Its first study’s result were published in 1962 in Scientific American titled: Population Density and Social Pathology. A second experiment was made in the 1970s with a 101-inch square cage and using mice, reaching a population about 2200 mice. The experiment was stopped when conditions of the rodents became too extreme even for the ethical standards of the time. After the experiment concluded, Calhoun took the 4 healthiest males and females out of the enclosure, but their behavior was altered to the point that they could no longer care for their young as their pups all died shortly after weening.The Probers would also cannibalize their young instead of caring for them. Group 3: The Probers The weirdest of the 3 groups, these rodents were hyperactive and hyper-sexual, pursuing females in heat despite suffering numerous injuries from the dominant rats.Group 2: The Somnabulists This group was simply described as fat, sleek, and healthy-looking, ignoring the other rats and being mostly ignored by them.Group 1: The Pansexuals These rats wouldn't compete for social standing, spending their time rummaging around the enclosure attempting to mate with any other rats regardless of age or gender, and their advances would often go unchallenged.Three groups of male non-dominant rats were defined:.These were called 'the beautiful ones' precisely because they spent a lot of time cleaning themselves.

Some rats just kept themselves isolated from the rest, not interacting with them nor trying to mate, dedicating most of their time to grooming.Some males were frequent recipients of violence that they had to endure because there was no escape. Some males still tried to mate even if that caused them to suffer violence from stronger males.Those who survived were often unprepared to care for their own offspring once they had them. Of those that did breed, strange behaviors started to show, like violently attacking the infants (this is not uncommon for some reason, rodents sometimes eat their offspring) or stopping caring for the offspring long before it was normal.

Some females made groups and created their own territories living together and violently rejecting any male approaching, with no apparent interest in breeding.Interestingly, the dominant males allowed some males to remain, although these males exhibited odd behavior, as rather than try to mate with the females, the males would attempt to mate with the dominant male, and the dominant male would let them. Some dominant males made harems, taking control over several female rats to mate with them.Some of the behaviors observed included:

With time, the rats started to show bizarre conducts. The case was divided in four interconnected rooms able to hold a dozen rats each. As the rats were provided with food, water, nesting materials and protection from predators it was called 'Utopia'. The first experiments were made between 19 using 32 to 56 Norwegian rats in a 10×14-foot case in a barn.

Mouse utopia experiment