There was a famous fly tier from up-state NY name of Art Flick. His light Hendrickson dry fly called for body dubbing of URINE BURNED Fox. Any one want to guess what color the white belly fur of a red fox turns when it becomes "urine burned"? If you guessed a pinkish color you are correct. This fur was highly prized by dry fly tiers and few, if any, commercial attempts yielded an accurate match.
I have noticed similar discolorations in bitch coyotes from time to time. It is somewhat un common...maybe a dietary issue with certain food stuffs effecting the "staining" quality of urine. This sort of thing (dye / pigment concentration in the urine) is pretty well understood. Various antibiotics, vitamins etc., are cleared so fast that incredible concentrations can be reached in bladder urine. In WWII we captured urine from soldiers being given penicillin, re-purified the penicillin and used it in the next patient.
Hope this helps some.
(note in press; just noticed cuckeo's post after posting since I typed this I'll leave it)