What does utopia look like for mice? According to a researcher who did most of his work in the 1950s through1970s, it might include limitless food (of course), multiple levels and secluded little rodent condos. These were all part of John Calhoun’s experiments to study the effects of population density on behavior. But what looked like rat utopias and mouse paradises at first quickly spiraled into out-of-control overcrowding, eventual population collapse and seemingly sinister behavior patterns.

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Article: Smithsonianmag.com