Nitrogen waste leaves as ammonia, urea or uric acid, depending on how much water an animal can spare. Fish flush out soluble ammonia in water, we humans make less toxic urea, and birds excrete dry uric acid to save weight. It’s all about the body’s balance of water and poison.
I think of my sewing machine—on rainy days, with no power, I cannot run it, so I do hand-stitching instead. The body, too, changes its design based on what resources it has. Water is like electricity here; when it’s plentiful, you use one method, when it’s scarce, you find another.
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