In the late 1960s and early 1970s, ethologist John B. Calhoun conducted a series of experiments on mice. One of the experiments, known as ‘Universe 25’, attempted to understand the impact of overpopulation on behaviour and societal structures, using colonies of mice as his subjects, in a rodent Garden of Eden.
Universe 25 was a carefully designed enclosure measuring nine feet square with 4.5-foot-high sides. Within this space, Calhoun created what was essentially a mouse utopia. The enclosure was divided into four equal sections, each with a central nesting area connected by ramps to multiple food and water dispensers. There were no predators in this world, and disease was minimised due to regular cleaning. With an unlimited supply of food and water, the mice were provided everything they needed to flourish.
But, on day 600 in this mouse paradise, the last baby was born. By day 920 the last remaining mouse died. So what happened to this thriving population?
At the beginning of the experiment, Calhoun introduced eight mice — four pairs of males and females — into this environment. The initial days in Universe 25 were marked by exponential growth, a period Calhoun referred to as the “strive period”. The population doubled approximately every 55 days. By Day 315, Universe 25 was home to 620 mice. This might sound like a big number but it was a mere fraction of the 3,840 the enclosure was designed to house.
However, it was at this point that hierarchies began to form.
Despite the ample resources and having everything they required provided to them, around Day 315, the growth rate began to decline, slowing more than what was expected based on the enclosure’s physical capacity. It was at this juncture that Calhoun observed behavioural changes indicative of a deep-seated societal breakdown.
Male mice, devoid of any need to defend territory or compete for resources, began to display heightened aggression. Violent encounters became common, often directed haphazardly, not only at other males but also at females and juveniles.
Calhoun called this breakdown of social order a “behavioural sink”.
On the other hand, some males, referred to by Calhoun as “the beautiful ones”, opted for complete societal withdrawal. These were mice that had been born into the chaos. They ceased to mate or fight, instead spending their time eating and obsessively grooming. These mice remained healthy and sleek but contributed nothing to the continuation of their society, instead displaying an obsessive focus on self-maintenance. They no longer interacted with their peers, instead preferring to spend all of their time alone. This withdrawal can be viewed as a form of psychological retreat, a response to the stressful social conditions in their environment.