Group size can vary from 2 to over 200 individuals. Both drills and mandrills are diurnal quadrupeds and can be arboreal or terrestrial. Adult males tend to be more terrestrial than females than females and juveniles, perhaps because of their larger size. Male drills forage at the rear of the group and herd the group away from danger. Mandrills will sleep in trees, in a different location each night.
Drills rely on visual and vocal communication, while mandrills also use scent marking.
They are one of the few Old World monkeys to have cutaneous glands. Alpha males scent mark the most frequently. Drills have a series of deep grunts used for close contact communication, to get their group together, and to begin movement. Juveniles alarm bark more than the others. Male mandrills have a 2-phase grunt or roar to round up their group.