osmoregulation by Vertebrates in Terrestrial Environments

Abstract

Abstract A terrestrial environment for vertebrates leads to both a passive water loss to the air and solute gain through food. Consequently, terrestrial environments would tend to decrease water content and increase the osmotic concentration of vertebrates. Natural selection has led to a variety of behavioural, physiological and anatomical adaptations to both minimise water loss and excrete solutes in order to maintain a constant osmotic concentration in terrestrial environments. Extant amphibians represent the ancestral colonisers of terrestrial environments. Their water‐permeable skin make them most prone to dehydration and have the greatest tolerance to dehydration and the most unique mechanisms to stabilise plasma volume with dehydration. Reptiles exploit very arid terrestrial environments by minimising cutaneous evaporative water loss. Birds and mammals have unique kidney capacities to concentrate solutes and minimise urinary water loss. This in conjunction with elaborate nasal turbinates to minimise respiratory water loss allows them to survive in arid terrestrial environments. Terrestrial environments favour water loss and challenge maintenance of body water volume and osmotic concentration. Water budgets and daily rates of water turnover are constructed to quantify avenues of exchange and propensity for water stress. Extant amphibians represent the ancestral group of vertebrates that colonised terrestrial environments. Amphibians have the greatest rates of daily water turnover, primarily a consequence of high rates of evaporative water loss resulting from high cutaneous permeability to water. Reptiles have the lowest rates of daily water turnover, primarily the result of low rates of cutaneous evaporative water loss. Endothermic classes (birds and mammals) have intermediate daily water turnover rates, the result of metabolic water production with their elevated aerobic metabolic rates. Endothermic classes can minimise urinary losses and better able stabilise osmotic concentrations the result of kidneys that can produce urine hyperosmotic to their own body fluid concentration. The threat of dehydration is greatest for amphibians, given the high daily water turnover rates. Dehydrational stress is handled by amphibians and some reptiles by first reabsorbing stored hypoosmotic urine from their urinary bladders. When stored bladder fluid is exhausted, the key element to handling dehydration is maintaining plasma volume at the expense of intracellular water and interstitial fluid. Anuran amphibians have the greatest capacity to maintain cardiac function with dehydration.