Urine stained chin???

I just noticed that my girls (about 4 months old) have urine colored bottoms. It's a pretty wide area around their privates. Is this normal? Anything I can do about it? They're both mosaics so it's pretty easy to tell. My last girl was ebony so I don't know if she was like that too & I just didn't notice.



I have the ferret nation cage with fleece lined trays & a Pyrex dish with pine shavings for their litter box, if that makes any difference.