Behavioral Sink: The Overpopulation Experiments Of John B. Calhoun

Paul Gwamanda
8 min readMay 14, 2021

Back in the 60s, American ethologist John B. Calhoun conducted a series of behavioral studies on captive mice that inhabited a nine-square-foot enclosure.

Within this enclosure known as Universe 25, several pairs of mice were allowed to breed, the conditions were simple:

  • No shortage of food, water and nesting material.
  • No predators.
  • Limited opportunities for transmissible disease.
  • The only adversity: space limitation — the size of the habitat was predicted to host 3840 mice.

When the study was completed, it confirmed Dr. Calhoun’s existing hypothesis: Overpopulation spawns a breakdown in social functions. That is, overpopulation inevitably leads to extinction.

Though wildly controversial when first made public, Calhoun’s theory has raised concern over the years that the social breakdown of Universe 25 could ultimately serve as a metaphor for the trajectory of the human race.

Consequently, the “rodent utopia project” has been a subject of interest among architects, city planning councils and government agencies ever since.

Early Rodent Studies

Calhoun began his experimental research on rodents in 1947, when he studied an enclosed group of Norway rats at a barn in Rockville, Maryland. Supplying the critters with unlimited food and water, he expected to see their population swell to 5,000 over the course of the 28-month experiment. However, the population capped out at 200 after subdividing into smaller groups, each of which comprised merely a dozen individuals.

Continuing with these studies during the 1950s, Calhoun set up a more complex enclosure to examine how further groups of rodents would behave in a sterilized, predator-free environment.

A Video record of his experiments show Calhoun with a pleased smile and a pipe in his mouth, with color-coded mice scurrying over his boots.

Over the course of these experiments, the same sequence of events would transpire each time:

  • The mice would meet, mate and breed in large quantities.
  • Eventually a leveling-off would occur.
  • After that, the rodents would develop either hostile and cliquish or passive and anti-social behaviors.
  • The population would eventually trail off to extinction.

In 1962, Scientific American published Calhoun’s observations from his research in the article “Population Density and Social Pathology,” wherein he coined the phrase “behavior sink” to describe the results of overcrowding — namely the breakdown of social functions and the collapse of populations — in the enclosed rodent environment. The article hit the public just as vast urban expansion saw growing numbers of college graduates flocking to large cities for work opportunities.

The new habitat:

Expanding on his earlier research, Calhoun devised his ultimate experiment. In Universe 25, a population of mice would live within a 2.7-square-meter enclosure consisting of four pens, 256 living compartments and 16 burrows that led to food and water supplies.

With a plague-free environment, a plenitude of comforts, a lack of predation and an unlimited supply of consumables, the mice would enjoy all the luxuries equivalent to modern human life. Calhoun initiated the experiment with four pairs of healthy mice, which were set loose into the enclosure to begin the new society.

During the first 104 days — a phase Calhoun dubbed the “strive period” — the mice adjusted to their new surroundings, marked their territory and began nesting. This was followed by the “exploit period,” which saw the population double every 55 days. By the 315th day, Universe 25 contained 620 mice.

Despite the abundance of space throughout the enclosure — each compartment could house up to 15 individuals, and the overall enclosure was built for a capacity of 3,000 — most mice were crowding select areas and eating from the same food sources. The act of eating, as it turned out, came to be viewed as a communal activity, which caused most of the mice to favor the same few compartments.

All of this huddling, however, led to a drop in mating, and the birthrate soon fell to a third of its former level. A social imbalance also took place among the mice:

  • One-third emerged as socially dominant.
  • The other two-thirds turned out less socially adept than their forbearers.
  • As bonding skills diminished among the mice, Universe 25 went into a slow but irreversible decline.

The society

By Day 315, behavior disparities between males of high and low status became more pronounced. Those at the bottom of the pecking order found themselves spurned from females and withdrew from mating altogether. Having no roles to fulfill within the society of mice, these outcast males wandered apart from the larger groups to eat and sleep alone — and sometimes fight among one another.

The alpha males, by contrast, became more aggressive and pugnacious, often launching into violence with no clear provocation or motive. At times, these males would roam around and indiscriminately rape other mice, regardless of gender. Meanwhile, the beta males — those ranked between the aggressive alphas and outcast omegas — grew timid and inert, and often wound up being the passive recipients of violence. In several instances, bloodbaths ended with a cannibalistic feast for the victors.

