Resident rats’ aggression toward intruders

A study was performed in which attacks by four different types of “resident” rat (males housed with fertile females, males housed with sterile females, paired males, and isolated males) on six different types of intruder (isolated males, grouped males, castrated males, isolated females, grouped females, and ovariectomized females) were investigated. The objective was to study features of resident and intruder rats that would allow the designing of an aggression test that used a minimum of animals and produced a rapid behavioral response. In some combinations of residents and intruders, attack was generated within a 10-min test period. Isolated resident males attacked as much as males housed with females; however, paired rats showed only low incidences of attack. The fertility of the female partner did not influence the male’s aggressiveness. Most male attacks were directed towards like-sexed intruders. Only isolated males differentiated between the different treatment types of male intruder, attacking group-housed and castrated rats less intensely than isolates. Of the females, only those that were fertile produced significant amounts of attack behavior and almost exclusively attacked female intruders. Group-housed intruder females received more attacks than isolates. The results suggest optimal conditions for generating two models of attack behavior in the laboratory rat.