In July 1968 four pairs of mice were introduced into the Utopian universe. The universe was a 2.7 m square metal pen with 1.4 m high sides. There was no shortage of food or water or nesting material. The only adversity was the limit on space.
Initially the population grew rapidly and reached 620 by day 315, after which the population growth dropped markedly. The last surviving birth was on day 600. This period between day 315 and day 600 saw a breakdown in social structure and in normal social behavior. After day 600, the social breakdown continued and the population declined toward extinction.
During this period females ceased to reproduce. Their male counterparts withdrew completely, never engaging in courtship or fighting. They ate, drank, slept, and groomed themselves – all solitary pursuits. Sleek, healthy coats and an absence of scars characterized these males.
The conclusions drawn from this experiment were that when all available space is taken and all social roles filled, competition and the stresses experienced by the individuals will result in a total breakdown in complex social behaviors, ultimately resulting in the demise of the population.