Drill Monkey Facts: Diet, Habitat and Threats

Estimated reading time: 14 minutes

Learn about interesting Drill monkey facts in this comprehensive article about one of the most visually arresting, emotionally complex, and endangered monkeys on Earth. Endemic to a narrow slice of rainforest in Nigeria, Cameroon, and Bioko Island, these intelligent primates have lost over 50% of their population in just three decades. They face a terrifying gauntlet of threats: palm oil plantations, hunting for the bushmeat trade and timber logging. Although rarely witnessed, Drills are capable of extraordinary behaviour. They form multi-species foraging alliances, mourn their dead, and navigating their crumbling world with grace and resilience. Only a few thousand of these precious #primates remain alive. Take action every time you shop, be #Vegan and #BoycottPalmOil #Boycott4Wildlife

Drill Mandrillus leucophaeus

IUCN Red List Status: Endangered

Location: Nigeria, Cameroon, Equatorial Guinea (Bioko Island)

Drill monkey appearance and behaviour

The Drill is an extraordinary primate, often described as a living mosaic of colour and power. Males exhibit large, richly hued rumps in shades of lilac, mauve and indigo, which serve as visual signals in social communication. Their massive canines and robust skulls hint at their physical strength. Male drill monkey size is three times bigger than females.

Shy and cryptic primate with bold colouration

Drills have muscular, barrel-chested bodies with long, sturdy limbs adapted to both terrestrial travel and arboreal sleeping. Their movements on the forest floor are purposeful and heavy-footed, but they climb gracefully at night to sleep in trees, often 10–20 metres above ground. Despite their strength and bold colouration, Drills are shy and cryptic, vanishing silently into the understorey when disturbed.

Recent research has revealed that Drill primates regularly form cooperative associations with other monkeys in Cameroon’s Korup National Park. They often forage in loose alliances with red-capped mangabeys Cercocebus torquatus, despite competing for similar foods.

This unusual cooperation may help them reduce predation risks or navigate foraging landscapes more efficiently. Each species uses different forest layers and movement strategies to avoid direct conflict, while benefitting from group vigilance (Astaras et al., 2011).

These associations with other monkey species suggest a high level of behavioural flexibility and intelligence.

Drill monkey diet

The drill monkey diet consists of 90% fruit. However, on Bioko Island and in montane regions, the drill monkey diet alters to include herbaceous stems, leaves, and fungi due to lower fruit availability.

This ability to adjust their feeding strategies to suit local conditions reveals their remarkable adaptability. Researchers have discoverd that drills also consume seeds, insects, and small vertebrates opportunistically. Their robust jaws and thick enamelled teeth are well suited to cracking hard seeds and tough vegetation.

Reproduction and mating

Drills live in complex, fission–fusion societies comprising multi-male, multi-female groups of 15–75 individuals. Mating is dominated by intensely colourful, high-ranking males, who gain the greatest access to fertile females. Sexual dimorphism in Drills is extreme: males possess vividly pigmented faces and rumps that function both as dominance signals and sexual ornaments. These visual cues are correlated with testosterone levels and social status rather than individual attractiveness alone (Marty et al., 2009). Females give birth to a single infant after a gestation of around 5–6 months, and maternal care is extensive and sensitive.

Drills have empathic mourning patterns

A remarkable insight into Drill social bonds comes from a 2023 thanatology study, revealed a mother caring for her dead infant over several days. She carefully groomed the dead infant, carried them gently. Then she made repeated efforts to engage their eyes. These behaviours indicate a profound emotional attachment. Group members also showed interest, inspecting the infant and sometimes sitting near the mother in what appeared to be mourning-like behaviour. Two days later, the mother began to eat the corpse (interestingly also performed by extinct hominids species like Neanderthals) a phenomenon seen in some other primates as a response to extreme stress or to recover nutrients after stillbirth. The act was not shared with others and appeared deliberate and solitary (Casetta et al., 2023).

These intelligent primates have complex emotional inner lives

These observations reveal that Drills are not only intelligent but deeply emotional animals capable of complex grief responses. Their inner lives, once invisible to science—are now starting to emerge through careful observation. Combined with their intricate dominance hierarchies, cooperative alliances, and nuanced communication, these findings make clear that the Drill is a primate of both immense ecological importance and profound emotional depth.

Drills are threatened by deforestation, particularly in Cameroon where multiple oil palm plantation projects are already underway or proposed (Morgan et al. 2013).

