Geographic Distribution and Habitat
Mandrills mostly reside in the tropical rainforests of equatorial Africa. The largest population lives in Gabon, where they inhabit dense, lowland jungles that range from sea level to steep hilly terrain. Smaller groups inhabit the rain forests, upland slopes, and bush of Cameroon, Equatorial Guinea, and the Democratic Republic of Congo. Despite their preference for thick woodland, mandrills spend their days on the ground and sometimes travel across savannas. The Sanaga, Ogooué, and Ivindo rivers bind their distribution to the north and east.
Mandrills belong to the Cercopithecidae family of primates. Initially, primatologists classified mandrills in the genus Papio alongside baboons, drills, and geladas. But recent research has determined that mandrills and drills warrant their own genus, Mandrillus. They are now considered the only two living species in the genus Mandrillus.
Size, Weight, and Lifespan
Mandrills are the largest monkeys in the world. Adult females (about 27 pounds/12 kg) tend to weigh two or three times less than males (about 70–119 pounds/32–54 kg), and usually measure roughly 22 inches (56 cm) to a male’s 32 inches (81 cm).
In the wild, mandrills live about 20 years. In captivity, with proper care, they often live up to 31 years, though the oldest captive mandrill on record lived to be 46.
Pairing olive-gray coats, blue noses adorned with red stripes, golden beards, and bright red lips with their remarkable stature and weight, mandrills distinguish themselves in both size and shading. Their unique rumps bear vibrant red, blue, or purple pads to support sitting, enhance visibility while moving through vegetation, and signal sexual interest and capacity to other mandrills. Exceptionally long arms and canines help them forage widely and omnivorously in the food-ridden forests. They also have long cheek pouches down the sides of their necks. When full, the pouches can hold close to a full stomach load of food.
Mandrills display sexual dimorphism—males are bigger and more colorful than females. The vibrancy of a male indicates their testosterone levels; consequently, dominant males are the most colorful, and their colors will fade if their status decreases. Females tend to prefer more colorful males. These differences appear in the face, rumps, and genitalia of male mandrills. Females indicate their fertility to males through a similarly visible display—red swellings on their rump.
When mandrills become excited, the coloring on their buttocks further brightens. And when a female flashes her bright rump at a male, it is typically a sign that she is in estrus and receptive to mating. However, rump flashing can also be an indication of submissiveness in both sexes, particularly between subordinate and dominant mandrills, and is considered a gesture of proper etiquette.
Up to 90% of a mandrill’s diet consists of figs and other fruits. They also eat seeds, insects, small vertebrates like lizards and rodents, and soil and clay. Researchers think this latter practice helps them add key minerals to their diet and neutralize the toxins of certain fruits.
While fruits, seeds, and small animals form the main staples of mandrill diets, mandrills will feed opportunistically on anything they can find. They irregularly scavenge on kills left behind by other predators, and occasionally, they have been observed eating other primates, though never other mandrills.
Behavior and Lifestyle
Mandrills sleep high up on tree branches to avoid predators like leopards and African rock pythons. At sunrise, they awake and move to the ground, where they spend most of the day foraging for food with breaks in between for resting, grooming, and socializing. While foraging, mandrills may dig in the ground for roots and tubers, climb trees to reach fruits and nuts, or search for small animals and insects on the forest floor. Adult females and mandrill young forage together. Adult males forage alone or clump in separate small groups. As the day progresses, mandrills become less active, and when night comes, they return to the trees and sleep.
Rafiki, Simba’s wise teacher in “The Lion King,” is commonly referred to as a baboon. But the colors on his face indicate that he’s actually a mandrill!
The colors on a male mandrill change depending on his status within his troop—more vibrant coloring on his face, rump, and genitals signals a higher status.
Mandrills are the largest monkeys in the world!
Mandrills live in multi-male/multi-female groups of roughly 40 members. These groups exist within larger groups called “troops” that may feature 600–800 mandrills. Within their smaller groups, mandrills maintain and rearrange their complex social networks and hierarchies through grooming, aggressive displays, bonding behaviors, and other forms of socialization. Every group contains distinct male and female hierarchies. A dominant male resides at the top of male hierarchies, while several adult females reign over female hierarchies.
The dominant male receives greater access to females during mating season. He is also responsible for protecting the group from threats and leading the group in foraging and investigating food sources. Because of his heightened status, the dominant male displays the most colorful facial, posterior, and genital markings. He maintains his dominance over the other males through threatening vocalizations, facial expressions, physical postures, scent markings, and displays of strength like lunging or charging. Since male dominance interactions rely on aggression, the dominant male is usually the largest and strongest in his group.
