The good mixed with the bad.

[Mike Cavanaugh]

Hey i just seen this on another forum, "If I find that I need to replace a female..or male...I only do this when I clean the tub. By adding the new rat and the rest of the colony to a clean tub with new fresh bedding...they do not fight with the new cage member. "
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I have learned first hand that this is NOT normally true. The established group will fight and possibly kill the newcommer.

The reason I was suggesting to feed off the moms is because unless i misunderstood, you just got them and don't know their age. it is better to have a breeding group all the same age... and it is best to keep track of that age.