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[Mighty Mouse]

Just to go back to the original question about housing mice. If you put male mice that are not familiar with each other together, especially if they have had the opportunity to establish themselves in a cage, you will get a lot of fighting. Ideally, when mice are weaned, you would put them with brothers. However, a dominance hierarchy eventually develops, and can sometimes lead to very aggressive mice. If you get mice shipped in, this can also happen. We have had times when there is an increased incidence of biters that we got from Jackson Labs for whatever reason. It's so hard to know with animals. But once cages are established I would strongly advise against mixing males from different cages as they will likely fight and attempt to establish dominance (even with enrichment).