Ethologist/psychologist, John Calhoun (1917-1995), conducted an experiment with a population of mice, to test the effects of overpopulation and crowding, where food resources were not the limiting factor for growth:
J. Calhoun, “Population Density and Social Pathology,” Scientific American, vol. 206, 1962, pp. 139-148.
He had an area which would permit 3,840 mice to feed and survive. However, he observed that as mice numbers grew, a breakdown in social structure happened, with an increased killing of young, copious quantities of mouse homosexual behaviour, and a rapid rise in the level of aggression. The population crashed and headed to extinction:
https://www.youtube.com/watch?v=0Z760XNy4VM
https://www.youtube.com/watch?v=_v7htvkqU_I
https://www.youtube.com/watch?v=VKY2Sx2a4rM
“Many [female rats] were unable to carry pregnancy to full term or to survive delivery of their litters if they did. An even greater number, after successfully giving birth, fell short in their maternal functions. Among the males, the behavior disturbances ranged from sexual deviation to cannibalism and from frenetic overactivity to a pathological withdrawal from which individuals would emerge to eat, drink and move about only when other members of the community were asleep. The social organization of the animals showed equal disruption. ...
The common source of these disturbances became most dramatically apparent in the populations of our first series of three experiments, in which we observed the development of what we called a behavioral sink. The animals would crowd together in greatest number in one of the four interconnecting pens in which the colony was maintained. As many as 60 of the 80 rats in each experimental population would assemble in one pen during periods of feeding. Individual rats would rarely eat except in the company of other rats. As a result extreme population densities developed in the pen adopted for eating, leaving the others with sparse populations.
... In the experiments in which the behavioral sink developed, infant mortality ran as high as 96 percent among the most disoriented groups in the population.”
J. Calhoun, “Population Density and Social pathology,” California Medicine, vol. 113, 1970, p. 54.
Although, like all experiments, there have been critics, the most important ramification of this work was that the results were directly applicable to human populations. We are seeing this daily in the cities across the world, which have become no more than gigantic rat nests for capitalism and hyper-consumption. The fate of man, reduced to the level of a lab rat, will be, logically enough, the fate of rats.