There are more than 260 species of monkeys, split into two major groups: New World monkeys found in Central and South America, and Old World monkeys found across Africa and Asia. These two groups differ in body structure, behavior, and habitat, and they range from a creature that fits in your palm to one that weighs as much as a large dog.
New World vs. Old World Monkeys
The simplest way to understand monkey diversity is through this geographic split. New World monkeys live in the Americas, from southern Mexico down to Argentina. Old World monkeys span sub-Saharan Africa, North Africa, and large parts of Asia, from India and China to Japan and Southeast Asia. One species, the Barbary macaque, even lives in small pockets of Algeria and Morocco, where about 77% of the surviving population is found in high cedar forests.
The physical differences between these two groups are easy to spot once you know what to look for. New World monkeys have broad, flat noses with nostrils that point sideways. Old World monkeys have narrower noses with downward-facing nostrils, more similar to what you’d see on a human face. Old World monkeys also have tough sitting pads on their rear ends, built-in calluses that develop before birth. New World monkeys lack these.
Tails tell another story. All New World monkeys have tails, and in five groups those tails are prehensile, meaning the monkey can grip branches with its tail like a fifth hand. Old World monkeys have tails too (sometimes very short ones), but none of them are prehensile. This makes New World species like spider monkeys remarkably acrobatic, hanging and swinging by their tails in ways no African or Asian monkey can.
The New World Monkey Families
New World monkeys are classified into three families. All of them are small to medium-sized, ranging from just under 100 grams (a few ounces) to a little over 10 kilograms (about 22 pounds). Most lack an opposable thumb, which sets them apart from Old World species and apes.
The family Cebidae includes capuchins and squirrel monkeys. Capuchins are among the most intelligent monkeys in the Americas, famous for using tools like rocks to crack open nuts. Squirrel monkeys are smaller, social, and live in some of the largest monkey troops in the New World. This family also includes marmosets and tamarins, tiny primates that often live in cooperative family groups where a single breeding female is supported by multiple males who help carry and care for infants.
The family Atelidae contains spider monkeys, howler monkeys, and woolly monkeys. Spider monkeys are long-limbed acrobats with powerful prehensile tails. Howler monkeys are famous for their booming calls, which can travel several miles through dense forest. Both groups face serious conservation pressure, with several species of howler monkeys and spider monkeys among the most endangered primates on Earth.
The family Pitheciidae includes titi monkeys, sakis, and uakaris. These are generally less well-known but no less interesting. Titi monkeys form lifelong pair bonds, sitting side by side with their tails intertwined. Uakaris are striking for their bright red, nearly hairless faces.
The Old World Monkey Subfamilies
Old World monkeys all belong to a single family, Cercopithecidae, but that family is divided into two very different subfamilies.
The cercopithecines include baboons, macaques, mandrills, vervet monkeys, and mangabeys. These monkeys have cheek pouches that let them stuff food into their mouths and store it for later, like built-in grocery bags. This group is widespread and adaptable. Macaques alone account for more than 20 species, found everywhere from the snowy mountains of Japan to the temples of Bali. Baboons dominate open savannas across Africa, living in large, complex social groups.
The colobines are the leaf-eating monkeys. This subfamily includes colobus monkeys in Africa and langurs and proboscis monkeys in Asia. Their digestive systems are specialized for breaking down tough plant material, with multi-chambered stomachs that function somewhat like a cow’s. Red colobus monkeys in Africa are under particular threat, with multiple species critically endangered due to habitat loss and hunting.
Smallest to Largest
The size range across monkey species is dramatic. The pygmy marmoset holds the title of smallest monkey, measuring about 5 to 6 inches long (not counting the tail) and weighing roughly 4 ounces. That’s lighter than a stick of butter. These tiny primates live in the Amazon basin and feed heavily on tree sap, gnawing holes in bark to get it flowing.
At the opposite extreme, the mandrill is the largest monkey. Males can weigh up to 110 pounds and measure over three feet from head to tail. They live in the rainforests of central West Africa and are instantly recognizable by their vivid blue and red facial markings, which become more intense in dominant males. Despite their size and power, mandrills are vulnerable to extinction due to deforestation and bushmeat hunting.
How Monkeys Differ From Apes
People often confuse monkeys with apes, but the differences are significant. The easiest distinction: monkeys have tails, apes do not. Gorillas, chimpanzees, orangutans, and gibbons are all apes, not monkeys. Apes also have broader chests, shorter spines, and extremely flexible shoulders that allow them to swing through trees with their arms in ways monkeys generally cannot. Ape brains are proportionally larger, with heavier, more robust skulls to protect them. Monkeys tend to run along the tops of branches on all fours, while apes are built for hanging and climbing.
What Monkeys Eat
Monkey diets fall into a few broad categories. Frugivores eat mostly fruit. Folivores eat mostly leaves. Many species are omnivores, mixing fruit, leaves, seeds, insects, and occasionally small animals. The label often depends on what makes up the majority of the diet. A fruit-eating species might still spend 13% of its feeding time on leaves, while a leaf-eating species might get 16% of its calories from fruit and another 22% from seeds.
Old World leaf-eating monkeys like colobus and langurs have evolved specialized guts to extract nutrition from fibrous leaves that would be indigestible to most other primates. New World marmosets and tamarins have a different specialization entirely, using their lower teeth to gouge holes in trees and feed on gum and sap. Capuchins and baboons are more generalist, eating nearly anything they can find, from fruit and roots to bird eggs and small vertebrates.
Social Lives and Group Structure
Monkeys are overwhelmingly social animals, but the way they organize themselves varies widely. The most common arrangement is the multi-male, multi-female group, where dozens or even hundreds of individuals live together with complex social hierarchies and kin relationships. Macaques, baboons, vervet monkeys, capuchins, and squirrel monkeys all live this way.
Some species form single-male groups, sometimes called harems, where one dominant male lives with several females and their offspring. These groups are always at risk of takeover by outside males, who often form all-male bachelor groups while waiting for their chance. Other species take the opposite approach: titi monkeys and owl monkeys form monogamous pair bonds, with both parents investing heavily in raising young. Marmosets and tamarins add a cooperative twist, with multiple males in the group helping a single breeding female raise her infants.
Conservation Threats
More than half of all primate species are threatened with extinction, according to the IUCN. Monkeys face pressure from habitat destruction, hunting, and the illegal pet trade. In Africa, red colobus monkeys are particularly at risk. In South America, several howler monkey and spider monkey species are in steep decline. The Roloway monkey, found only in Ghana and Ivory Coast, is believed to be on the very verge of extinction.
Deforestation is the single biggest driver. Monkeys depend on forests for food, shelter, and movement corridors between populations. When forests are fragmented into small patches, monkey populations become isolated, making them more vulnerable to disease, inbreeding, and local extinction from events like storms or fires.