The purpose of this series of studies was to provide a detailed description of aggression in hamsters during pregnancy, on the day of parturition, in the presence or absence of pups, and during the estrous cycle. In the initial experiment, intrasexual aggression was examined on days 1, 4, 6, 12, 13, and 15 of pregnancy and on the day of parturition. Measures of aggression during the 10 min home cage tests included the number of attacks, fights, chases, and intruder retreats. Aggression was low during early pregnancy, significantly increased between days 4-12, and steadily declined until day 15, when levels of aggression were as low as those occurring during early pregnancy and among cycling controls. Aniials tested on the day of parturition were significantly more aggressive than animals tested 24 hr prepartum, animals in early pregnancy, and cycling control animals.
The second experiment showed that the high levels of aggression occurring on the day of parturition were significantly decreased when pups were immediately removed at parturition and females were tested 6 8 hr later.
The last experiment examined intrasexual aggression in female hamsters during the estrous cycle. Four different groups of animals corresponding to the 4 days of the cycle were tested. Although peak levels were reached on diestrus days 1 and 2 and low levels of aggression occurred during proestrus and vaginal estrus, no significant differences were found.