Drill, Mandrillus leucophaeus
DRILL
Mandrillus leucophaeus
Geographic Distribution and Habitat
The drill (Mandrillus leucophaeus) is a monkey species that occurs across three African countries: Nigeria, Cameroon, and Equatorial Guinea’s Bioko Island. On the mainland, with a geographical range smaller than 7,722 square miles (20,000 sq km), their area of occupancy is slightly smaller than the state of West Virginia. Populations occur in fragmented pockets as far south as the Sanaga River in Cameroon and as far west as the Cross River in Nigeria with a total population of about 4,000 mature individuals.
The drills of Bioko Island occur in lowland coastal forests and have been observed on beaches along Bioko’s southern coast, as well as submontane and montane forests. Most of the population is found in the remote southwestern sector of the Gran Caldera Scientific Reserve. This area is considered a “monsoon forest”, receiving over 393 inches (1,000 cm) of rainfall per year. It has a very small human footprint. Near human settlements, Bioko Island’s drills persist in low densities and have been observed raiding crops and fruit trees. The total population of Bioko Island’s drill is estimated at 600 mature individuals.
Drills live in a variety of different climates and habitats, including forests, savanna, inland cliffs, and mountain peaks. Throughout Nigeria, Cameroon, and Bioko, there is typically one long, dry season that lasts from December to January and has very minimal rainfall, often less than 3.94 inches (100 mm) per month. Southern and coastal areas tend to be cooler than other geographical locations, and the average temperatures typically range between 77 to 86 degrees F (25 to 30. degrees C).
Formally considered “forest-dwelling baboons”, drills (Mandrillus leucophaeus) have been observed by scientists on Africa’s mainland since 1962, and on Bioko since 1990 when Gail Hean visited the island with two primatologists. Although drills were previously grouped by some into the baboon genus Papio, genetic analyses concluded that the genus Mandrillus is more closely related to the mangabey genus Cercocebus than it is to Papio.
Across Nigeria, Cameroon, and Bioko, scientists also discovered that there are two subspecies of Mandrillus leucophaeus: M. l. leucophaeus on the mainland, and M. l. poensis on Bioko. In order to further support these discoveries, ongoing morphological research is crucial, now more than ever.
Size, Weight, and Lifespan
Drills are highly sexually dimorphic and males often exhibit secondary sexual characteristics that females do not have. Males, which are significantly heavier and slightly longer than females, weigh 71.2 pounds (32.3 kg) and are 32.3 inches (82 cm) long, while females weigh 25.79 pounds (11.7 kg) and are 26 inches (66 cm) long. Males typically also have larger paranasal muzzle swellings than their female counterparts. A paranasal muzzle swelling refers to the noticeable swelling on the muzzle of an animal around their nostrils, which is caused by enlarged paranasal sinuses, or hollow spaces within the facial bone next to the nasal cavity.
In a 2006 study, primatologists compared the relationship between the paranasal muzzle swellings of drills and their body mass in order to determine biological differences between males and females. Through the research, primatologists discovered that the muzzle swellings in males appeared to be independent of their body mass, while a female’s muzzle swelling seemed to be directly correlated to her mass. Upon this discovery, scientists determined that the growth of the muzzle, and paranasal swellings, is very likely to be influenced by androgen production, or steroid hormones produced in the testes, ovaries, and adrenal glands. Considering how androgens may affect a male’s testes size and sperm motility, or the ability for sperm to move successfully through a female’s reproductive tract, the study concluded that large paranasal muzzle swelling may be a clear indicator of a male’s reproductive quality. This observation is particularly significant when studying how males attract mates, especially when it comes to displaying behaviors related to dominance, intimidation, and attraction.
In the wild, the average drill may live for up to 10 years.
Appearance
Drills are large, short-tailed, long-snouted monkeys with chocolate-brown eyes. Besides their face, most of their body is covered by thick, brownish-gray hair. Their faces are black, shiny, and decorated with white, fringe-like hair on the outer edge. They have chiseled nasal ridges and large, fatty cheek pads on either side of their face.
