Drill

Overview

The Drill (Mandrillus leucophaeus) is one of the most endangered, majestic, and poorly known primates on the African continent. Closely related to the famously multicolored mandrill, this powerful forest monkey is restricted to a tiny, highly fragmented pocket of primary tropical rainforest in southeastern Nigeria, southwestern Cameroon, and the volcanic island of Bioko. With its jet-black obsidian-like face, snowy-white cheek ruff, and stocky, muscular build, the male drill cuts an imposing figure on the forest floor. Semi-terrestrial by nature, drills move through the dense rainforest understory in massive, vocal multi-family troops, playing an essential role in dispersing large forest seeds.

Biology and Physical Characteristics

Mandrillus leucophaeus exhibits extreme sexual dimorphism: adult males are more than double the size and triple the weight of mature females, possessing immense canine teeth that rival those of a leopard. The drill’s body is covered in olive-brown to charcoal agouti fur that pales on the underparts. Its face is completely hairless, pitch-black, and characterized by pronounced bony ridges along the muzzle, fringed by a striking, fluffy white chin and neck ruff that accentuates facial expressions. Mature dominant males display vivid lilac, magenta, and scarlet coloration on their naked rump and perianal region, which functions as a brilliant social billboard that subordinate troop members can follow through the dimly lit forest understory.

Behavior and Ecology

Drills are semi-terrestrial, diurnal primates that spend the majority of their daylight hours foraging methodically along the rainforest floor, flipping over stones and decomposing logs in search of roots, invertebrates, and fallen fruits, before climbing into high canopy forks to sleep safely at night. They live in large, cohesive social groups of 20 to 50 individuals, which regularly coalesce into massive ‘super-troops’ of over 100 individuals during seasons of high fruit abundance. Communication is complex and nuanced: drills utilize quiet contact grunts, loud two-phase ‘crow’ barks, and an intricate repertoire of facial displays, most notably the ‘grin’ or teeth-baring smile, which serves as an appeasement and greeting signal rather than aggression.

Conservation and Human Impact

The Drill is categorized as Endangered on the IUCN Red List, with fewer than 4,000 individuals estimated to remain in the wild, making it one of Africa’s most imperiled primates. The primary and catastrophic driver of their decline is commercial bushmeat hunting. Because drills are large-bodied, terrestrial, and travel in noisy groups, hunters using shotguns and tracking dogs easily locate and wipe out entire social troops. Rapid deforestation for timber concessions, palm oil plantations, and infrastructure further fragments their shrinking habitat. Major conservation strongholds include Nigeria’s Gashaka-Gumti and Cross River National Parks, the Drill Ranch sanctuary, and Cameroon’s Korup National Park.

Interesting Factoid

When a male drill opens its mouth wide to flash its massive, two-inch-long fangs in a grimacing ‘smile’, it is not threatening an attack; in drill social etiquette, this facial expression is actually a friendly, peaceful greeting designed to reassure other troop members.