American ecologist and animal behaviorist John Calhoun conducted a series of experiments on rodents to explore the effects of population density on social behavior, even when all essential resources were available. His most famous experiment, known as “Universe 25”, was carried out around 60 years ago. Calhoun created a utopian environment for a colony of mice, one that was free from disease and predators, with unlimited access to food, water, nesting materials, and more than enough space to support a population far larger than the initial group.
The experiment unfolded in several phases:
1. Strive period: The mice adapted to their new environment, began reproducing, and making use of the resources.
2. Exploit period: Lasting from approximately day 105 to day 315, the population grew rapidly, doubling approximately every 55 days.
3. Stagnation phase and the emergence of the Behavioral Sink: This phase lasts about 240 days, during which population growth slows dramatically despite the continued availability of resources. During this time, strange and destructive social behaviors begin to surface. Male behavior deteriorates, and they stop defending their territories and females. Some males become aggressive, attacking females and young, and exhibit abnormal sexual behaviors. Cannibalism has also been observed among the mice, despite the ample food supply.
4. Social Isolation phase: Mice become unable to form normal social bonds or engage in complex behaviors such as courtship, mating, and raising their young. Another group of mice becomes completely isolated, refusing all social and sexual interaction, spending their time solely eating, drinking, sleeping, and excessively grooming their fur. This is followed by females growing more aggressive, abandoning or even attacking their young. Some females fail to carry pregnancies to term, and mortality rates among the young rise sharply. The natural social structures collapse as social roles become unclear, and essential behaviors like mating and raising offspring cease to function effectively.
5. Death phase: This phase begins around day 600. Although some individuals remain physically capable of reproduction, the few surviving females stop bearing offspring. The last mouse died less than five years after the experiment began, in 1973.
Calhoun concluded that despite abundant material resources, excessive population density and overcrowding lead to the collapse of social structures and the emergence of a series of destructive and abnormal behaviors, which he termed the Behavioral Sink. This “behavioral degeneration,” as Calhoun called it, eventually led to the extinction of the entire population.
He believed the experiment offered important lessons for humans, warning of the dangers of overpopulation and its potential effects on social behavior and psychological well-being, even in a world of material abundance.
The Universe 25 experiment sparked much controversy and debate, with some questioning how applicable its findings are to complex human societies.
Nonetheless, it remains one of the most notable studies in animal behavior and the impact of environment and population density. The primary goal was to examine how overcrowding affects social behavior in rodents living in an ideal environment, free from predators and disease, with unlimited food, water, and nesting materials, to understand how overcrowding alone could trigger social collapse, even in the absence of other environmental pressures.
By Ahmad alsarraf
email: [email protected]