IUCN red list

Drill monkey habitat

Drill monkey habitat is in a narrow strip of rainforests of southeastern Nigeria (Cross River region), southwestern Cameroon (Korup and Ebo regions), and Bioko Island off the coast of Equatorial Guinea.

The total range of drill monkey habitat is estimated at just over 63,000 km², with an area of living below 20,000 km². Most of this area is fragmented by palm oil plantations and roads (Morgan et al., 2013).

Cameroon is home to roughly 75% of the global population, with estimates suggesting 3,000–5,000 Drills remain there. Nigeria supports fewer than 1,200 individuals, while Bioko’s population has declined by over 70% and may now number fewer than 1,000 (Cronin, pers. comm., 2016).

Drill monkey threats

Habitat loss from palm oil, rubber and tea agriculture and logging

Across Cameroon and Nigeria, massive areas of rainforest are destroyed for oil palm, rubber, banana, and tea plantations. Logging concessions, even those supposedly regulated, continue to expand into primary Drill monkey habitat. For example, in the Mount Cameroon region and the Ebo corridor, drill monkey threats have multiplied due to the decimation of forests. Drills are barely surviving in isolated fragments of forest. (Morgan et al., 2013; Astaras, 2009).

Road, mining and infrastructure projects

New roads, often tied to palm oil plantation and mining expansion, now dissect once-pristine Drill monkey habitat. These roads do more than fragment forests: they act as conduits for hunters and settlers, opening up remote areas to poaching and development. Planned road networks in Cross River State, the Ebo region, and Bioko threaten to divide the last strongholds of this species permanently (Linder & Oates, 2011).

Hunting and the bushmeat trade

Drills are one of the most sought-after primates in the bushmeat trade. Adult males are especially prized for their fatty flesh. When a group is spotted, hunters with dogs may target the entire troop, killing multiple individuals in one hunt. Though once hunted opportunistically, Drills are now commercially targeted for sale in urban markets, further escalating population decline. (Gadsby, 1990; Astaras, 2009).

Genetic fragmentation and isolation

Because of a wicked combination of Drill monkey threats, the species is now fragmented into at least 10 isolated populations. They have no natural corridors to link between groups. For example the Bioko’s subspecies is completely cut off. Genetic isolation increases vulnerability to disease, reduces fertility, and limits the adaptability of remaining groups, hastening the extinction spiral (Morgan et al., 2013).

Weak law enforcement and corruption in the illegal wildlife trade

Even though legal protections exist in all range countries, authorities enforcing the law are largely absent. As a result, Drill monkey habitat is rarely patrolled. Additionally corruption is rife and economic pressures drive local communities to encroach on wildlife zones. Without empowered, well-funded, and community-supported conservation efforts, these laws are worthless. (Linder & Oates, 2011).

Take Action!

The gentle and misunderstood Drill is a primate symbol of rainforest vitality, social intelligence, and emotional complexity. The extinction of these precious primates would be a moral and ecological tragedy. Take action every time you shop and boycott palm oil! Support indigenous-led forest protection in Nigeria, Cameroon, and Bioko. We must demand that large infrastructure projects and palm oil plantations are halted before more forest is lost. The time to act is now. #BoycottPalmOil #Boycott4Wildlife, be #Vegan #BoycottMeat

FAQs: drill monkey facts

How many Drills are alive in the wild?

The global population of the drill monkey has collapsed catastrophically. Conservationists estimate that only 4,000 to 6,000 individuals remain in the wild today. This includes up to 5,000 in Cameroon, roughly 1,000 in Nigeria, and fewer than 1,000 clinging to survival on Bioko Island. One of the biggest drill monkey threats is aggressive palm oil expansion and logging. This has destroyed their drill monkey habitat and puts scattered drill monkey populations at dangerous risk.

Do Drills form alliances with other species?

Yes. In Korup National Park, Cameroon, Drills forms cooperative foraging groups with red-capped mangabeys and other monkeys. These temporary alliances in Drill monkey habitat may help reduce predation risk and improve foraging efficiency by sharing lookout roles and occupying different layers of the forest (Astaras et al., 2011).

Explain Drill monkey mourning rituals

The most interesting of drill monkey facts comes from a 2023 study into drill mourning. In one instance, a Drill mother was grooming and attempting to revive her deceased infant for several days before eventually consuming the body. Group members also engaged in quiet observation. Drill behaviour like this shows that they have a profound capacity for grief, emotional bonds and possibly stress-related coping strategies (Casetta et al., 2023).

Why do hunters target Drills?

