.......The research began at Johns Hopkins in 1946 and continued through the 60s, when Calhoun, then a research psychologist, initiated his studies of population dynamics under natural conditions, in a disused land in Towson, Maryland, by introducing five pregnant Norway rats in a quarter acre outdoor enclosure with plenty of food and no predation issues. He observed them for 28 months. The population expanded to conform a ¨rat city¨, it never exceeded 200 individuals and at last stabilized at 150, as social fights were so disruptive to normal maternal care, that only a few of the young survived. The point is that female rats, in a natural environment could have produced 50000 progeny in 28 months, but available space could not have accommodated this number........

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