Population density and social pathology.

Abstract

In a series of experiments normal colonies of Norway rats were allowed to grow unchecked in a limited environment. In each set experiments after an extremely crowded condition of 80 adults was reached removal of infants that survived birth and weaning held the populations steady. Although by the 16th month the number of surviving infants would have held the population constant the total breakdown in behavior being noted would have led the colonies to die out. 4 distinct types of behavior emerged: 1) In the end pens which were easily defended dominant males set up harems. Since other males were easily repelled the population in these pens were kept low and nesting raising of young and social activity remained fairly normal. 2) In the 2 middle pens the less dominant males broke down into 2 groups. 1 group became very aggressive chasing estrous females and forcing attentions upon them resulting in high complications of pregnancy and high maternity and infant mortality. These aggressive males also ate the young and attacked other males. 3) The females in the overcrowded pens gradually lost their ability to build nests usually just scattering a few strips of paper on the floor of the box and finally giving birth in the bare sawdust. They also failed to nurse their young and mortality rates were ove 90% among the pups. 4) Eating gradually became the major social activity in the crowded pens. In fact the food hopper became a behavioral sink with rats refusing to eat unless other rats were present and rats gradually abandoning other food hoppers to crowd up at their favorite hopper. The females were distributed equally among the pens but due to the dominant males most of the remaining males were crowded into the middle 2 pens. Females raised in the crowded pens did not regain their nest-building and nurturing ability even when transferred to uncrowded living quarters.