Rats and mice when cramped together will kill and eat each other.
My wife is from Eastern Ukraine. Everyone lives cramped together in apartments over there --drab, concrete apartments that don't look any different that the WWII-era housing complexes that were built in our inner cities. They'll reside in the same apartment well into their mid 20s until they get married and move into their own drab, concrete apartment --likely in the same building.
They aren't killing each other.