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Could a desert animal, with the right symbiotic microbes, avert the need to urinate by creating ammonia and having bacteria in its blood/kidneys convert it into nitrate and then back into amino acids? Or is nitrification too slow a process to ever entirely replace urination?

If required, the animal can spare some of its own energy to allow the symbiotes to adapt for nitrification specifically. The animal lives is humanoid physiologically, and eats approximately the same amount of calories as modern people. The symbiotes' nitrification processes cannot be more efficient than is seen in reality, but they can do the processes more if energetically feasible

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  • $\begingroup$ I'm sure there's chemistry that would break down ammonia for reuse as proteins (although I've my suspicions about biological viability), but I'm wondering about the value of avoiding urination. You could redirect all the other poisons to defecation, but how do you avoid water intoxication? $\endgroup$
    – JBH
    Feb 20, 2023 at 5:05
  • $\begingroup$ I've heard this phenomena has been at least once described in a human, so the answer is a technical yes. And water levels could be regulated by breathing, and possibly by sweating, a desert animal might require extra cooling to actually mitigate the need of lowering internal water level by urinating. $\endgroup$
    – Vesper
    Feb 20, 2023 at 7:23
  • $\begingroup$ I removed the Nitrogen tag because (a) this is the only question that uses it and (b) you don't use the word anywhere else in the post. (It'll be auto-deleted in a couple of weeks.) Generally, it's not preferred to invent new tags unless you feel confident that they will be used frequently. Considering there are 118 elements currently in the Periodic Table, it's unreasonable to have 118 new tags. If you disagree, that would make a good meta question for community consensus. $\endgroup$
    – JBH
    Feb 21, 2023 at 4:30
  • $\begingroup$ You could just vent ammonia to the atmosphere, ammonia boils at about -33 C so it could just be vented. Ammonia is a small component of the waste products in human urine, I would assume similar low-levels in most animals. $\endgroup$ Feb 21, 2023 at 4:51
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    $\begingroup$ there are much MUCH easier way to make it so an animal does not need to urinate. many animals do not urinate. $\endgroup$
    – John
    Feb 21, 2023 at 22:06

2 Answers 2

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The first and obvious problem is that urination removes many toxic chemicals from the body, only two of which are ammonia or urea. At best, this plan can reduce the rate of urination by reducing how much waste needs to be removed.

The second problem is that what you are describing is basically the entire nitrogen cycle taking place inside a single organism. Although this is possible, it is rather incredulous. You see, any chemical cycle uses energy. The laws of thermodynamics simply require it. This means that the organism would need less water, but more food.

There are animals that make this trade-off, such as when bears enter hibernation [1, 2]. However, bears can only do this because during the summer they can eat enough to build huge fat stores. If your desert had a highly predictable rainy season, then your animals copy the bears life history by 'hibernating' through the dry season and eating large amounts during the rainy season. However, that environment sounds more like a particularly harsh savanna to me than a proper desert.

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    $\begingroup$ Maybe at the cost of loads of food, you could concentrate the nitrates to absurd levels and sweat enough out to remove the toxins that absolutely must leave. $\endgroup$
    – user86462
    Feb 20, 2023 at 8:52
  • $\begingroup$ In a desert, you don't have "loads of food". There is barely any primary production in deserts, so no matter what type of food this creature eats it simply cannot consume enough to justify spending so much energy on recycling nitrogen compounds. $\endgroup$
    – E Tam
    Feb 23, 2023 at 3:36
  • $\begingroup$ @E Tam Yes, but its waste levels are proportionally lower too. There's just less metabolism going on. 'Loads' in this case means 'An expensive fraction of what you eat'. It potentially saves water, remember. $\endgroup$
    – user86462
    Feb 23, 2023 at 6:03
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Half a frame challenge:

Use uric acid.

Forming uric acid (birds, reptiles) rather then urea(mammals) is a viable option to reduce water output upon excretion. Uric acid is more expensive metabolically then urea but its plus point its much less water loss/use.

The whole point of excreting urea./urea is to remove excess nitrogen that is not currently required. Presumably because every meal has excess Nitrogen, which would negate the need for recycling of Nitrogen.

This is not recycling as per question, but it is avoiding urination portion with desired reduction of water use.

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  • $\begingroup$ Not recycling but it achieves the goal, animals that use uric acid don't need to urinate. Urination is mostly only widespread in mammals. $\endgroup$
    – John
    Feb 21, 2023 at 22:05

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