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As others pointed out, mice have less nutritional content....more specifically, much less fat. Rats contain a very high percentage of fat that allows speedy growth in snakes. However, I believe you can feed most snakes a different diet and be perfectly ok.....in fact maybe 10 years ago there was an enormous push to feed African Soft Fur rats to BP's and some keepers still do.....but most people abandoned the idea once they discovered ASF's grow very slowly, eat through tubs, are hard to find, and have nasty little attitudes.

So in summary, a BP's diet would naturally be ASF rats, but people fed normal rats instead for convenience reasons while maintaining nutritional value. BUT mice do lack nutritional value so unless the animal won't eat them, there's no logical reason to feed them
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I did mention ASF's actually......