In freshwater the caiman gains water at the rate of 3.5 ml/kg hr or 1.1 μl/cm2 hr. Of this 70% is the result of uptake through the skin, the rest being due to drinking. This gain in water is excreted by the kidneys as a hypotonic urine. Urine sodium losses are reduced by active sodium uptake from the urine during storage for 3 to 4 hours in the cloaca. In vitro and in vivo experiments indicate that the skin is somewhat permeable to sodium, which moves along concentration gradients.
In air the caiman loses water by evaporation (1.2 μl/cm2 hr). About 75% of this takes place from the skin.
In 3.3% NaCl solution caimans show elevated serum sodium concentrations and 20% of them die within 24 hours. The sodium accumulation is mostly due to drinking (4.4% of the body weight/day), and only to a minor extent to osmotic water loss through the skin (2.5% of the body weight/day or 0.6 μl/cm2 hr). The permeability of the caiman is compared to that of fishes, Amphibia and other reptiles.