![]() ![]() Social disintegration soon occurred: factions of rats divided into different groups with modified sex ratios, congregated in certain pens while leaving other areas sparse, ate rarely or only in the company of other members, and eventually crowded together in a pen as one big mob. Males became more frenzied, aggressive, resorted to sexual deviation, even cannibalism, followed by morbid withdrawal, where they would only come out to feed or roam about when others in the community were asleep. Females were largely unable to carry a pregnancy to full term, died after delivery if they did, or neglected their litters if they survived. ![]() As the population density increased, behavioural pathology emerged among both females and males. This observation in the wild led to a series of more controlled experiments using domesticated rats indoors. Despite an adequate amount of space, stress from social interaction led to disruptions in maternal functions where very few young survived. One reason was an extremely high infant mortality rate. But by the end of 27 months, the population stabilized at only 150 adults, despite the fact that reproduction rates were predicted to produce 5000 by that time. Soon a colony formed and the population boomed. He confined wild Norway rats to a quarter-acre enclosure where they had a bounty of food, plenty of space to mill about, removed any source of predation or conditions for disease, and he watched. ![]() In the 1940s, ethologist John Calhoun began to study the effects of population density on rats. ![]()
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