Behavioral Sink and the Effects of Overpopulation

Behavioral Sink and the Effects of Overpopulation

Written By Robert Luscomb

Artwork By Rola Abdelwahab

In the 1960s, an American scientist working for the National Institute of Mental Health began capturing the imaginations of the people nationwide through his experiments on overpopulation. [7] He experimented with rats in specially built communities where food, water, shelter, and anything else that the rats would need would be provided. John began to see changes in the rats´ behavior over time, which led to his coining of the term “behavioral sink” to explain animal’s change in behavior within an extremely condensed environment. 

The experiments all started at John’s Maryland home where he coaxed his neighbor to let him build a “rat city” in his neighbor’s wooded land. This sanctuary for the rats was built to hold approximately 5,000 rats, but the population of rats never surpassed 200. This test was just the beginning of his experiments, though. Succeeding this initial experiment, Calhoun was hired by NIMH (The National Institute of Mental Health) and began conducting more research to see why the rat’s behavior was changing so drastically. 

The following data was collected by Calhoun after the 8 years of experimentation were concluded. The rat utopia was a 10 x 14 ft enclosure split into four sections by an electric fence. Some bridges allowed the rats to cross these fences, but they were only present on three of the fences, creating a secluded space for the corners with only one entrance/exit. There were food and water stations in each section of the enclosure along with nesting areas featuring spiral staircases leading up to them. Two experiments were conducted in this enclosure with one consisting of 32 rats split equally by sex and another with 52. This enclosure was fit to hold 40 rats but could contain 80 at maximum capacity. 

Female rats began dispersing equally among all four of the quadrants of the pen while male rats exhibited a completely different story. Dominance amongst the male rats was naturally set which caused the less dominant rats to begin waking earlier to forage for food while the dominant rats would protect their quadrants of the enclosure. These alpha males took over the sections with only one entrance barring any other male into their sections. This issue left an influx of male rats trapped in the two leftover quadrants on the south of the pen. What resulted was that the dominant males were protecting their flock of female rats with a few male rats leftover in their enclosure. These leftover beta male rats would hide in the burrows of the female rat’s nests but would never try to mate with them. Instead, they would try to mate with the dominant male who would not make any attempt to fight them off. 

There were two different types of feeders in the enclosure: one with powdered food and one with food pellets behind a wire fence. The food that was behind the wire fence required a substantially longer process to retrieve than the powdered food, causing the male rats to feed together which conditioned the rats to be reliant on the social nature of eating. They began only eating in groups and never alone. Since the dominant males refused to let the less dominant rats into the northern pens, the southern two pens slowly became more and more condensed. This is when the term “behavioral sink” comes into play. The definition of a behavioral sink is the destructive behaviors among nonhuman animals that occur with overcrowding [2].

The behavioral sink was first exhibited in the female rats when the mothers would slowly pay less attention to their young. They would set aside their children in the pen, instead favoring more social behaviors. Such behavior meant they were building less sparse nests that could not properly hold their young, leading to a high mortality rate amongst the young rats. At one point, the mortality rate reached 96%. Owing to this high mortality rate, the rats began cannibalizing the dead young in the nests.

The male’s behavioral sink was very different from the females. The dominant males would attack other male rats due to stress. Less dominant males were characterized by three groups: Pansexuals, Somnambulists, and Probers. The Pansexual male rats would not compete for any sort of dominance or ranking within the pen. They would only try to mate with any other rat in the enclosure regardless of sex. The Somnambulists would move slowly in the enclosure, not interacting with any of the other rats. The Probers were hypersexual rats that would pursue anyone and anything for mating. They would follow female rats into their burrows and mate with them even though they knew the dominant rats would viciously attack them afterward. This was where Calhoun stepped in and ended the experiment. He took the four most healthy rats of each gender and let them mate, but because of their behavioral changes, none of their offspring survived. 

Calhoun conducted one final grand experiment in which he once again provided food, water, and shelter to the mice. There were some changes made, such as the replacement of rats with albino house mice. This enclosure, which he named Universe 25, was a more vertical design where there were 16 “apartment” buildings built for the mice. At the bottom of each complex, there was an unlimited supply of food and water. This enclosure would be able to house 3,048 mice. 

