Imagine a world with infinite food, water, and shelter, no threat of death by hunger, disease, or predator. Sounds like paradise. In 1968, one scientist built exactly that but for a colony of mice. Within four years, they were all dead.
In the summer of 1968, American scientist John B. Calhoun set out to create a controlled mouse habitat at the National Institute of Mental Health in Bethesda, Maryland. In a 1973 research paper, ‘Death Squared,’ published in the Proceedings of the Royal Society of Medicine, Calhoun detailed how he constructed a pen with four metal walls 54 inches (about 4.5 feet) high and a square with 101 inches (approximately 8.4 feet) sides. Inside were 256 mouse “apartments,” food hoppers that were always full, water that never ran out, and endless nesting material, with no predators, no bad weather, and mice screened for disease beforehand. Calhoun called it the "Mortality Inhibiting Environment for Mice," and this specific run was dubbed "Universe 25."
The paper notes he brought eight mice, four males and four females, on July 9, 1968, and waited to see what would happen when survival was not a worry.
The boom nobody could stop, at first
The paper says the first 104 days were mainly spent with the mice getting to know each other. The population exploded when litters arrived, roughly doubling in number every 55 days, from about 20 mice to more than 600 in a year. Within the pen, small “neighborhoods” developed, each centered around a dominant male, his females and their young. The paper says the civilization seemed to be doing well until about day 315, when the growth suddenly slowed to a doubling time of about 145 days.
