Old World monkeys are a large family of primates, called Cercopithecidae, that live across Africa and Asia. They are the closest monkey relatives to apes and humans, sharing the same dental formula: two incisors, one canine, two premolars, and three molars on each side of each jaw. With over 130 recognized species split into two distinct subfamilies, they are one of the most diverse and widespread primate groups on the planet.
How Old World Monkeys Differ From New World Monkeys
The “Old World” label refers to Africa and Asia, as opposed to the “New World” of the Americas. But the differences between the two groups go well beyond geography. Old World monkeys belong to a group called catarrhines, meaning their nostrils point downward and sit close together. New World monkeys have nostrils that are wider apart and face sideways.
Perhaps the most practical difference is the tail. Nearly all Old World monkeys have tails, but those tails are almost never prehensile, meaning they can’t grip branches the way a spider monkey’s tail can. Old World monkey tails are typically used for balance and are often short. The one exception to having a tail at all is the Barbary macaque, which is tailless. Another distinctive trait: all Old World monkeys develop hardened sitting pads on their rear ends, called ischial callosities, even before birth. These built-in cushions allow them to sit comfortably on branches and rocky surfaces for long periods.
The Two Subfamilies
Cercopithecidae splits neatly into two subfamilies with very different body plans and lifestyles.
Cercopithecines: The Cheek-Pouch Monkeys
This group includes about 45 species: macaques, baboons, mandrills, geladas, guenons, patas monkeys, and vervets, among others. Their signature feature is a pair of cheek pouches that work like built-in grocery bags, letting them stuff food away quickly and chew it later in safety. They have simple stomachs and short digestive tracts, suited to an omnivorous diet of fruit, seeds, insects, and small animals.
Cercopithecines split further into two groups by body type. The smaller, lighter species (guenons, patas monkeys, talapoins, vervets, and Allen’s swamp monkey, roughly 35 species across five genera) tend to be more agile and arboreal. The larger, heavier species (macaques, baboons, mandrills, geladas, and mangabeys, about 44 species in seven genera) are the most ground-dwelling of all Old World monkeys, with limb proportions and joint structures adapted for walking on land.
Colobines: The Leaf Eaters
The second subfamily contains around 63 species in 11 genera, including colobus monkeys, langurs, doucs, proboscis monkeys, and snub-nosed monkeys. Instead of cheek pouches, colobines have a large, multi-chambered stomach that functions a bit like a cow’s, allowing them to break down tough cellulose from leaves. Their small intestine is also notably long, maximizing nutrient absorption from a low-calorie diet.
That said, “leaf-eating monkey” is somewhat misleading. Colobines adjust their diet by season and regularly eat flowers, fruit, and seeds alongside leaves. They are generally more arboreal than their cercopithecine cousins, though grey langurs of South Asia are a notable exception, spending significant time on the ground.
Where They Live
Old World monkeys occupy an enormous geographic range. In Africa, they inhabit nearly every forest south of the Sahara. Blue monkeys and DeBrazza’s monkeys are widespread across sub-Saharan forests. Black-and-white colobus monkeys stretch from Guinea to Ethiopia and occupy the widest variety of forest habitats of any African colobine. Red colobus monkeys include 16 species distributed from Senegal all the way to Zanzibar. At the other extreme, geladas live only on the high, grassy plateaus of Ethiopia.
In Asia, the range extends from Sri Lanka and India through Southeast Asia and into China and Japan. Grey langurs occupy habitat from tropical forests to conifer forests and even temple grounds across western Asia. Surilis and lutongs live in the forests and rubber plantations of Indonesia and Malaysia. Snub-nosed monkeys push into some of the harshest environments of any primate, inhabiting montane forests in China, Vietnam, and Myanmar at high elevations. Japanese macaques famously survive snowy winters, making them the northernmost non-human primates in the world.
