Types of Nitrogenous Waste

Metabolic processes produce waste, and a great deal of this is in the form of nitrogen.
This combines with water to produce toxic ammonia.

Animals must safely excrete these toxins, which is the function of the excretory system.


  • contains only one nitrogen per molecule
  • is highly toxic
  • requires large amounts of water for dilution and flushing from the body.
  • Animals that have constant access to fresh water, such as freshwater fish and some amphibian larvae, excrete much of their nitrogenous waste as ammonia.


Terrestrial animals have less access to water, and have been under selective pressure to (1) repackage ammonia as less toxic compounds and (2) flush those compounds with less water. Urea...
  • requires metabolic energy to build (as opposed to ammonia)
  • contains two nitrogen atoms
  • is less toxic than ammonia
  • requires less water to dilute and flush from the body
  • The primary nitrogenous waste product in mammal urine is urea.
  • Many marine fish (particularly sharks) convert ammonia waste into urea.

Uric acid

Animals that evolved in very dry environments were selected to excrete nitrogen using even less water. Uric acid...
  • requires more metabolic energy to build than either ammonia or urea
  • contains four nitrogen atoms per molecule
  • has very low toxicity, so does not need to be diluted
  • is not readily water soluble, so requires little water for flushing from the body
  • is the primary nitrogenous waste product of reptiles (including birds).

The white, crystalline substance you see in a typical bird or lizard poop is actually urine: a little pool of uric acid crystals in a very small amount of water. The brown part is the fecal matter.

The evolutionary history and natural selective pressure of any given taxon of vertebrates contributes to which method the animals use.

Which environments would you suspect to exert selective pressure for each of the three types of excretion above?