Medial hypothalamic lesions in the rat enhance reactivity and mouse killing but not social aggression

In rats subjected to lesions of the medial hypothalamus, 8 of 11 animals became mouse killers and all manifested some degree of hyperreactivity to the experimenter. When introduced as intruders into a mixed sex colony group, the lesioned rats did not manifest piloerection nor did they emit flank attacks characteristic of social aggressive behavior. However, the lesioned animals did respond to the attacks of the resident male rats with significantly more counterattacks and biting than did sham-lesioned control rats. Spontaneous mouse killing rats also failed to show a level of social aggressive behavior significantly greater than that of sham-operated control rats, but the spontaneous mouse killers did kill rat pups in the colony groups. Alpha-male rats when introduced as an intruder into a colony group did exhibit piloerection and emit flank attacks characteristic of social aggressive behavior. These results demonstrate that the mouse killing and heightened reactivity associated with medial hypothalamic lesions do not represent an indiscriminant release of all forms of agonistic behavior.