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Biologist John Calhoun built the colony at the National Institute of Mental Health in Maryland in 1968. It was a large pena 4-foot cubewith everything a mouse could ever desire: plenty of food and water; a perfect climate; reams of paper to make cozy nests; and 256 separate apartments, accessible via mesh tubes bolted to the walls. Calhoun also screened the mice to eliminate disease. Free from predators and other worries, a mouse could theoretically live to an extraordinarily old age there, without a single worry. This was called Universe 25.

At the peak population, most mice spent every living second in the company of hundreds of other mice. They gathered in the main squares, waiting to be fed and occasionally attacking each other. Few females carried pregnancies to term, and the ones that did seemed to simply forget about their babies. They'd move half their litter away from danger and forget the rest. Sometimes they'd drop and abandon a baby while they were carrying it.
The few secluded spaces housed a population Calhoun called, "the beautiful ones." Generally guarded by one male, the femalesand few malesinside the space didn't breed or fight or do anything but eat and groom and sleep. When the population started declining the beautiful ones were spared from violence and death, but had completely lost touch with social behaviors, including having sex or caring for their young.
Gradually, the mice that refused to mate or engage in society came to outnumber those that formed gangs, raped and plundered, and fed off their own. The last known conception in Universe 25 occurred on Day 920, at which point the population was capped at 2,200, well short of the enclosure's 3,000 capacity.
An endless supply of food, water, and other resources were still there for the mice, but it didn't matter. The behavior sink had set in, and there was no stopping Universe 25 from careening to its self-made demise. Soon enough, there was not a single living mouse left in the enclosure..

(sources: https://www.smithsonianmag.com/smart-news/how-mouse-utopias-1960s-led-grim-predictions-humans-180954423/; https://www.sciencehistory.org/distillations/mouse-heaven-or-mouse-hell; https://www.victorpest.com/articles/what-humans-can-learn-from-calhouns-rodent-utopia)