The experiment started with 8 mice – four female and four male. There were four phases to the experiment: Adjustment (day 0-104), exploitation (day 105-315), stagnation (day 316-560), and death (day 561- X). The adjustment period was characterized by the establishment of dominance and the beginning of mating. The exploitation phase of the experiment was when the mice would double in number every 55 days until the stagnation phase ensued where the doubling rate would slow to every 145 days. Again, the mice formed population-dense areas where a vast majority of the mice would live in a specific area of the enclosure. 

During the stagnation phase, male mice would begin to get frustrated and attack each other out of stress. Because there was nowhere to run the mice would sit there and take all of the attacks, resulting in several injuries. The female mice would go to the tops of their apartments and hide in secluded areas away from the other mice. Once again, the mice began to become socially dependent around food hoppers with some being overfed through social feeding and others being left hungry. As the population kept growing, the dominant males would have to fend off more and more males from reaching their female counterparts. They would eventually be overwhelmed and the females would be left to fight off males while also caring for their young. This led to the mother mice becoming aggressive, often harming their young. Due to the stress of fending off males from their nests, mothers would sometimes reabsorb their young which raised the young mortality rate even more. 

During the final phase, the mice did not know how to socialize normally because they would be driven out of their nests at a young age by their aggressive mothers. Thus, the new generation of female mice did not know how to care for their children because they lacked the instincts to do so. The male mice, also being socially stunted, would not participate in any sexual or aggressive tendencies which led to a drop-off in population. Due to no sexual tendencies, the population growth stopped completely [8].

“I shall largely speak of mice, but my thoughts are on man, on healing, on life and its evolution. Threatening life and evolution are the two deaths, death of the spirit and death of the body.”- John B Calhoun  [3]

These experiments were reflected unto human life where studies were conducted on dense physical and social populations like dormitories, hospitals, and the most similar experience –  prisons. Paul Paulus’s studies summarize the results of a research program on the effects of crowding in prisons and jails. The relation of crowding to rates of suicides, violent and nonviolent deaths, psychiatric commitments, inmate-on-inmate assaults, disciplinary infractions, and attempted suicides and self-mutilation as evidenced in archival records of 175,000+ inmates from 4 state prison systems were examined [5]. These are all reflective of the events that unfolded in Calhoun’s Universe 25. 

The behavioral sink is more prevalent now than ever. In big cities all over the world, there is senseless violence similar to the situations presented in Calhoun’s studies. Fighting for dominance and ranking has become a priority to many people while others struggle with mental issues exactly like the rats and mice. With the world’s population consistently growing as a result of advancements in medicine and technology, a behavioral sink is bound to happen. 

Human communities are extremely dependent on social interaction like the rats and mice were in the experiments. If humans follow in the footsteps of these experiments we already know what will follow suit.  These experiments were done in 1965-1973. There was no correlation found between these rat and mice colonies and human life, but times have changed and the consequences of overpopulation are inevitable. All of this being said, humanity is in store for an interesting future. 


1. Anastasia Bendebury, Ph.D. “John B. Calhoun and His Rat Utopia.” DEMYSTIFYING SCIENCE, DEMYSTIFYING SCIENCE, 23 July 2020,

2. “APA Dictionary of Psychology.” American Psychological Association, American Psychological Association, 2021,

3. Calhoun, J B. “Death Squared: the Explosive Growth and Demise of a Mouse Population.” Proceedings of the Royal Society of Medicine, U.S. National Library of Medicine, Jan. 1973,

5. Cox, Verne & Paulus, Paul & McCain, Garvin. (1984). Prison crowding research: The relevance for prison housing standards and a general approach regarding crowding phenomena. American Psychologist. 39. 1148-1160. 10.1037/0003-066X.39.10.1148. 

7. Kunkle, Fredrick. “The Researcher Who Loved Rats and Fueled Our Doomsday Fears.” The Washington Post, WP Company, 12 June 2020,

8. “The Mouse Utopia Experiments | Down the Rabbit Hole.” YouTube, YouTube, 7 Oct. 2017, 

Behavioral Sink and the Effects of Overpopulation

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