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.
Females weren’t immune to these behavioural shifts either. They became hyper-aggressive, excessively protective of their offspring, and would sometimes attack other mice encroaching on their space, often leading to injury or death. Paradoxically, these hyper-protective females also started neglecting or outright abandoning their nests and offspring, which significantly increased the infant mortality rate.
Calhoun said the mice were “trapped in an infantile state of early development”.
Even though Universe 25 was merely half full, the society was on a downward spiral towards extinction. Calhoun identified this period as the “death phase”. Mice no longer wanted to mate and fertility dropped. The birth-rate fell dramatically and the society as a whole became listless and inactive. The final birth in Universe 25 was recorded on Day 600, and by Day 920, the last resident of this mouse utopia had died, marking the end of the Universe 25 experiment.
He repeated these experiments over and over again, always with the same result.
Calhoun’s Universe 25 experiment sent ripples through the scientific community and beyond. It provided a stark picture of a society imploding due to overpopulation, leading to chaos, societal breakdown and ultimately extinction. Some interpreted it as a warning about the potential fate of humanity in the face of unchecked growth and resource consumption.
Some might argue, exactly what the neo-Malthusians wanted people to worry would happen!
Others argued that Universe 25 was less about overpopulation and more about a lack of social roles and stimuli. Calhoun’s mice had no challenges to overcome, no new territory to explore and no threats to unite against. Their existence was reduced to eating, drinking and grooming, devoid of any meaningful engagement or social structure.
Do you see parallels with today’s society? Are we living in Universe 25? Or could it be that neo-Malthusians, alarmed by the experiment’s outcomes, felt compelled to intervene and prevent a hypothetical societal collapse, one that, in reality would never happen in a far more complex society of humans?
This piece was originally published on the Naked Emperor’s Substack. You can subscribe here.
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