It depends in part on male vs female; male rats and mice mark territory (and sometimes even their favorite walking paths, so beware if you let them roam) with urine. Back when I kept rats and mice, the female cages would always smell better than the male cages, no matter what I did.
Now I keep pythons, and both male and female cages don’t stink lol. I have to admit that the smell is the biggest thing I don’t miss about having pet rodents.
Or just walk through an old graveyard. There’s a pioneer cemetery near my old place with so many children’s graves. One family gravesite has the mother’s name, the father’s name, a couple of their kids, some young, some adults… and one is just titled ‘babies’.
Like, so many babies died for that mother and father they just put them all in one grave, not even names to remember them by…