Population density and social pathology in mice. Differences in catecholamine metabolism associated with differences in behavior

A population of mice was allowed to reproduce freely in a designed habitat containing an abundance of food and water as well as a variety of living space. As the population approached maximum density, overt behavioral pathologies emerged. Two groups of male mice, representing opposite ends of the behavioral spectrum, were selected for studies on catecholamine metabolism. One group was socially withdrawn and the contrasting group showed relatively normal behavior, participating in defense of territory. When contrasted with mice showing territorial behavior, the socially withdrawn mice showed a higher tyrosine hydroxylase (TOH) activity in brain and phenylethanolamine-N-methyl transferase (PNMT) activity in adrenal glands accompanied by an increased turnover of catecholamines in these organs. This increased rate of catecholamine metabolism in socially withdrawn mice suggests that socially withdrawn mice were under greater stress, even though they were far less active physically and socially than the territorial mice. While the relationship between catecholamine metabolism and behavior is as yet unclear, our findings nevertheless suggest that when a population exceeds a certain density within a closed system, the increased frequency and variety of social contacts may constitute an important source of psychosocial stimulation which could lead to sustained behavioral and biochemical changes when prolonged.