Excretion: choosing the right nitrogen waste

A fish dumps toxic ammonia straight out through its gills into the river, no processing needed. Do that in a human body and the blood would be poisoned within minutes. Same nitrogen waste, two very different solutions.

Proteins and nucleic acids break down and release an amino group, which quickly becomes ammonia. Ammonia is highly toxic, so it must leave the body, one way or another, before it builds up.

What it costs to remove it, in water, decides which product an animal actually uses.