I am aware of the different ways animals excrete excess nitrogen (mainly ammonium, urea, uric acid). My question is: why do we excrete excess nitrogen instead of recycling it? How can we explain that there has been no selection for more nitrogen recycling in animals, as this would probably constitue a selective advantage?
I have read here that "In the quest for sufficient food energy to meet caloric requirements, animals ingest more nitrogen, largely as amino acids, than they require. Accordingly, the excess nitrogen ingested must be excreted in some form.". This is exemplified by this paper: a low protein diet comes with lower urea levels.