following his earlier experiments with rats, in 1972 calhoun would later create his "mortality-inhibiting environment for mice": a 101-inch square cage for mice with food and water replenished to support any increase in population, which took his experimental approach to its limits. in his most famous experiment in the series, "universe 25", population peaked at 2,200 mice and thereafter exhibited a variety of abnormal, often destructive behaviors. by the 600th day, the population was on its way to extinction.