The drill is found in the moist evergreen forests of Bioko and between the Cross and Sanaga rivers. They live in the Cameroon mountains up to 1,000 m and also in areas with both forest and savannah components. Drills live in groups of up to 25. Sometimes multiple groups rendezvous forming bands of up to 200 animals. The groups consist of a single dominant adult male and multiple females and their offspring.
Groups travel on the ground, but feed and sleep in the trees.
Drills utter a sharp alarm bark and grunt while foraging. Other sounds include the marked crowing call for contact between groups and the croaking call of distressed juveniles. The dominant male will shake branches and bob his head as a threat display. Drills have also been reported to eat giant land snails, coconuts, sea turtle eggs, and infrequently injured duikers. In fact, only the small group of drills at the Hanover, Germany zoo reproduce regularly, with several females raising their young together.