With male mice abandoning their traditional roles in Universe 25, the females were left to fend for their nests. Consequently, many females adopted more aggressive forms of behavior, which would sometimes spill over into violence toward their young. Others would refrain from motherly duties altogether, banishing their unraised litters and withdrawing from further mating, resulting in serious consequences:

  • In some compartments, the infant mortality rate topped 90 percent.
  • Calhoun named this the “stagnation phase,” alternately known as the “equilibrium period.”
  • He attributed the overly aggressive and passive behavioral patterns to the breakdown of social roles and rampant over-clustering.

A Spike in Mortality

By the 560th day, the population increase had ceased altogether as the mortality rate hovered at 100 percent. This marked the start of the “death phase” — aka the “die period” — in which the rodent utopia slid toward extinction. Amidst the violence, hostility and lack of mating, a younger generation of mice reached maturity, having never been exposed to examples of normal, healthy relations. With no concept of mating, parenting or marking territory, this generation of mice spent all of their waking hours eating, drinking and grooming themselves.

In reference to their perfected, unruffled appearances, Calhoun called these mice the “beautiful ones.” Living in seclusion from the other mice, they were spared the violence and conflict that waged in the crowded areas, yet made no social contributions.

According to Calhoun, the death phase consisted of two stages: the “first death” and “second death.” The former was characterized by the loss of purpose in life beyond mere existence — no desire to mate, raise young or establish a role within society. This first death was represented by the lackadaisical lives of the beautiful ones, whereas the second death was marked by the literal end of life and the extinction of Universe 25.

The beginning of the end

Extending on his observations of the beautiful ones, Calhoun later opined that mice, as humans, thrive on a sense of identity and purpose within the world at large. He argued experiences such as tension, stress, anxiety and the need to survive make it necessary to engage in society.

When all needs are accounted for, and no conflict exists, the act of living is stripped to its barest physiological essentials of food and sleep. In Calhoun’s view:

  • Herein is the paradox of a life without work or conflict.
  • When all sense of necessity is stripped from the life of an individual, life ceases to have purpose.
  • The individual dies in spirit.

Gradually, the mice that refused to mate or engage in society came to outnumber those that formed gangs, raped and plundered, and fed off their own. The last known conception in Universe 25 occurred on Day 920, at which point the population was capped at 2,200, well short of the enclosure’s 3,000 capacity.

An endless supply of food, water and other resources were still there for the mice, but it didn’t matter. The behavior sink had set in, and there was no stopping Universe 25 from careening to its self-made demise. Soon enough, there was not a single living mouse left in the enclosure..

Extinction event

Before the rodent utopia imploded entirely, Calhoun removed some of the beautiful ones to see whether they would live more productive lives if released into a new society, free of social strife and carnage. Placing these mice in a fresh setting with few pre-existing residents — a scenario similar to that which greeted the initial pairs placed in Universe 25 — he expected the beautiful ones to awake from their asocial haze and answer nature’s call to populate the barren environment.

However, the relocated mice showed no signs of change from their earlier behavioral patterns. Refusing to mate or even interact among their new peers, the reclusive mice eventually died of natural causes, and the fledgling society folded without a single new birth.

In Calhoun’s view, the rise and fall of Universe 25 proved five basic points about mice, as well as humans:

  • The mouse is a simple creature, but it must develop the skills for courtship, child-rearing, territorial defense and personal role fulfillment on the domestic and communal front. If such skills fail to develop, the individual will neither reproduce nor find a productive role within society.
  • As with mice, all species will grow older and gradually die out. There is nothing to suggest human society isn’t prone to the same developments that led to the demise of Universe 25.
  • If the number of qualified individuals exceeds the number of openings in society, chaos and alienation will be the inevitable outcomes.
  • Individuals raised under the latter conditions will lack any relation to the real world. Physiological fulfillment will be their only drive in life.
  • Mice are dependent on a set of complex behaviors to survive, qualities such as empathy and compassion are vital to the continuance of their species. Likewise, man’s concern for others is vital to his continuance as a species, and the loss of these attributes within a civilization could lead to its eventual collapse.Sources:



Paul Gwamanda

“Either write something worth reading, or do something worth writing.” Ben Franklin