23.6: Comparative Excretory Systems
Animals have evolved different strategies for excretion, the removal of waste from the body. Most waste must be dissolved in water to be excreted, so an animal’s excretory strategy directly affects its water balance.
Nitrogenous wastes are some of the most significant forms of animal waste. Nitrogen is released when proteins and nucleic acids are broken down for energy or conversion into carbohydrates and fats. Proteins are broken down into amino acids and nucleic acids into nitrogenous bases. The nitrogen-containing amino groups of amino acids and nitrogenous bases are then converted into nitrogenous wastes.
Typical nitrogenous wastes released by animals include ammonia, urea, and uric acid. These excretory strategies involve tradeoffs between conserving energy and water.
The various nitrogenous wastes reflect distinct habitats and evolutionary histories. For example, most aquatic animals are ammonotelic, meaning they directly excrete ammonia. This approach is less energy-intensive than converting ammonia into urea or uric acid before excretion, but also requires more water. For terrestrial organisms, which face perhaps no more significant regulatory threat than dehydration, water conservation is worth the extra energy cost.
Ureotelic animals, like mammals and sharks, convert ammonia into urea before excretion. Urea is less toxic than ammonia and requires less water for removal from the body. Many amphibians that move from aquatic to terrestrial habitats excrete ammonia primarily as tadpoles but excrete mostly urea as adults on land.
Uricotelic organisms, including reptiles, birds, and many insects, convert ammonia into uric acid before excretion. Uric acid is not water-soluble and is excreted as a paste or powder, using very little water. Uric acid is even less toxic than urea. However, converting ammonia into uric acid requires even more energy than conversion into urea.
These different excretory strategies allow animals to meet the unique water and energy demands of their environments.