ABSTRACT
Amphibians were the first vertebrates to emerge from aquatic habitats, and modern species have evolved a variety of mechanisms to regulate water and electrolyte homeostasis. Fossil evidence for terrestrial vertebrates first appears in the upper Devonian in the form of tetrapods such as Ichthyostega and Ancanthostega. The extreme tolerance of amphibians to hydration and dehydration is associated with the large and variable extracellular volume and rapid fluid exchange between plasma and lymph space which secure the blood flow despite large variations of total body water content. Food intake initiates drinking behavior in the terrestrial Bufo bufo. Following a meal, the toad visits a water source and takes up water at an amount exceeding the mass of the food eaten. The water intake is proportional to the size of the meal and may amount to as much as 15% of the body mass measured with empty urinary bladder.
