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A common fact is that Ammonia, Urea and Uric acid are the most common excreted metabolites and their removal needs varying amount of water, highest for ammonia (hence suited only for aquatic animals) and lowest for uric acid (suited for desert animals). Why is this so? Is it because of different solubility of these compounds in water or their toxicity? If it is the latter, how would excess water reduce the toxicity of a compound? This is a bit over-simplistic question, but I want to understand the tradeoffs involved in selecting the mode of excretion, ammonotelic, ureotelic, uricotelic.

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you can also look at it in a molecular point of view.. Uric acid has more atoms of nitrogen (4) per mole; then comes urea (2) and then ammonia (1) –  WYSIWYG Dec 6 '13 at 10:58
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Ammonia is more water soluble and also more toxic than either urea or uric acid but it is energetically less expensive. So, in regions where deficiency of water is not a problem, ammonia is an excretory product. Where water is scarce urea or uric acid is the excretory product but it is energetically more expensive to manufacture them. So, you can see an obvious trade off between energy expenditure and water conservation.

You can refer to any introductory biology book like campbell for further reading.

UPDATE :

Ammonia : Most toxic , Easily Soluble , Less Energy Requirement

Urea :Moderately toxic , Moderately soluble , More energy requirement(As needed to be processed from ammonia

Uric Acid:Least toxic , Least soluble , Highest energy requirement ( As needed to be processed from ammonia )

It is obvious now what tradeoffs are involved.

The only tradeoff not deducible from above is that of the eggs/youngs. In birds, eggs are not permeable for water so the best option is to excrete waste in form of uric acid so that high concentration of waste does not build up. Uric acid is excreted in form of solid palette which can stay in the egg without any harm.

We can also think of tadpoles which live exclusively in water and so they excrete ammonia but the frogs live both in land and water and so while on land they excrete urea as dessication is the main problem there.

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it would be nice if you provide some references (or explain a little bit). –  WYSIWYG Dec 6 '13 at 11:02
    
This was helpful, and settles the tradeoffs part. But, Can you elaborate on why ammonia is the most toxic, and uric acid the least? –  Satwik Pasani Dec 11 '13 at 15:08
    
@satwik pasani Well, I asked this here at stack exchange but couldn't get an answer. –  biogirl Dec 11 '13 at 15:39
    
By the way, I do remember reading about ammonia toxicity in Lehninger biochemistry. I will try finding more about it ! –  biogirl Dec 11 '13 at 15:40
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One more thing, I haven't ever even thought about why uric acid is less toxic. Interesting question ! Will search it out. –  biogirl Dec 11 '13 at 15:41
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