Geographic Distribution and Habitat
Found in the dense, ancient rainforests of a geographically limited area of western Africa, the drill is a little-studied Old World monkey in great need of conservation. The endangered drill species comprises two subspecies: Mandrillus leucophaeus, found north of the Sanaga River in Cameroon and Nigeria, and Mandrillus leucophaesus poensis, found only on Bioko Island, located about 20 miles (32 km) off the west coast of Africa. Both subspecies, along with their colorful and slightly larger mandrill cousins Mandrillus sphinx, are within the genus Mandrillus.
Once thought to be a type of forest baboon, drills are more closely related to white-eyelid mangabeys. Cameroon’s Korup National Park, with its mature lowland forest habitat, is home to the largest protected population of this spectacular species. Although drills have a restricted geographical range, they can be found in coastal lowland and riverine tropical rainforests as well as in foothills and higher mountainous regions.
Size, Weight, and Lifespan
As one of the world’s largest monkeys, drills display extreme sexual dimorphism, with males growing to double the size of females. Sexual dimorphism is distinct differences in size or appearance between the sexes of an animal in addition to differences between the reproductive organs themselves. On average, a male drill can grow to about 70 lbs (32 kg) with his more petite female counterpart weighing at about 26 lbs (12 kg). Average length at maturity is 22 in (56 cm).
A drill in captivity can live on average about 28 years. Females tend to have longer lifespans than males; the oldest known drill lived to be 46 years old.
Rarely seen in captivity—and even more rarely in the wild—the elusive drill’s visage is striking: a chiseled, shiny black face with penetrating amber honey eyes, a bright red lower lip, and prominent ridges along its nose. Drills from the mainland and Bioko share the same olive-brown coat with a pale white underbelly, though they can be distinguished by the hair on the sides of their crown: a Bioko drill has yellowish brown hair with a black tip while a mainland drill’s crown is ringed yellow and black.
The male drill, defender of family, has large canine teeth—sometimes more than two inches long—that he uses for protection and to compete with other male drills. Unlike his mandrill cousin, who is immediately recognizable for his bright red nose flanked with blue ridges, a mature male drill is perhaps more distinguished by the view of his rear: his bottom develops remarkable red, pink, and blue hues that grow more vibrant as he becomes excited. When a female drill is ready to mate, her genitals swell up, and her bottom becomes a deep red when she is pregnant.
Drills—members of the Cercopithecinae subfamily—have a short nub of a tail and possess deep cheek pouches that they use for storing food. Both males and females have scent glands on their chests or sternum, which they use for communication and territory marking. These sternal glands are a rare find in Old World monkeys, and are far more developed in males, who put them to excellent use during breeding season when they rub their chests on nearby trees to leave their scent and proclaim their dominance.
The omnivorous drill loves fruits (and may be considered a frugivore), but since fruit can be hard to find, she spends a great deal of her time foraging the forest floor, using her nimble hands to pluck through leaf litter, peek under logs, and peck away at bark in search of protein-rich seeds, insects, plants, eggs, and small mammals. She’s not an unnecessarily picky eater: the drill will enjoy what she can find, including sea turtle eggs, crabs, and domesticated crops. Researchers surmise that the drill’s tough molars are particularly well-suited to cracking open seeds and hard-shelled fruits.
Behavior and Lifestyle
The drill’s social structure and lifestyle are not fully understood due, in part, to lack of access: the dense, primary rainforests in which drills live make it difficult for researchers to conduct detailed studies of this shy species. In fact, the drill is one of the least-studied monkeys in Africa.
Bioko Island is also called Fernando Po. The island was first sighted by Fernão do Pó, a Portuguese explorer, in 1472. With its richly diverse and unique ecosystem, the island was originally named Formosa, which means beautiful.
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.
Drills make a great deal of noise in the forest. Researchers believe the drill’s regular grunts, screams, and calls allow groups to remain cohesive as they traverse their dense habitat; unfortunately, their vocalizations also make them easy targets for hunters.
During breeding season, male drills secrete a scented liquid from their sternal glands and will rub their chests against trees, which sends a territory-marking signal to other members of the group.
Drills also communicate with facial expressions: when a drill opens his mouth in a big wide grin, it’s not meant to be a threat as seen with other primate species, but rather a friendly appeasement. In addition, as with most primates, grooming is an important aspect of drills’ social bonding.
Male drills compete intensely for dominance and the right to breed. The bigger and fiercer a male drill is, the better his chances of success; this has also resulted in the significant size differences seen between males and female drills.
Female drills give birth seasonally (sometime between December and April) after a gestational period of about five to six months. Mother drills are the primary caregivers and protectors of offspring, but sisters and brothers also play a role in bringing up baby by helping to carry, groom, and play with their siblings.
Female drills are thought to remain with their natal group while males venture out on their own by the time they become sexually mature at five to seven years old.
As a primate species that eats fruits and seeds, drills help disperse seeds throughout their habitat, which is critical to forest maintenance.
The Bioko drill, found only on Bioko Island, is thought to play an especially vital role in maintaining balance in the diverse, delicate ecosystem of Bioko Island, home to some of Africa’s rarest primates. Seed dispersers like the drill help trees and plants spread their seeds and regenerate the rapidly disappearing forest.
Drill numbers have declined by over 50% in the last 30 years, and both subspecies of drills are considered Endangered by the International Union for Conservation of Nature (IUCN, 2017). Dwindling habitat, habitat fragmentation, and illegal bushmeat hunting continue to threaten drills’ survival. Drills have also fallen prey to growers of bananas, cocoa, and manioc, who view the elusive monkeys as pests to be eradicated. Often, hunters will use dogs to drive large troops of drills into trees, where they can be easily shot en masse.
Today, approximately 80% of all drills live in Cameroon, where the human populations has surged and drill habitat has yielded to oil palm plantations, oil exploration, and other economic developments.
On Bioko Island, drill numbers have fallen below 5,000, with its range shrinking to only 136 miles (220 km) today, down from 326 miles (525 km) in 1986. The entire space that all drills inhabit is roughly the size of Switzerland.
Drills are protected under the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) and although it is illegal to hunt drills, enforcement of laws is non-existent or difficult to implement, even in protected areas.
However, some promising programs to protect and raise awareness about this species are in place, including The Drill Project on Bioko Island, where conservationists are working to use film to educate the world about the drill’s precarious existence and the rare, rich biodiversity of Bioko Island. In Nigeria, Pandrillus operates a drill rehabilitation and breeding center that provides recovery, rehabilitation, and captive breeding of orphaned drills with goal of eventual release of drill groups. Korup National Park in Cameroon offers the world’s only protected area for drill populations.
- Redmond, Ian. The Primate Family Tree. New York: Firefly, 2008.
- Setchell, J. M. 2016. Drills and Mandrills. The International Encyclopedia of Primatology. 1–3.
Written by Christine Ragan Davi, August 2017. Conservation status updated July 2020.