Female hierarchies are more complicated. Dominant females are usually the oldest and most experienced group members, and the status of other females depends on their familial relationship to these figures. The sisters and offspring of dominant females also hold or receive higher status—in other words, female mandrill hierarchies are “matrilineal.” Dominant females receive greater access to food and high-status males. Unlike males, who tend to operate on their own, females sometimes form alliances to increase their social strength and access to resources.
Grooming is crucial in mandrill groups, as it helps maintain dominance hierarchies, resolve conflicts, and strengthen social bonds. Dominant males or females tend to groom their subordinates, including mandrills of a different sex and/or age. Usually, grooming mandrills face one another in a seated position and use their fingers to remove parasites or dirt from their partners’ fur.
Mandrills partially communicate through complex vocalizations, including grunts, barks, and screams. Mandrill grunts are low-pitched and used to indicate contentment, to remain in contact with the group, and sometimes as a form of greeting. Barks are louder and higher-pitched. They warn other mandrills of potential threats and signal aggression. Screams are the loudest and highest-pitched—they indicate fear or alarm.
Body language and facial expressions also help mandrills communicate their social status and intentions. A mandrill’s subtle movements and posturing can inform their peers of intentions or emotional states—for instance, by slowly crouching to indicate submission. Less subtle bodily behaviors, like charging and fleeing, serve a similar (if more explicit) purpose. In adult males, facial coloration communicates the owner’s hormonal state—more color means more testosterone—while specific facial expressions, like baring teeth or showing the whites of their eyes, directly convey aggression or submission.
Mandrills also use chemical markers produced by glands on their chest, rump, and genitals to mark their territory with scents that convey information about their sex, hierarchical rank, and fertility. Primatologists think that “anogenital presentation”—the presentation of one’s genitals to a peer—helps mandrills convey these olfactory signals.
Mandrills usually engage in sexual activity twice each year, when females enter their 32-day menstrual cycle. During the cycle, the rumps of females swell and grow red to signal fertility. The swellings last about a week. Males and females both tend to mate with multiple partners. Dominant males and females receive greater access to similarly high-ranked partners.
Pregnancy lasts roughly six months, after which females give birth to one or two infants. For a few weeks, infants cling to their mothers’ fur and nurse frequently and are typically carried for a few months further. Males tend not to assist with caregiving, but unrelated females may assist with child-rearing—this practice is known as “alloparenting.”
As they become more mobile, the newborns begin exploring their environment and soon familiarize themselves with the social dynamics of their groups by interacting with other members. After about 2 years, they become independent from their mothers. Females reach sexual maturity at 4 years of age, while males mature by age 7.
By eating and digesting various fruits, mandrills disperse fruit seeds to different areas of their environment. Their foraging behavior also helps to maintain the forest understory, while their hunting behavior controls populations of certain insects and vertebrates. Despite their intimidating size and large groups, mandrills also serve as prey for leopards, African rock pythons, and a few other predatory species.
The International Union for Conservation of Nature lists mandrills as Vulnerable (IUCN, 2016), appearing on the IUCN Red List of Threatened Species. This means the species is likely to become endangered if greater efforts to conserve their population are not made.
The rainforests of equatorial Africa, where most mandrills live, are being rapidly cleared for mining, farming, logging, and human settlement. Their destruction splits mandrill groups and makes it more difficult for the primates to find food and mates. Hunting further weakens mandrill populations—mandrill meat is considered a delicacy in Africa, and their body parts are sometimes used in traditional medicine. Human diseases can also harm mandrills, and their spread has grown more common as human settlements increasingly encroach on mandrill habitats. Climate change has had a similarly potent effect by changing weather patterns, which in turn shifts the distribution of many common mandrill foods and changes the structure of the rainforest.
Mandrills are 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.
NGOs and local governments are working to secure mandrill populations through habitat protection, anti-hunting measures, breeding programs, and education/awareness efforts. In 2021, the U.N.-backed Central African Forest Initiative (CAFI) agreed to pay the nation of Gabon—where most mandrills live—$150 million to protect their rainforests. Gabon has been highly proactive in protecting their forests, which cover 90% of the country and reducing carbon emissions. Mandrill tourism, sponsored by NGOs like WCS Gabon in collaboration with the National Parks Authority, has also helped protect mandrills by reducing poaching through rigorous surveillance programs, raising public awareness, and providing primatologists with opportunities to study mandrill populations in-depth.
Efforts to breed mandrills in captivity and reintroduce them into wild spaces have also proved successful—in 2008, for instance, 36 captive-bred mandrills were released into Lékédi Park, Gabon, and after two years their survival and reproduction rates were stable.
Written by Eli Elster, January 2023