Physical Adaptations
The sheer variety of habitats Old World monkeys occupy has produced a wide range of physical adaptations. Species that spend most of their time in trees tend to have more rounded hip joints that allow flexible movement through the canopy, along with relatively longer arms for climbing. Ground-dwelling species like baboons have more elongated limb proportions, stronger muscle attachments at the joints, and reduced joint mobility, all features that favor efficient walking over uneven terrain.
Body size varies dramatically across the family. Talapoins, the smallest Old World monkeys, weigh around 1 kilogram. Male mandrills, the largest, can exceed 30 kilograms. This range reflects the diversity of ecological niches the family fills, from dense tropical canopy to open savanna.
Diet Across Species
Old World monkeys as a group are omnivorous, though individual species lean toward different food sources. Cercopithecines eat the broadest diet: fruit, seeds, flowers, bark, insects, birds’ eggs, lizards, and small mammals all appear on the menu depending on the species and the season. Baboons and macaques are especially opportunistic, raiding crops and scavenging in human settlements when natural food is scarce.
Colobines are more specialized, relying heavily on leaves and unripe fruit that other monkeys can’t efficiently digest. Their chambered stomach hosts bacteria that ferment plant cellulose, extracting energy from food sources most primates would ignore. But even colobines are flexible, incorporating flowers, ripe fruit, and seeds when available.
Social Lives
Old World monkeys are highly social, living in groups that range from small family units to troops of several hundred. Most species organize around a core of related females who remain in their birth group for life, while males disperse to join new groups at maturity. This matrilineal structure means that a female’s social rank passes to her daughters, creating stable hierarchies that can persist across generations.
Geladas form some of the largest aggregations of any primate, with herds of over 600 individuals gathering on Ethiopian grasslands. Baboons live in multi-male, multi-female troops where complex alliances and rivalries shape daily life. Some colobines, by contrast, live in smaller groups of one male with several females. Grooming is the social currency across nearly all species, reinforcing bonds, resolving tension, and maintaining group cohesion.
Reproduction and Development
Gestation in Old World monkeys typically lasts five to six months, with most species giving birth to a single infant. Twins are rare. Mothers carry and nurse their young for an extended period. In rhesus macaques, one of the best-studied species, infants nurse for roughly 6 to 12 months before weaning. Earlier weaning (around 6 months) tends to allow mothers to reproduce again sooner, while later weaning delays the next pregnancy but doesn’t appear to harm infant health.
Young monkeys depend on their mothers not just for nutrition but for learning social rules, foraging techniques, and predator awareness. In matrilineal species, a mother’s rank directly shapes the resources and social opportunities her offspring will have access to throughout life.
Conservation Concerns
Old World monkeys face serious threats. According to IUCN Red List data, 88 species or subspecies within the family are critically endangered, 146 are endangered, and 115 are classified as vulnerable. Habitat loss from deforestation, agricultural expansion, and urbanization is the primary driver, compounded by hunting for bushmeat and the illegal pet trade.
Some species survive in alarmingly small numbers. The grey-shanked douc of Vietnam has fewer than 700 individuals remaining in the wild. The dryas monkey of the central Congo basin and the kipunji of Tanzania are both critically endangered. The pig-tailed langur, confined to Indonesia’s Mentawai island chain, and the proboscis monkey of Borneo’s peat swamps and mangrove forests are also at serious risk. Among red colobus monkeys, at least one species may already be extinct. Snub-nosed monkeys, living in remote montane forests, are all endangered, though new species are still occasionally being discovered in these hard-to-reach habitats.
Relationship to Apes and Humans
Old World monkeys share a more recent common ancestor with apes and humans than New World monkeys do. This is reflected in their identical dental formula, similar nostril structure, and closer genetic relationship. However, Old World monkeys diverged from the ape lineage roughly 25 million years ago and have followed their own evolutionary path since, diversifying into the wide array of species seen today. They are not ancestors of apes or humans but rather evolutionary cousins, occupying a parallel branch on the primate family tree.