Early exposures to intermale aggression increases the adult aggression of male rats.

Intact litters of rats were exposed to three brief episodes of intense paternal aggression against an unfamiliar adult conspecific prior to weaning on day 31. Control litters were not exposed to paternal conflict. Male offspring of both groups were given a long-term test of their aggression against an unfamiliar intruder when the subjects were 103-114 days old. During the initial 10-minute observational phase of the aggression test, few animals in either group attacked the intruder, but intruders lost more bodyweight during the 221 2 -hour test when placed into the cages with the aggression-exposed subjects. Thus, early exposure to aggressive exchanges between adult animals increases the use of an aggressive strategy during subsequent confrontations with unfamiliar conspecifics. Two mechanisms possibly underlying this phenomenon are discussed.