Drill

The drill (Mandrillus leucophaeus) is a large Old World monkey belonging to the family Cercopithecidae and the genus Mandrillus, which it shares with the closely related mandrill. Endemic to the lowland rainforests of western Central Africa, its range spans southern Nigeria, southwestern Cameroon, Equatorial Guinea, Gabon, and the Republic of Congo. Drills inhabit primary and secondary tropical forests, swamp forests, and riverine habitats, preferring dense understory with abundant fruiting trees. Adult males are strikingly colorful, displaying a black face with a white beard and a bright blue patch on the nose, while females and juveniles have more subdued brownish‑gray pelage. They are among the largest guenons, with males weighing up to 25 kg and females up to 20 kg, and a head‑body length of 50–70 cm. Drills are diurnal and arboreal, spending much of their day foraging in the mid‑canopy but also descending to the forest floor to feed. Their social groups typically consist of 10–30 individuals, organized into one‑male–multiple‑female units that may associate with other groups. The species is listed as Endangered by the IUCN due to rapid habitat loss and hunting pressure, with an estimated total population of fewer than 10,000 mature individuals. Conservation efforts focus on habitat protection, anti‑poaching patrols, and community‑based initiatives to reduce bushmeat hunting.

Drills live in cohesive, multimale–multifemale troops that can number from a few individuals up to 30 or more, especially when several one‑male units temporarily merge. Within a troop, a dominant male monopolizes access to receptive females, while subordinate males may form coalitions or act as escorts. Females maintain strong matrilineal bonds, grooming each other frequently and cooperating in infant care. The species is primarily diurnal, with peak activity in the early morning and late afternoon; they rest in dense foliage during the hottest part of the day. Home ranges are estimated at 5–10 km² in continuous forest, and groups patrol overlapping territories using vocalizations, scent marking, and visual displays. Communication includes a repertoire of loud “barks,” soft “purrs,” and facial expressions such as teeth‑showing and nose‑flaring, which convey aggression, submission, or alarm. Drills are also known for their intelligence, solving simple problems and using tools in captive studies, indicating a high degree of behavioral flexibility.

Drills are omnivorous, with a diet that reflects the seasonal availability of forest resources. Fruit constitutes the bulk of their intake, especially figs (Ficus spp.), drupes, and berries, providing up to 60 % of caloric intake during peak fruiting periods. They supplement fruit with large seeds, nuts, and tubers, which they crack open with their strong jaws. Invertebrates such as insects, termites, and beetle larvae are important protein sources, particularly during the dry season when fruit is scarce. Occasionally, drills consume small vertebrates, including birds, eggs, and even small mammals. Daily food consumption averages 2–3 kg of fresh matter per adult, and they spend up to 6 hours foraging each day. Foraging strategies involve both arboreal searching and terrestrial digging; drills are adept at using their dexterous hands to extract hidden insects from rotting wood or leaf litter. Their broad diet and flexible foraging behavior enable them to exploit a wide range of forest habitats, but also bring them into competition with other frugivores such as chimpanzees and forest elephants.

Female drills reach sexual maturity at about 4–5 years, while males mature later, around 6–7 years, when they acquire the size and coloration needed to compete for mates. Breeding is seasonal, coinciding with the rainy season when fruit abundance peaks, typically from March to June in the southern part of their range. After a gestation period of 160–170 days, a single infant is born, weighing roughly 600–800 g. Newborns are altricial, with closed eyes and sparse fur, and are carried ventrally by the mother for the first few weeks. Maternal care is intensive; the mother nurses the infant for 6–12 months, and the young remain within the natal group for up to 5 years before dispersing. Males may assist in infant protection, especially dominant individuals who benefit from increased group cohesion. Birth intervals average 2–3 years, reflecting the high energetic cost of rearing offspring in a competitive forest environment.

The IUCN classifies the drill as Endangered, with the total population estimated at fewer than 10,000 mature individuals and declining at an estimated rate of 30 % over the past three generations. The principal threats are habitat loss and fragmentation caused by logging, oil‑palm plantation expansion, and infrastructure development, which reduce the availability of continuous forest needed for their large home ranges. Additionally, drills are heavily targeted by bushmeat hunters for their meat, and their striking appearance makes them attractive for the illegal wildlife trade. Hunting pressure is especially acute near forest edges and in areas lacking effective law enforcement. Climate change poses a longer‑term risk by altering fruiting phenology and increasing the frequency of extreme weather events. Conservation actions include the establishment and management of protected areas such as the Cross River National Park and the Monte Alén National Park, community‑based anti‑poaching patrols, and awareness campaigns that promote sustainable livelihoods. Ongoing research on population genetics and habitat use informs management plans aimed at maintaining connectivity between forest fragments.

• Drills are the only primate species where the male’s bright blue facial coloration is used to signal dominance and health.
• They have one of the longest gestation periods among guenons, lasting about 5½ months.
• Unlike many monkeys, drills can crack open hard nuts using a combination of strong jaws and specialized molars.
• Their vocal repertoire includes a distinctive “screech” that can travel up to 500 m through dense forest.
• Drills are capable of using simple tools in captivity, such as sticks to retrieve out‑of‑reach food items.