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It occurred to me (while urinating) that this would seem to be selected against because water is a scarce resource. Why are we constantly losing water we don't need to through urination? What is it about the chemistry of urine and the waste products eliminated that make urination necessary as opposed to eliminating them through defecation and recovering the water on the way out?

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Actually this process (in the kidney) helps saving a lot of water. The primary urine is concentrated about 200x before it is excreted into the urinal bladder. –  Chris Apr 17 at 15:43
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1 Answer 1

Toilets have always been a great place to think about biology, I agree $\ddot \smile$.

In short, urine contains the waste from our blood while defecation is just the stuff that we haven't digested. Kidneys are the organs responsible for draining wastes (mostly nitrogen-containing, or nitrogenous, wastes) from our blood.

You're correct that the loss of water through urination is a considerable cost for an organism (especially those living in dry environments). But the amount of water used to excrete nitrogenous wastes is negatively correlated with the energy it costs to perform this excretion. In other words, there is a trade-off between water and energy loss during nitrogen excretion. Also the question of toxicity is important.

Animals basically have three choices to excrete nitrogenous wastes:

  • Uric acid (excreted by uricothelic organisms)

    • Solid (crystal) with low water solubility
    • Low toxicity
    • Little water is needed
    • Lots of energy is needed
  • Ammonia (excreted by aminothelic organisms)

    • Highly soluble in water
    • High toxicity
    • Lots of water is needed to dilute it because of the toxicity
    • Not much energy is needed
  • Urea (excreted by ureothelic organisms)

    • Solid but highly soluble in water
    • "medium" amount of water is needed
    • "medium" toxicity
    • "medium" amount energy is needed

Not only the amount of available water in the environment should be considered in order to understand why a species is ureothelic, aminothelic or uricothelic. For example, birds are uricothelic (uric acid) probably because during the development of the embryo the wastes cannot be excreted outside the egg and therefore, excreting uric acid allows to greatly decrease the toxicity of the embryo's environment.


Note: This domain is not at all my field of study and I have no reference, typically for what concerns the toxicity, water amount and energy amount. One should not take my words for granted!

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This is an interesting answer. But it doesn't explain why nitrogen can't be excreted in solid or gaseous, rather than aqueous, form. –  ThePopMachine Apr 17 at 18:30
    
@ThePopMachine Well, Amonia is a gaz, Urea is liquid and Uric acid is solid (it makes cristals if I'm not mistaken). So, yes it can be excreted through solid form but it cost lots of energy (ATP). But you're right that my answer does not explain why it is more costly. But I know nothing about physiology and I can't give this answer $\ddot \smile$ I'll let someone else to give info about that if you're waiting for them. –  Remi.b Apr 18 at 11:22
    
@ThePopMachine You might want to create another post and to ask a more specific question about the physiology of Nitrogen excretion and why different solutions (Ammonia, Uric acid and Urea) are differentially costly in terms of energy. Consider this post as it had answered the evolutionary part of the question. –  Remi.b Apr 18 at 11:26
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