What have you been feeding them? I have heard of this happening in conjunction with overly fatty foods being used (lab rats or mice, or jumbo mice). It's due to nutritional issues, when that is the cause.

If they were mine, I would start supplementing with powdered vitamins, and consider a different source for feeders. If you're raising the feeders, what are you feeding your rodents?


It could also be the IMG effect (increasing melanin gene)--are your two animals siblings?