4 pairs of females, or 5?

[Stewart_Reptiles]

How long do female rats live/breed for? Not sure when to get new breeders ready

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Optimum production is variable, as rule rather than use age I determine retirement based on the number of rats produced anything under 8/10 and it's retirement, since they all retire at different stage every time one retires one is being added (sometime 2 depending on the season)