Drills, like mandrills, are highly social animals living in multi-male, multi-female groups of about 20 individuals, with one male who dominates breeding choices. Researchers believe that drill group sizes can vary greatly depending on the group and season. Drills are thought to have a fission-fusion society in which larger groups—with hundreds of individuals—often break off into smaller groups to forage for food and spend time with other community members (fission), and then assemble once again (fusion).

Scientists classify drills as diurnal, digitigrade quadrupeds, and semi-terrestrial. That is to say, drills are highly active during the day and walk on all four limbs, on the flats of their hands and feet, and while drills are good climbers, they are more likely to be found on the forest floor doing the hard work of foraging for their food of choice. Members of the troop will venture into the canopy at night to avoid predators. Using specialized arm muscles that allow them to cling and swing from tree to tree, male drills tend to stay in the lower canopy while females and the young, perhaps due to their smaller size, may climb higher.

Much of an adult drill’s day is spent searching for fallen food on the forest floor, while weaned young drills play and explore the forest under the watchful eye of their mothers and siblings. Drills use their deep cheek pouches to store their fruits, seeds, and other spoils—sometimes enough to fill their entire stomach—for a later time.

In this intensely male-to-male competitive group, a male drill will fatten up as mating season approaches so that he can spend less time foraging and more time competing for rank and status.