Behavioral Science

The Rat Utopia Experiments That Predicted Humanity’s Dark Future

John Calhoun Inside Universe 25 – The Biggest Mouse Utopia. Credit: Public Domain

Global population estimates exceeded 8 billion people in November 2022. That’s more people than have ever existed! In fact, a jaw-dropping 7pc of people who have ever lived are alive today. Surely, such a startling statistic should be front-page news. Yet it was hardly a blip. The soaring trajectory of human population growth over the past two centuries is now an accepted fact – an ascending rollercoaster without any sense of down.

That wasn’t always so. In the mid-twentieth century, the rampant population growth worried many who feared imminent overcrowding, overpopulation, and societal collapse. Short stories like Billennuim warned of a world of box-like apartments and pedestrian congestion lasting days, where every inch of land is devoted to housing or farming. Known as the neo-Malthusians (after 18th-century scholar Thomas Malthus), they predicted a terrifying future if the rollercoaster of human population growth rose ever higher; they dared to look down.

Ethologist John Calhoun’s Rat Utopia experiments fueled these fears, pointing towards a spiraling degradation of normal social interactions. He coined the term “behavioral sink” to describe the so-called aberrant behaviors resulting from overcrowded population densities. It was the stuff of nightmares – at least to the nervous 1970s audience.

In the mid-twentieth century, the rampant population growth worried many who feared imminent overcrowding, overpopulation, and societal collapse.

Calhoun’s early experiments involved a 28-month study of a colony of Norway rats in a 10,000-square-foot outdoor pen. Beginning with five females, he predicted an exponential population rise leading to a theoretical 5,000 healthy progeny by the experiment’s end. In fact, the population never exceeded 200 individuals, stabilizing at 150. Surprisingly, the rats congregated in groups of roughly a dozen – the rat’s social limit.

Like the brutalist architects of the era, Calhoun’s next experiment was bigger and bolder. Inside the second floor of a huge barn, his team constructed a series of four rooms connected by three bridges, forming a U-shaped space. Inside each room was a drinking and feeding station.

Detailing the shocking findings in a Scientific American article, Calhoun revealed how the effects of density led to a brutal battle among males. In the earliest stages, the male rats struggle for status. Once they fixed their place in the social hierarchy, they ruled among the two rooms with only one bridge – the end rooms. This became a dominant male’s territory where females were kept. Most male rats, therefore, congregated in the middle two rooms.

Early in the morning, before the dominant males awoke, the subordinate males would roam the various rooms. However, if in the middle rooms after he awoke, subordinate males found it exceedingly difficult to reach their original quarters. They were trapped. After several defeats, these males would never attempt to cross the bridge. The dominant male would only tolerate subordinate males so long as they never attempted to mate with the members of his harem – and they never did. Rather these subordinate males would often attempt to mount the dominant male, which he generally tolerated.

Diagram detailing the arrangement of the four-roomed rat habitat in Calhoun’s second series of experiments.

Females living in the middle rooms, meanwhile, fell into a behavioral sink. They became less adept at building nests before stopping completely. Nor did they move their litter to safety when necessary – unlike the females in the harems. Infants were frequently abandoned and would die when they were dropped. High rates of maternal miscarriage and infant mortality followed.

Connections were naturally drawn to the sink estates and neighborhoods arising in the US and UK. But Calhoun was far from done.

Universe 25 would be his most ambitious experiment; involving a vast space suitable for thousands of mice, Calhoun ensured the mice had ample food, water, and nesting material.

Between day 1 and day 315, the population doubled every 55 days, reaching 620 mice. Following day 315, population growth doubled only every 145 days. During this period, normal mice social behavior broke down. Young mice were cast aside by mothers before they weaned; homosexual behavior became common; dominant males struggled to maintain their territory leading to increasingly aggressive female behavior.

As Calhoun described:

“At the peak population, most mice … [waited] to be fed and occasionally attacked each other. Few females carried pregnancies to term, and the ones that did seemed to simply forget about their babies. They’d move half their litter away from danger and forget the rest. Sometimes they’d drop and abandon a baby while they were carrying it.”

In the final stages of mouse utopia, some young male mice never engaged in sex or fighting. They devoted their time solely to grooming, eating, and sleeping, becoming known as “the beautiful ones.” Despite appearances, these maladaptive mice could not cope with the stresses of life.

By day 600, the mice lost all social skills required to mate. Females ceased to reproduce, and the beautiful ones grew in number, engaging solely in solitary pursuits. When Calhoun wrote the experiment, the final breeding male had died – the mouse utopia was marching toward extinction.

While many have criticized the experiment for its social ramifications, it’s hard not to be alarmed. The parallels with modern-day society are stark. Yet social scientists who have attempted to replicate the findings in humans struggled to find any association between social density and behavior. Whatever your opinion, Calhoun started a conversation that very much continues today.


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