The dominant male initiates all movements of the troop. A major function of high-ranking males is group defense. If a predator is encountered, the troop flees into nearby trees. If that is not possible, one or more males defend females and young. At night, all sleep in trees for security.
Drills communicate through a complex set of social signals.
Vocal and visual greetings, social grooming, threat displays, and submissive postures have been identified in their repertoire. Two distinct grunt calls have been identified that are used to gather or disperse the drill troop in the forest. Unfortunately, these calls also signal to hunters that drills are nearby. Scent marking by male drills can be seen on bark, as well as smelled. Makes it easier for hunting dogs to track them.