I separate mine. I have 4 females and one male. When a female is clearly pregnant or births a litter (if i didnt notice pregnancy) i separate her and leave her separated till the litter is weaned. Then back in with the group she goes and the pups get separated by sex. I do this with a total of 6 tubs. I always have different sizes available and have yet to run out of space. Ive actually had to keep the male separated longer lately as i only have 4 BPs and am producing more rats than i really need. Ive yet to have any little killed off.

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