Maternal aggression in hamsters: effects of stage of lactation, presence of pups, and repeated testing.

This study investigated maternal aggression in hamsters and examined the effects of early versus late lactation, presence or absence of the litters during the tests, and prior aggressive encounters. There was a total of six experimental groups. Two groups were tested in the presence of their litters on both Days 5 and 15 of lactation; two groups were presented with intruders after a 6-hr interval of mother-litter separation on Days 5 and 15 of lactation. The last two experimental groups were tested on Day 15 only in the presence of their litters or after a 6-hr separation from their litters. Estrous-cycling animals were also tested twice (10 days apart) or once to control for periods of social isolation. Animals were tested in their home cages for 10 min with weight-matched estrous-cycling intruders. Sexually receptive females were not used as controls or intruders. Measures of aggression included fights, attacks, chases, and intruder retreats. Lactating animals initiated significantly higher levels of all measures of aggression than cycling controls. There were no differences in aggression between Days 5 and 15 of lactation or between the groups tested in the presence or absence of their litters. Prior testing on Day 5 had little effect on aggressive responses on Day 15. The results are discussed in terms of comparisons to other species and the factors responsible for high levels of aggression during lactation.