I figured out what kind of rats I have as well: 3 hooded females and the rest are REW. Are the REWs prone to megacolon? The sites I saw on megacolon named the blaze and ones with white areas to be the most prone to it, but since apparently megacolon development is linked to pigment migration, I would assume that there's just no pigment in the REWs, not that it is a genetic issue?

Thanks for the advice on looking up megacolon: that information is definitely good to know!