According to what experienced breeders have told me and posted here on BPNet either mice or rats will do just as well. Certainly you'd have to feed multiple mice to get the same volume as a single rat (one adult mouse is about equivalent to a big rat pup). Since no one knows the actual nutrient values of a rat vs a mouse or what exactly is a BP's weekly requirement for specific nutrients all we can go on is experience. Experience has shown that they seem to grow, reproduce, etc. just as well on either rats or mice.