Aggressive behavior in inbred strains of mice during pregnancy

Female aggressive behavior toward adult male ICR/JCl mice was compared for virgin and pregnant mice of five inbred strains (BALB/c, C3H/He, C57BL/6, DBA/2J, and AKR/J). Pregnant females from four strains except BALB exhibited intense aggressive behavior, whereas only virgin AKR females were aggressive. Aggressive behavior began in early pregnancy, was highest in midpregnancy, and declined slightly thereafter until the day of parturition. The level of aggressive behavior showed significant strain differences. The topography of aggressive behavior was also different among the four strains. DBA females showed marked contrast with the other three strains in the temporal changes of aggression in the first phase of encounter. Furthermore, strain specific behavioral pattern of aggression was demonstrated based on six behavioral acts ( Darting , Chasing, Attack, Biting, Wrestling, and Boxing). Virgin and pregnant AKR females showed the identical behavioral pattern of aggression.