Drills have never been seen outside of forests, while mandrills may sometimes venture onto the savanna. Both are omnivorous, eating fruit, seeds, vegetation, and small animal prey. Mandrills have been known to raid crops and plantations. It has been suggested that mandrills live in a comparatively food-poor zone, the lower levels of the forest which are the least productive part, which accounts for their long daily travels. Their travels are similar to hamadryas baboons, which inhabit desert regions.
Drills live in harem groups.
One male can can as many as 20 females in his group. During the dry season several groups may come together to form one large group. As many as 200 individuals together have been observed. Males will emigrate and also can remain solitary. Mandrills normally form large multimale-multifemale groups with a distinct hierarchy.