Rodent Overpopulation

“John B. Calhoun,” Wikipedia

Rat colonies are known to adjust their populations according to the food supply. When food is plentiful, they have larger and more frequent liters. When food is scarce, fewer pups are produced. What would happen, someone thought, if you gave laboratory rats an unrestricted food supply, but kept them in their usual, small cages. How would this affect their behavior? Would breeding stop as they became overpopulated?

Callhoon and his mice

Calhoun and his mice

For long time, I vaguely recalled reading about this seminal psychological study of rat overpopulation conducted in the 1960s.   I recently found the study by a Google search.

My memory was close, but not perfect. The study was done on mice, not rats, in 1968, at the National Institute of Mental Health by John B. Calhoun.  His study became well-known and was described in Scientific American, where I probably read about it.

He introduced four pairs of mice into a large pen (not as small as I remembered). The mice had no restriction of food, water or nesting material. There were no predators. The only adversity was the limit on space.

Initially, the population grew rapidly, doubling every 55 days until it reached a peak of 620 in less than a year, and then continually declined, even as the overcrowding abated. The last surviving birth came another year later.

The decline was accompanied by a continuing breakdown of normal social behavior, not just in mating. Among the aberrations were:

  • Expulsion of young before weaning was complete.
  • Wounding of the young.
  • Inability of dominant males to defend their territory and harems.
  • Male sexual deviation (homosexuality).
  • Aggressive behavior of females.
  • Increased attacks by non-dominant males on each other with no responding defense.

The social breakdown continued after the last surviving birth. The overpopulation triggered a social response that continued even when the population dwindled.  During this period, females ceased to reproduce at all. The males withdrew completely, never engaging in courtship or fighting. They only ate, drank, slept, and groomed themselves—all solitary pursuits. They looked so exceptionally well-groomed and sleek, the researchers called them “the beautiful ones.”  (See posting Hikikomori and Otaku, 12/22/2013)

“The conclusions drawn from this experiment were that when all available space is taken and all social roles filled, competition and the stresses experienced by the individuals will result in a total breakdown in complex social behaviors, ultimately resulting in the demise of the population.”  —Wikipedia

Calhoun repeated the experiments several times with rats (as did others), and the results were essentially the same.

Is this same breakdown of social behavior happening now in our human population?  Sure seems so, I kid you not.


About Roger Walck

My reasons for writing this blog are spelled out in the posting of 10/1/2012, Montaigne's Essays. They are probably not what you think.
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