Although their facial features are worthy of a Vogue Magazine cover spread, drills have other, more colorful features that make them even more exciting. While most of a drill’s body is covered in fur, males in particular have chins, buttocks, and groin areas that are very vibrant in color: red markings paint their chin and bottom lips, while their buttocks and groin are stained various shades of red, blue, lilac, pink, and white.
According to a behavioral study conducted in 2009, researchers observed that the brightest colored males had the highest ranking within a troop’s social structure. These particular males were also observed to be the most sexually active troop members. Further considering the male-female associations in the troop, researchers also discovered females’ lack of mating preference for the most brightly colored males. This observation suggests that male-male interactions are more affected by coloration differences than male-female interactions.
Although the chin, buttocks, and groin are generally brown and much more subdued in females, when she becomes pregnant or is ready to mate, her genitals swell up and turn bright red.
Diet
Drills are omnivorous and opportunistic eaters. Depending on the distribution of fruiting trees during the rainy season, forest fruit is their most preferred food source. Considering the geography of their natural habitats, fruit and plant species they may frequently eat from include the fiddle-leaf fig (Ficus lyraata), the cola palm tree (Oncocalamus), and the African corkwood tree (Musanga cecropioides). During the dry season, other foods such as leaves and grasses take priority. In general, a drill’s main diet primarily consists of various types of seeds, fruits, mushrooms, plants, and some invertebrate species. Ants, just like seeds, provide a rich source of protein for drills and are easy to digest thanks to their specialized dental system. Their long canines, which are 2 inches (5.08 cm) in length, and hard molar teeth are evolutionarily designed to accommodate their wide selection of food sources.
Behavior and Lifestyle
Drills are seasonal, semi-nomadic primates and are most likely to socialize at dawn and dusk, particularly when the sun peaks out above the horizon and when the sky is illuminated by a soft, hazy glow.
Depending on the season, led by the dominant male, troops may undergo a migration in which the entire unit travels from one region to the next in search of accessible resources, motivated by warmer climates. As a semi-terrestrial species, drills will travel quadrupedally and walk on all four limbs. Once a new home is established, members will remain within 1.93 square miles (5 square km) of their designated sleeping tree for an entire season; drills leave their tree each day in search of food and other resources.
In order to find resources, drills spend most of their day foraging on the ground for prey and fallen foods, in addition to climbing trees in search of specific flower types, or fruit. Although they are often incorrectly referred to as terrestrial primates, drills are actually very good climbers, as supported by their specialized forearm muscles that allow them to cling vertically to tree trunks and forest vines. Since so much of their day is spent looking for food, foraging, and climbing may provide intense exercising opportunities and engaging mental stimulation.
At night, regardless of whether they are living within lowland forests, premontane forests, submontane forests, montane forests, or montane savannas, drills find shelter and protection within the trees, especially when avoiding predators.
When confronted by predators, rather than physically fighting, drills often prefer to flee danger by climbing high into the forest canopy. Typically, these heights begin at 32.8 feet (10 m) and continue to the top of the canopy, sometimes reaching 100 feet (30.4 m) high! That being said, if the situation calls for it, drills have the capacity to fight fiercely and furiously. Besides humans, the natural predators of the drill include the leopard (Panthera pardus), crowned eagle (Stephanoaetus coronatus), and the Central African rock python (Python sebae).
Do drill monkeys seem familiar to you? Well, if you’ve ever seen the sequel to the “Hunger Games”, you may recognize these colorful, formidable creatures: drills were one of the inspirations for the aggressive monkeys Katniss Everdeen fights off in the movie “The Hunger Games: Catching Fire”.
According to the film’s visual effects supervisor Janek Sirrs, the design of the movie’s murderous primates was inspired by both drills (Mandrillus leucophaeus) and mandrills (Mandrillus sphinx). In an interview, Sirrs revealed that during filming, when the production team visited a zoo in Atlanta, Georgia, they were so inspired by a drill they observed in captivity that they decided to replicate its appearance for the movie. Although the movie does not portray an accurate account of a drill’s natural behavior, it is cool to see the role that it played in such a highly coveted movie franchise!
Drills live in multi-male, multi-female troops that consist of between 15 to 75 individuals. To put the dynamic into perspective, for every 20 troop members, there is one adult male. On occasion, multiple drill troops may come together to form giant “super groups” that are made up of more than 100 individuals. Unfortunately, more scientific research is necessary to explain why this phenomenon may occur.