Disgustingly, hunters target Drill primates in the bushmeat trade, especially large males for their meat. Hunting methods include dogs and firearms, often decimating entire groups in one raid. Most of this hunting is for commercial sale in cities rather than for subsistence (Astaras, 2009).

What do the colours of Drills signify?

Male Drill primates display intense facial and rump colouration that becomes more vibrant with dominance and testosterone levels. The Drill monkey’s colourful appearance is used for social signalling and mate competition. Although studies suggest rank is more important than colour alone in determining reproductive success (Marty et al., 2009).

Do Drills grieve their dead?

A 2023 scientific study indicates that yes they do grieve their loved ones. The study documented a Drill mother gently carrying and carefully grooming her dead infant for two days, refusing to let go. Other Drills gathered around, quietly observing and touching the mother. On the third day, in an act both shocking and intimate, the mother began to eat the infant’s body in private.

Researchers believe this behaviour may help the mother to cope emotionally or recover nutrients after her traumatic loss. This rare observation shows that Drills, like other primates and humans, experience grief, maternal love, and perhaps even an awareness of death itself (Casetta et al., 2023). They are emotional beings and they are vanishing.

What are the natural Drill monkey predators?

The most dangerous of drill monkey predators are humans. Another primary Drill monkey predator is the leopard Panthera pardus, who stalks them through the dense West African rainforests. To escape a leopard, an entire troop of drills will startle and scramble high into the nearest canopy. However, human hunters actively exploit this exact instinct. Poachers use dogs to frighten the drills into the trees, where the monkeys are subsequently trapped and easily shot en masse with firearms to supply the commercial bushmeat trade. Absolutely disgusting!

What is the difference in a Drill monkey vs mandrill?

While they belong to the same genus Mandrillus and share a similar heavy, ground-dwelling body type, there is a stark visual difference in a Drill monkey vs a mandrill. The mandrill is famous for an incredibly colourful face featuring a bright red midline flanked by deeply grooved blue ridges. Conversely, the Drill monkey has a sleek, ungrooved, jet-black face with only a vivid red stripe beneath the lower lip. Furthermore, mandrills inhabit a broader range across Central Africa, while drills are highly restricted to a tiny sliver of forest spanning Nigeria and Cameroon.

Why do Drill monkeys show their teeth?

A male Drill monkey shows his teeth primarily as a threat display to assert dominance over rival males. Their canine teeth are formidable weapons, growing up to two inches long and rivalling those of large predatory cats. During a “threat yawn,” a male will deliberately gape his jaw wide, exposing these massive canines and demonstrating his physical jaw strength to competitors, without actually initiating a violent fight.

Support the conservation of this species

The Drill Project

WCS Nigeria

Further Information

Gadsby, E.L., Cronin, D.T., Astaras, C. & Imong, I. 2020. Mandrillus leucophaeus. The IUCN Red List of Threatened Species 2020: e.T12753A17952490. https://dx.doi.org/10.2305/IUCN.UK.2020-2.RLTS.T12753A17952490.en. Downloaded on 12 March 2021.

Astaras, C., Krause, S., Mattner, L., Rehse, C., & Waltert, M. (2011). Associations between the Drill (Mandrillus leucophaeus) and sympatric monkeys in Korup National Park, Cameroon. American Journal of Primatology, 73(2), 127–134. https://doi.org/10.1002/ajp.20877

Casetta, G., Nolfo, A. P., & Palagi, E. (2023). Record of thanatology and cannibalism in drills (Mandrillus leucophaeus). Primates, 64, 475–481. https://doi.org/10.1007/s10329-023-01075-8

Marty, J. S., Higham, J. P., Gadsby, E. L., & Ross, C. (2009). Dominance, coloration, and sexual behaviour in male Drills. International Journal of Primatology, 30(6), 807–823. https://doi.org/10.1007/s10764-009-9382-x

Morgan, B. J., Abwe, E. E., Dixson, A. F., & Astaras, C. (2013). The distribution, status, and conservation outlook of the Drill (Mandrillus leucophaeus) in Cameroon. International Journal of Primatology, 34, 281–302. DOI https://doi.org/10.1007/s10764-013-9661-4

National Geographic – Drills

Owens, J. R., Honarvar, S., Nessel, M., & Hearn, G. W. (2015). From frugivore to folivore: Altitudinal variation in the diet of the Bioko Island Drill. American Journal of Primatology, 77(11), 1263–1275. https://doi.org/10.1002/ajp.22479

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