When mice are put into enclosures with limitless resources, their social behaviour degenerates dramatically.
You always hear about scientists getting mice to run around in some complicated maze in search of a single cube of cheese or pouring burning hot liquids into their eyes. But in 1972 John B Calhoun deployed a seemingly more mouse-friendly experiment called Mortality-Inhibiting Environment for Mice.
He built a mouse’s utopia (a pa-rat-ice ;] ) called Universe 25. It was a 101 square inch tank equipped with 37 inch climbing walls and an abundance of food, water, and nesting material. There were no predators, the temperature was kept at a steady 68 degrees, and all the mice were disease-free.
Naturally, one of the first things the mice did in their enviable new environment was reproduce; the population doubled every 55 days. This seemed fine at first, but eventually more proved to not be so merry; within 315 days the Universe 25 was crowded with 600 mice, each struggling to cope with the overpopulation (there was still an abundant amount of food and water in the Universe, but little space). Soon enough, there were more mice than available social roles.
The mice lost their ability to form social bonds; some male rats would randomly attack the others, some didn’t do anything but eat and groom, females attacked their own nursing young, etc. With all this, the population and consequently, the entire mouse society, collapsed.
As the number 25 will indicate, this wasn’t the first Universe experiment Calhoun had conducted. All the other rat societies he’d experimented with also spiraled into dystopia. In 1962, Calhoun used these results to publish a paper called “Population Density and Social Pathology” in Scientific American in which he stated that; “overpopulation means social collapse followed by extinction.” Well, I guess this is why the Disney Universe only contains 2 mice.