In addition to adult males, troops are made up of females, juveniles, subadults, and infants.
When a dominant male is ready to breed with a female, in addition to grinning and smacking lips at an attractive female, the males will fight for dominance, and the control of breeding opportunities, by performing “mate guarding”. Mate guarding is a behavior in which the male guards the female away from other males and is aggressive toward those who approach the female. This behavior creates a monopoly and actively excludes other competitors from accessing the female.
Among the troop’s adult males, dominance lasts about three years. Once this time has passed, males will compete against one another through a series of displays, evidently attempting to show off. Although actual fights are rare, when males do engage in aggressive contact, they usually begin when one male holds out a hand to the other in an attempt to grab him. If contact is made, each competitor will begin to fight. Whoever inflicts the most damage will be deemed the winner, and will soon have the support of the troops majority. Although juveniles do not get involved in the actual fight, they will instead sit nearby and follow the fight on the sidelines. For them, only popcorn and a wide-screen television could make the experience even better!
Later, when a lesser male is confronted by a dominant male, the lesser male may submit to their defeat by displaying an appeasement grin. These grins, or smiles, are behavioral tactics used in an attempt to diffuse tension and keep peace among troop members. Just like for humans, the act of smiling, even if forced, can release feel-good neurotransmitters that can physically reduce stress and anxiety, making one appear more approachable and calming in potentially tense situations.
For drills, the average gestation length, or the period of time in which a female is pregnant, is 179 to 182 days. After this period, females give birth to a single infant. Until an infant becomes independent, adult females are the primary caregivers, although juveniles may play a significant role in parenting alongside the adults, especially when it comes to grooming and socializing their siblings.
Young drills are officially weaned around 15 to 16 months old, and no longer rely on others for daily activities. Males, as they move towards adulthood, begin spending less and less time with their mothers and sisters, and become more independent. Since subadult males quickly become the target of a dominant male’s aggression, they are typically pushed to the bottom of their troop hierarchy or totem poll. Eventually, when he becomes sexually mature, he will abandon his home troop entirely in the hope of creating his own. Although solitary males have been observed in the wild, this is not a typical behavior.
For females, sexual maturity is reached at the age of three, while sexual maturity for males is reached around the age of six.
Due to their opportunistic dietary preferences, drills help disperse various plant and seed products throughout their home range, fulfilling a critical ecological niche within their ecosystem. Whether plucking fruit from trees or digestively passing seeds through their system, when drills drop seeds throughout a forest, they are participating in seed dispersal. Seed dispersal allows plant parents to spread their seed across a wide area of land in order to avoid competition with other plant species. In turn, this increases a seedling’s chance of survival and promotes the general biodiversity of the area in which they are growing.
The International Union for Conservation of Nature lists drills as Endangered (IUCN, 2017), appearing on the IUCN Red List of Threatened Species.
According to the IUCN Redlist, concern for the drill’s survival was noted as early as 1962 for populations in Nigeria and 1968 for populations in Cameroon. In addition to having a very limited range within highly human-populated areas, hunting, habitat loss, and habitat fragmentation are the largest threats to drills and their conservation status.
Habitat loss is a result of forest clearance for farms, particularly in Nigeria where land is cleared for large-scale plantation projects. In Cameroon, commercial logging is the largest threat to the ecosystem, especially since it opened and provided new road access to hunters and cultivators.
The drill is listed in Appendix I of the Convention on International Trade in Endangered Species (CITES), an international agreement between governments whose goal is to ensure that international trade in specimens of wild animals and plants does not threaten their survival.
Although the species currently faces high hunting pressures, field studies have been carried out to find the best way to protect this species. In Cameroon, drill monkeys occur and are protected in Korup, Takamanda, Bakossi, and Mount Cameroon National Parks. In Nigeria, they are protected across the Oban and Okwangwo Divisions of Cross River National Park, Afi Mountain Wildlife Sanctuary, and the Mbe Mountains Community Forest. On Bioko Island, drills are protected in Gran Caldera de Luba Scientific Reserve and are also believed to persist in the Pico Basile National Park.
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Written by Brooklynne Mitchell-Arno, December 2024