Uricotelism

Uricotelism is the excretion of uric acid (and its salts) as the main nitrogenous waste, common in birds, reptiles, insects, and some land snails/crustaceans — but not the most common strategy in all terrestrial animals.

Nitrogen Metabolism and Excretion

Animals catabolise organic molecules to release energy, and atoms from these molecules appear in a variety of catabolic end products. During aerobic catabolism, the three most abundant atoms generated are carbon, hydrogen, and oxygen. The fourth most abundant atom is nitrogen, which is the major constituent of proteins and nucleic acids.

The disposition of nitrogen atoms from catabolism is not as simple as that of carbon, hydrogen, and oxygen, and some of the compounds produced as a result are ammonia, urea, uric acid, creatine, creatinine, etc. The relation between nitrogen excretion and kidney function varies throughout the animal groups.

Uric acid requires considerably more energy per nitrogen atom for its synthesis than urea. The biochemical pathways employed for the synthesis of uric acid or related compounds from protein or nitrogen are complicated. However, they in fact are only relatively small modifications of very ancient and universal pathways for the synthesis of the purine constituents of DNA.

Excretion in Uricotelic Animals

If uric acid is the primary nitrogenous end product, along with dihydrate of uric acid, urate salts, or a mix of all these purine compounds, the animal is uricotelic. All of these organic compounds have low toxicities and are poorly soluble in water, compared to urea or ammonia.

As uric acid has low solubility, cations like Na⁺, K⁺, NH₄⁺, Ca²⁺, and Mg²⁺ can be incorporated into uric acid excrement. Uncertainty exists over the chemical form assumed by these ions, but most likely they present as urate salts.
Regardless, the cations are removed from solution, and this state can appreciably reduce the water demands of excretion. Thus, uricotelic animals excrete semisolid pastes, dry pellets and even powders.
Uric acid and urates not only permit nitrogen to be excreted with little water but also provide advantages during a water crisis, when urine production is curtailed. If a uricotelic animal stops producing urine, uric acid and urates are deposited as precipitates within its body.

Examples of Uricotelic Animals

Birds, lizards, and snakes are uricotelic.
Turtles inhabiting dry terrestrial environments tend toward uricotelism.
Most terrestrial invertebrates employ purine derivatives as their primary nitrogenous end products.
Most terrestrial insects employ uric acid or allantoic acid as their principal nitrogenous excretion. Temporary or permanent storage of purines has been observed in many insects and snails
and in certain land crabs.

Several terrestrial phyla have converged on uricotelism, which testifies to the advantages of poorly soluble nitrogenous end products for terrestrial existence.

Uric Acid Metabolism and Excretion In Humans

Uric acid is formed due to the catabolism of purines (obtained from the diet). Humans are primarily ureotelic, but also excrete uric acid in measurable amounts (~0.2–1 g/day), which can cause gout/kidney stones when elevated. Defective metabolism of uric acid results in the following disorders -

Gout: Excess uric acid is accumulated as crystals that deposit in the joints, causing a painful arthritis (inflammation of joints).
Renal Calculi or Kidney Stones: Crystals of uric acid precipitate in kidney tubules, causing their blockage and leading to renal colic pain. In some cases, stones can occur in the urinary bladder, causing painful urination and the presence of blood in the urine.

Frequently Asked Questions (FAQs)

Q1. Why are mammals not uricotelic, even after being terrestrial?

A. Without known exception, all mammals are ureotelic, even though terrestrial organisms prefer uricotelism. There is no absolute answer to this riddle, but the probable hypotheses are-

The elaborately developed countercurrent multiplication system of the mammalian metanephric kidney has a unique trait for concentrating urea to a greater extent, accompanied by low water loss. Thus, the selective advantage of uricotelism has been blunted.
The reverse argument is that for some reason, mammals were unable to evolve the biochemical and physiological attributes required for uricotelism. Being tied to ureotelism, the mammals experienced great selective pressures to evolve exceptional urea-concentrating abilities.

Q2. What is Guano?

A. Guano is the uricotelic excrement of birds and bats, which represents the white coloured part of their faecal matter. Guano can be used as a fertiliser as it contains high nitrogen, phosphorus and potassium.

Q3. How does uricotelism conserve water?

A. Uric acid is poorly soluble and excreted as a paste or solid, requiring minimal water compared to ammonia or urea.

Q4. Which animals show all three strategies (ammonotelism, ureotelism, uricotelism)?

A. Some amphibians/fish switch between ammonotelism and ureotelism; reptiles/insects rely on uricotelism. No group exhibits all three strategies fully.

Q5. Why is uric acid considered both beneficial and problematic?

A. In animals, it conserves water and reduces toxicity; in humans, excess uric acid leads to disorders like gout and kidney stones.