Crowding pregnant mice affects attack and threat behavior of male offspring.

Attack and threat behavior of adult male offspring of female mice crowded during the final third of pregnancy was investigated. In 5-min test pairings with an anosmic "standard opponent" which had 50 microliter of male mouse urine applied to its fur, the prenatally stressed group of males showed significantly less attack behavior; attack latency was longer and number of attacks, bites, amount of time spent attacking, and composite aggression scores were all lower, compared with the control group. Similarly, less threat behavior was observed in offspring from crowded dams; there were lower frequencies of tail rattles, rough grooms, and upright threats. Additionally, proportionally fewer males in the prenatally stressed group attacked or displayed threats. A second experiment was designed to investigate the effects of exogenous androgen on the aggressiveness of males from crowded mice: testosterone propionate administration (500 micrograms/animal/day, for 5 days prior to testing) abolished differences both in the proportion of males from crowded mice that fought and also apparently abolished differences in intensities of attack and threat behavior between groups. However, trends toward reduced aggression in prenatally crowded males remained. More detailed analysis of these responses, based only on animals that displayed aggression, revealed significantly reduced intensity of aggression in offspring from females crowded during pregnancy, indicating that testosterone propionate therapy did not completely restore this behavior. In order to reduce postnatal effects due to possible differences in mothering, all offspring were fostered to untreated mothers at birth. The results are discussed in terms of in utero exposure of male fetuses of crowded dams to stress-liberated adrenal steroids of maternal origin, and the possible consequences for the endocrine integrity of these offspring.