The effects of orbitofrontal lesions on the aggressive behavior of vervet monkeys (Cercopithecus aethiops sabaeus)

This study examined the role of the orbitofrontal cortex in the integration and expression of aggressive behavior in the vervet monkey ( Cercopithecus aethiops sabaeus ). Operated and unoperated controls were observed in two settings: In one they were confronted by a human observer and in the other they were members of a social group of conspecifics. Nine animals (four males and five females) underwent orbitofrontal lesions. Operated males and females threatened a human observer more postoperatively than preoperatively, but exhibited no change in the rate of aggressive behavior expressed toward conspecifics. However, operated monkeys were less effective in their aggressive encounters with conspecifics. Operated males lost a greater proportion of dyadic encounters; operated females participated less successfully in coalition-mediated aggression. The postoperative changes exhibited by vervets differed from those reported in rhesus monkeys ( Macaca mulatta ), which suggests that there may be considerable interspecific variability among Old World monkeys in the effects of orbital frontal lesions on aggressive behavior.