Effects of Chlorpromazine on Aggression in Laboratory Populations of Wild House Mice

Three confined, freely growing populations of wild house mice (Mus musculus) were studies to determine possible functions of aggressive behavior in regulating population size. Effects of a tranquilizer, chlorpromazine, on aggression and population parameters were measured. In singly caged pairs of mice chlorpromazine, mixed with the food at a rate of 0.75 mg/g food, significantly increased the number of days between litters. Also, significantly fewer young were born and weaned when compared with pairs given a control diet. Thus chlorpromazine produced no direct enhancement of reproduction. At the same dose chlorpromazine reduced fighting among C57 male mice. The growth curves of the populations prior to treatment were generally sigmoid, upper limits varying in each population. Birth and survival rates of infants declined with increasing population size. Mortality rates of adults did not vary significantly with population size. Due to a higher mortality of post—weaned males than females, sex ratios generally favored females. Aggressive rates (interactions per min) increased as the population increased, then leveled off after the after the population reaches a peak. When adjusted for population size, the rates followed the same pattern. Adult males established territories in all three cages, up to seven such males being present in a cage at one time. These males significantly heavier than non—territorial males of similar age and initiated up to 8% of the fights. Adrenal glands of territorial males were significantly lighter and thymuses significantly heavier than those of non—territorial males. Females did not defend territories but accounted for up to 50% of the fighting. Chlorpromazine (0.75 mg) was given to one population for 111 days and to a second population for 60 days after each had leveled off. Renewed population growth occurred during drug administration and aggressive rates were markedly reduced. The only effect of the drug on measured population parameters was increased infant survival. When the drug was removed from one population the numbers of mice declined and the aggressive rate increased. Reduced aggression must have produced its effect on population size through some factor(s) influencing infant survival.