The major nitrogenous waste excreted by vertebrates is primarily dependent on
Aquatic animals usually excrete ammonia while the terrestrial animals produce lesser toxic nitrogenous wastes like urea and uric acid for conservation of water. Hence, habitat plays an important role in the type of nitrogenous waster product excreted among the vertebrates.
Ammonia is a small molecule that readily diffuses across skin and gills if sufficient water is available, but the kidneys inefficiently excrete ammonia. Ammonia is highly toxic, and animals cannot survive even moderate ammonia concentrations in their body fluids. For this reason during the transition to land, selection favored the excretion of a less toxic form of nitrogen, such as urea or uric acid. Uric acid is the least toxic nitrogenous by-product.
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