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. "

I know when i was younger, we had puppies that we fostered to another mom dog. When we did it we put baby powder on them and put them all together and the mom did not know which ones were hers. I have not tried this yet put could be a possibility.