The ‘Universe’ was surrounded by 16 tunnels leading to food, water and burrows. No predators, no scarcity, the mice would have to be blind to not see the utopia around them. At least it began as Utopia. Four breeding pairs of mice were introduced into Universe 1. After 104 days they adjusted to the new world and the population began to grow, doubling every 55 days. By day 315 the population reached 620. Then is stopped. The population grew much more slowly as the mice came against the limit of space, their only limiting frontier.
Society broke. Young were expelled before they had been properly weaned and were arbitrarily attacked by excessive aggressive male mice. Females became more aggressive, non-dominant males became passive, not retaliating to attacks. The last healthy birth came on the 600th day. Then there were no new mice. Then there were none.
I thought this was an unbelievable cool, frightening, depressing, and wild article about when a scientist, John B. Calhoun, tried to simulate over-population. The full article is here at mostlyodd.com which I believe I found via Surrogate Self. There’s also an online PDF about the experiment called Death Squared here. I love that he called the barn the rats lived in “Universe”. Creepy.