Effects of cohabitation with another female on aggressive behaviour in pregnant mice

Pregnant, nonpregnant (but mated) and virgin females were individually housed or lived with a pregnant or nonpregnant cagemate. They were subjected to a series of successive daily encounters with a male intruder, the cagemate being excluded. The most aggressive subjects were pregnant females living with a pregnant cagemate. Isolated pregnant females were as aggressive as pregnant females with a nonpregnant cagemate. Nonpregnant and virgin females were the least aggressive. Nevertheless nonpregnant subjects living with a pregnant cagemate generated higher aggressive scores than nonpregnant females living alone. The way in which a pregnant cagemate influences the aggressive behaviour of another pregnant or nonpregnant female is discussed.