I talked to the hobbyist/breeder that I got my collection from earlier today, he said he does the same thing, replaces males every 6-7 months and females every 8 months. The mice I had froze from the winter weather. Not properly insulated rack. My fault. I have 2 male rats and 2 females. Both males are brothers just a little over a year old. One male prefers a solitary life. Kills anything put in with him. So he is just a pet. Females are a few months apart. Just put the breeder male in with younger female. Thank you for advice

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