The rat populations of NIMH: Density, reproduction, and the neocortex

Abstract

Investigations of the effects of crowding on social behavior has long been a major focus of the research on rodent populations conducted in the laboratory of John B. Calhoun, at the National Institute of Mental Health. In these studies, rats living under crowded conditions exhibited many behavioral abnormalities, including poor parental behavior, and excessive aggression. These behavior patterns are similar to those displayed by rats with neocortex damage except that the latter were tested under controlled, uncrowded conditions. An experiment was designed to examine the concomitant effects of forebrain lesions and population density upon a rat's capacity to cope with social complexity. The behaviors of rats with less than 50 percent of the normal neocortex, resulting from prenatal chemical treatment, were compared at two population densities to similar groups of normal rats. The micrencephalic rats were initially and generally less capable parents and were more aggressive than normals. In both types of rats, females in populations of eight bisexual pairs reared proportionally fewer of their offspring to weaning age than females in populations of four pairs. The overall reproductive success of normal rats at the higher population density was as poor as that of the micrencephalic rats at the lower population density. These results have been interpreted as showing that rats are sensitive to differences in population density even at relatively low absolute population sizes, and that increased population density interferes with the capacity of the neocortex to cope with environment complexity. © 1987 by the American Society of Zoologists.