 One such road runs through the Gran Caldera Southern Highlands Reserve, a protected area in the south of Bioko Island, where the drill's population is most dense.This also poses an issue to habitat range of the Bioko drill as their population density is lowest around roads.The diet of the Bioko drill is largely dependant on their location on the island, with lowland drills primarily consuming a frugivorous diet, similar to the mainland drill, whereas drills living in more mountainous areas preferably feed on herbaceous materials such as pith, leaves and fungi, though these are significantly less nutritious.