John B. Calhoun’s “Universe 25” experiments — often called the Rat Utopia or Mouse Utopia studies.
John B. Calhoun was an American ethologist at the National Institute of Mental Health (NIMH) between the 1940s and 1970s.
The experiment:
Calhoun built an enclosed “utopia.” The environment was literally a rodent paradise.
- unlimited food
- unlimited water
- no predators
- perfect temperature
- plenty of nesting materials
- no disease
- no scarcity of any kind
At first, everything was good. The population boomed. Social structure went smoothly.
This didn’t last….
Once the population got dense enough, things took a bad turn. Even though food, water, and nesting sites were abundant, social behavior completely disintegrated:
Phase 1 – Growth
Normal mating, normal nesting, normal aggression levels.
Phase 2 – Stress
As social space became increasingly limited, the rodents were unable to maintain stable territories or normal social roles.
Behavioral malfunctions began:
- Males withdrew and stopped defending territory
- Hyper-aggressive males attacked others indiscriminately, even females and young
- Female aggression rose. Mothers ceased building proper nests, abandoned or attacked their own pups, or carried them around aimlessly.
- Infant mortality increased – It skyrocketed (from <10% early on to nearly 100% in late stages).
- Homosexual behavior, hypersexuality, and pansexuality appeared in some individuals, while others became completely asexual.
- Cannibalism & other bizarre behaviors manifested – (mice spinning in circles, etc.)
- “Autistic” or socially detached behavior
- “The Beautiful Ones” — A group of males (and some females) withdrew completely from social/sexual interaction. They spent almost all their time grooming themselves, eating, and sleeping in isolated areas. They were physically healthy and beautiful, but never reproduced.
Endgame
Even after providing the mice (mice in the Universe 25 study) with MORE space and resources, the colony didn’t recover its health.
The last fertile female was born around Day 920; the last birth occurred around Day 1,200. By ~Day 1,580 the colony was extinct. Not from starvation or disease, but from behavioral and social collapse.
Calhoun’s Interpretation
Calhoun coined the term “behavioral sink” to describe the total breakdown of normal social roles under extreme crowding, even when physical resources were unlimited. He believed it showed that too much social interaction (too many contacts per day) overwhelmed the animals’ ability to form meaningful roles and relationships, leading to pathological behavior and eventual extinction. He extrapolated this to humans, warning that modern cities could produce similar social pathology if density became too high and social roles became too complex or meaningless.
While the direct analogy to human society is heavily debated, the experiment is still considered a classic demonstration of how social environment can override even the most favorable physical conditions and drive a population to extinction.