why birds excrete uric acid

Drinking Water is vital to all living things, and birds depend on it to survive. Birds do not have specialized sweat glands, but they still lose water through their skin and evaporation from the moist lining of their lungs during breathing. Water is also lost when waste products are excreted. The rate of water loss depends on several factors. A key one, of course, is weather. Water loss is significant in hot, dry weather because birds use evaporation to stay cool. Another factor is the size of the bird. Due to their larger surface area relative to volume, small birds lose water more than large birds do. This phenomenon is similar to heat loss in small birds. The amount that a bird flies compared to how much it rests, for example, will determine how quickly water is lost. It may help to explain the apparent “sky-pointing” behavior in long-billed species like curlews. Most birds drink to make up for the loss, and they do so by dipping the bill and then tipping the head back to let the water run down into the throat to be swallowed. Many small birds use dewdrops as a source of water. When it rains, pelicans will occasionally open their beaks to drink. Doves, pigeons, and northern fulmars drink more like horses do—they submerge their bills and suck up the water. Not all land birds need to drink water, however. Due to their mostly liquid nectar diet, hummingbirds typically experience flooding rather than dehydration. Birds in certain areas may either never drink at all or go for extended periods of time without drinking. Like all living things, they accomplish this in part by producing water during the process of “burning” their food (cellular respiration). They also store water and get it from their food, which includes some water even in dry seeds. The primary job of a bird’s kidneys is to eliminate nitrogen-containing waste products from the blood that are produced when proteins break down, all the while preserving the ideal ratio of water to salt and other substances in the body. In certain habitats, birds are able to eliminate these waste products with minimal water in their urine. The majority of mammals excrete these wastes mostly as urea, which is a highly toxic substance that needs to be diluted with a lot of water. Uric acid is excreted by birds and is relatively nontoxic, nearly dry, and does not dissolve easily in water. However, compared to mammals, birds require a lot more energy to produce uric acid than they do urea. Thus they pay a price for their efficient water retention. Reptiles expede uric acid and incur a high energy cost, just like birds do. It is likely that the evolution of uric acid excretion in both groups was necessary for the laying of terrestrial eggs. Ammonia and urea, two water-soluble nitrogen compounds, can be transferred from fish and amphibian eggs into the water they are submerged in. In order to prevent self-poisoning, reptile and bird embryos must store their nitrogenous wastes inside the egg and produce uric acid. Reptiles and their avian offspring were able to access enormously expanded terrestrial habitats, making the embryo’s energetic cost of producing uric acid a good deal. Then, evolution merely appropriated its “invention” for use in adult reptiles and birds as well. See also: Hummingbirds, Water, and Nectar; Metabolism; Eggs and Their Evolution; Temperature Regulation and Behavior Copyright ® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl Wheye.

Proteins and nucleic acids are two of the four main macromolecules found in biological systems that contain nitrogen. Carbon, hydrogen, and oxygen are taken out of nitrogen-containing macromolecules during their breakdown, or catabolism, and are then stored as fats and carbohydrates. Excess nitrogen is excreted from the body. The pH of bodily fluids is raised by toxic ammonia, which is typically formed by nitrogenous wastes. Large amounts of water and energy in the form of ATP are needed to dilute the ammonia that is formed in a biological system.

Purines, which are present in nucleic acids, are analogous to uric acid. It tends to form a white paste or powder and is insoluble in water. When compared to the processing of other nitrogenous wastes like urea (from the urea cycle) or ammonia, the production of uric acid requires a complex metabolic pathway that is more energy-intensive; however, it has the benefit of minimizing water loss, which lowers the requirement for water.

Terrestrial animals have developed unique systems to remove the hazardous ammonia from their systems, whereas aquatic animals can readily release ammonia into their aquatic environments. The animals have to transform ammonia into a form that is comparatively nontoxic, like urea or uric acid, in order to detoxify it.

Uric acid is also less toxic than ammonia or urea. It has four nitrogen atoms, so excreting it just requires a tiny amount of water. Out of solute, it precipitates and forms crystals. Xanthine and hypoxanthine, which are derived from other purines, are converted into uric acid by the enzyme xanthine oxidase. The large enzyme xanthine oxidase has molybdenum bound to sulfur and oxygen in its active site. Uric acid is released in hypoxic conditions.

The majority of terrestrial arthropods, including insects, birds, and reptiles are referred to as uricothelic organisms because they instead of urea, convert toxic ammonia to uric acid or the closely related compound guanine (guano). Mammals, including humans, on the other hand, use ammonia to produce urea; however, during the breakdown of nucleic acids, they also form some uric acid. Here, as in the case of birds and reptiles, uric acid is expelled in urine rather than feces.

FAQ

Why do birds excrete uric acid as their nitrogenous waste?

Uric acid is excreted by the animals which conserve water, at least during parts of their life cycle, such as birds, terrestrial reptiles, insects and some snails. In this case ammonia is converted into uric acid because it is less toxic than ammonia but it is insoluble in water.

Why would birds need to excrete urine as uric acid as opposed to urea when it is so energetically expensive to make?

Uricotely in Birds and Reptiles This pathway is much more energy expensive than the urea cycle of mammals with the main advantage being the excretion of relatively insoluble nitrogen-rich uric acid which is not reabsorbed from the cloaca in the adult or from the allantois in the avian embryo.

What is the benefit of discharging uric acid as main excretory product in the birds?

Uric acid as metabolic waste Organisms that excrete nitrogen primarily as uric acid (uricotelics), such as birds and some terrestrial reptiles, are excellent water conservers (14, 15), because they excrete some or all uric acid as crystals (the white color in bird droppings).

Why do insects excrete uric acid?

Although a few insect species, discussed below, excrete nitrogenous waste as ammonia, most insects are uricotelic and excrete compounds with low solubility, such as uric acid, or allantoin, as a means of conserving water.