Terrestrial organisms have evolved significantly in terms of the mechanisms to excrete nitrogenous wastes. The animals must detoxify the most toxic ammonia by converting it into relatively less toxic form like urea or uric acid. Mammals, including humans, rabbits, etc. produce urea, whereas the reptiles and many terrestrial invertebrates including all the insects produce uric acid as their primary nitrogenous waste. Urea is the main by-product of ammonia metabolism in the vertebrate animals. Animals excreting urea is called uerotelic animals. Urea is soluble in water and is relatively less toxic than ammonia. Uric acid is the least toxic, almost water-insoluble and tends to form a white paste or powder or semisolid form. Uric acid is excreted by birds, insects, and most of the reptiles. These animals are referred to as uricotelic animals. The conversion of ammonia to uric acid requires the maximum energy and is a much more complex process when compared to the conversion of ammonia to urea.