There are more than 260 species of monkeys, split into two major groups: New World monkeys found in the Americas and Old World monkeys found across Africa and Asia. These two groups differ in anatomy, behavior, and habitat, and they range from a creature small enough to fit in your palm to one that weighs more than 75 pounds.
The Two Main Groups
Every monkey species falls into one of two broad categories based on where it evolved. New World monkeys live in Central and South America. Old World monkeys live in Africa and Asia. The split between these lineages happened tens of millions of years ago, and the two groups have been on separate evolutionary paths ever since.
The easiest way to tell them apart is the nose. New World monkeys have flat noses with wide, sideways-facing nostrils. Old World monkeys have narrower noses with downward-facing nostrils, more similar to what you’d see on a human face. Other differences run deeper: many New World monkeys have prehensile tails that grip branches like a fifth hand, while no Old World monkey does. Old World monkeys often have tough sitting pads on their rear ends (called ischial callosities) and expandable cheek pouches for storing food. New World monkeys have neither.
One quick distinction worth knowing: monkeys are not apes. The simplest way to tell them apart is the tail. Monkeys typically have tails, while apes (gorillas, chimpanzees, orangutans, gibbons, and humans) do not. Monkeys also have arms and legs of roughly equal length, whereas apes have arms noticeably longer than their legs.
New World Monkeys
New World monkeys range from southern Mexico through Central America and deep into South America, mostly inhabiting tropical rainforests. Every species in this group is arboreal, meaning they spend their lives in trees. Nearly all are active during the day, with one exception: owl monkeys (the genus Aotus), which are the only nocturnal monkeys on Earth.
Within this group, three major clusters stand out:
- Marmosets and tamarins: These are the smallest monkeys. They have claw-like nails instead of the flat nails most primates have, and many species sport dramatic facial hair, manes, or ear tufts. The pygmy marmoset holds the record as the smallest monkey in the world, weighing just about 3 ounces (90 grams) as an adult. Tamarins include the striking golden lion tamarin of Brazil’s Atlantic Forest, one of the most recognizable endangered primates.
- Capuchins and squirrel monkeys: Capuchins are famously clever, known for using rocks to crack open nuts and for their complex social behavior. Squirrel monkeys are smaller and form some of the largest troops of any New World monkey, sometimes numbering in the hundreds.
- Howler monkeys, spider monkeys, and woolly monkeys: These are the large-bodied New World monkeys, and they all possess fully prehensile tails strong enough to support their entire body weight. Howler monkeys produce one of the loudest calls of any land animal, audible from miles away. Spider monkeys are remarkably agile, swinging through the canopy with long limbs and a tail that functions like an extra arm.
A separate cluster, the sakis and uakaris, are specialized seed predators with powerful jaws built for cracking open hard-shelled fruits that other monkeys can’t access. Uakaris are instantly recognizable by their bright red, hairless faces.
Old World Monkeys
Old World monkeys belong to a single large family that spans two subfamilies with very different lifestyles. Together, they occupy an enormous geographic range, from the rainforests of equatorial Africa to the snowy mountains of Japan, the mangrove swamps of Southeast Asia, and even cities across South and East Asia.
Cheek-Pouched Monkeys
The first subfamily includes baboons, macaques, guenons, mangabeys, and patas monkeys. Most of these species have expandable cheek pouches that let them stuff food away quickly and eat it later in safety. They tend to be omnivorous, eating fruit, seeds, insects, and occasionally small animals.
Baboons are among the most terrestrial of all monkeys, spending much of their time on the ground across African savannas and rocky terrain. Mandrills, closely related to baboons, are the largest monkeys in the world. An adult male mandrill can reach about 36 inches long and weigh up to 77 pounds (35 kilograms), roughly 400 times heavier than a pygmy marmoset. Males sport vivid blue and red facial coloring that intensifies with social dominance.
Macaques are the most widespread Old World monkeys geographically. The Japanese macaque (snow monkey) survives harsh winters by soaking in volcanic hot springs. Rhesus macaques thrive in cities across India and Southeast Asia. The Barbary macaque, native to North Africa and Gibraltar, is the only wild monkey in Europe.
Leaf-Eating Monkeys
The second subfamily consists of colobines, the leaf specialists. These include the colobus monkeys of Africa and the langurs, proboscis monkeys, and snub-nosed monkeys of Asia. Their digestive systems have evolved a specialized, multi-chambered stomach that ferments tough leaves, similar in principle to how a cow digests grass. This adaptation allows them to thrive on a diet that would be nutritionally useless to most other primates.
Proboscis monkeys of Borneo are famous for the male’s enormous, pendulous nose. Snub-nosed monkeys in China live at elevations above 13,000 feet, enduring freezing temperatures that would challenge most tropical primates. Black-and-white colobus monkeys of Africa are striking in appearance and unusual among monkeys for having only a small thumb stub, an adaptation to swinging through branches.
What Monkeys Eat
Monkey diets vary enormously across species. Primatologists generally categorize them by their primary food source: frugivores eat mostly fruit, folivores eat mostly leaves, and some species are true omnivores that eat across multiple food types including insects, eggs, and small vertebrates.
These dietary differences shape the body. Leaf-eating monkeys tend to have longer, more complex digestive tracts, specialized teeth for shearing fibrous plant material, and gut bacteria adapted to break down cellulose. Fruit-eating monkeys have simpler digestive systems and faster gut passage. Some species, like the sakis, have evolved powerful jaws specifically for cracking seeds that other monkeys avoid. Marmosets and tamarins gouge holes in tree bark to feed on sap and gum, a food source called exudate that few other primates exploit.
Social Lives and Group Structure
Monkeys are highly social animals, but the way they organize their groups varies considerably. One of the most common arrangements is the one-male group, where a single adult male lives with and defends a group of related females. This structure is typical of most colobus monkeys, guenons, patas monkeys, and howler monkeys. The resident male has exclusive mating access, but his position is always under threat. Males who haven’t secured a group often form roaming all-male bands, waiting for an opportunity to challenge and replace a resident male.
Many other species, including macaques and baboons, live in multi-male, multi-female groups where both sexes mate with multiple partners. These groups can be large and socially complex, with hierarchies maintained through grooming alliances, displays of dominance, and sometimes aggression. Capuchins and spider monkeys show yet another pattern: spider monkeys form fluid groups that split apart and reunite throughout the day, while capuchins maintain stable groups with clear pecking orders.
Marmosets and tamarins are unusual among monkeys for their cooperative breeding system. Typically only one female in the group reproduces, and the other group members, including the father and older siblings, help carry and care for the infants.
Conservation Outlook
Roughly 60% of all primate species are now threatened with extinction, and about 75% have populations that are actively declining. Monkeys make up the bulk of those numbers. Habitat destruction, particularly tropical deforestation for agriculture, is the primary driver. Hunting for bushmeat, the illegal pet trade, and climate change compound the problem.
Some of the most endangered monkeys include several species of langur in Southeast Asia, the golden lion tamarin in Brazil, and the drill of West Africa. Many species exist in fragmented patches of forest so small that a single road or farm expansion can cut off populations from one another permanently. Conservation efforts range from protected reserves and reforestation projects to captive breeding programs that have, in some cases, pulled species back from the edge. The golden lion tamarin, once down to a few hundred individuals in the wild, has rebounded to several thousand thanks to decades of targeted conservation work.