Mammals

Monkey

Simiiformes (excluding apes) · also called monkeys, New World monkeys, Old World monkeys

Not Evaluated ↓ omnivoreherbivore Rainforest Temperate forest Savanna & grassland Temperate grassland

A monkey is a primate with a tail, roughly speaking, and there are about 260 kinds. They divide into two great branches: the New World monkeys of Central and South America, flat-nosed and often with a tail that grips like a fifth hand, and the Old World monkeys of Africa and Asia, narrow-nosed, tail never grasping, and — surprisingly — the closer relatives of apes, including us.

Overview

Monkeys are simian primates found across the tropics of the Americas, Africa, and Asia, from lowland rainforest to the snow country of Japan and the edges of big cities. They are mostly omnivores leaning on fruit and leaves, with insects, gum, eggs, and small animals filling in. They range from the 100 g pygmy marmoset to the 35 kg mandrill.

Two branches, one grip

New World monkeys (platyrrhines) have broad, outward-facing nostrils, and some — spider monkeys, howlers, woolly monkeys — have a true prehensile tail. Old World monkeys (catarrhines) have narrow, downward nostrils, never a grasping tail, and often cheek pouches and hardened sitting pads. Because Old World monkeys are closer to apes than to New World monkeys, “monkey” is not a natural group — apes are technically nested inside it.

Societies

Most monkeys live in groups, from marmoset family units to baboon troops over a hundred strong, with hierarchies, alliances, grooming networks, and local traditions — like the Japanese macaques that wash food and bathe in hot springs.

Status

Not assessed as one group, but primates are among the most threatened mammals, with over half of monkey species declining from forest loss, bushmeat hunting, and the pet trade. Several lion tamarins, langurs, and red colobus are among the world’s most endangered primates. Monkeys are not suitable pets.

Monkey: common questions

What is the difference between a monkey and an ape?

The quickest field mark is the tail — almost all monkeys have one, and no ape does. Beyond that, apes, which are the gibbons, orangutans, gorillas, chimpanzees, bonobos, and humans, are generally larger, have broader chests and shoulder joints built for hanging and swinging beneath branches, walk more upright, take longer to grow up, live longer, and have larger, more complex brains relative to body size. Monkeys typically have a narrower chest, a more horizontal back, run along the tops of branches on all fours, and mature faster. In evolutionary terms apes are a branch that arose from within the Old World monkey group, so an ape is, in a sense, a tailless monkey that got bigger and smarter, but in everyday usage the two are kept separate.

What is the difference between New World and Old World monkeys?

They are the two main branches of living monkeys and have been separate for around 40 million years, since monkeys reached South America, probably by rafting across an Atlantic that was then narrower. New World monkeys live in the Americas, have broad flat noses with nostrils that face outward, usually have three premolar teeth on each side of each jaw, and include the only monkeys with a prehensile tail, along with the marmosets and tamarins, which have claws instead of nails. Old World monkeys live in Africa and Asia, have narrow noses with downward-facing nostrils, have two premolars, never have a grasping tail, and many have cheek pouches and hardened sitting pads. Old World monkeys are also the group that apes evolved from.

Are monkeys good pets?

No. Keeping a monkey as a pet is widely discouraged by primatologists, veterinarians, and animal-welfare organisations, and it is illegal or restricted in many countries and US states. Monkeys are intensely social, long-lived, and intelligent, and a hand-reared monkey kept alone becomes psychologically damaged, and as it matures it typically becomes unpredictable and aggressive, with a strong bite, and it cannot be safely rehomed. They can transmit diseases to people, including herpes B from macaques, which is often fatal, and hepatitis and tuberculosis. The pet trade also drives the capture of wild infants, usually after killing the mother. A monkey belongs in a troop of its own species, in the wild or in a properly run sanctuary.

Which is the smallest and which is the largest monkey?

The smallest is the pygmy marmoset of the upper Amazon basin, which weighs about 100 grams and is roughly the size of a large mouse with a long tail; it feeds mainly on tree gum that it gouges out with specialised lower teeth. The largest is the mandrill of the rainforests of Cameroon, Gabon, and the Congo, in which a dominant adult male can weigh 30 to 35 kilograms, more than three times the female, and develops brilliant blue and red ridges on the muzzle and matching colours on the rump, brightest in the highest-ranking males. The drill, a close relative, is nearly as large.

Sources & further reading

Written by HideShark Editorial Team · reviewed by HideShark Science Desk. Published September 10, 2026. Last updated September 10, 2026.

Quick facts

Scientific name: Simiiformes (excluding apes)
Diet: Mostly fruit and leaves supplemented by seeds, flowers, nectar, gum, insects, spiders, bird eggs, and small vertebrates, with the exact mix varying widely, from the almost entirely leaf-eating colobus monkeys to the insect and small-animal hunting of some capuchins and the gum-gouging of marmosets
Two great branches: Living monkeys fall into two groups that separated around 40 million years ago, the platyrrhines or New World monkeys of Central and South America, with broad flat noses and outward-facing nostrils, and the catarrhines or Old World monkeys of Africa and Asia, with narrow noses and downward-facing nostrils
Only New World monkeys have a gripping tail: A prehensile tail that can grasp branches and even pick up small objects has evolved only in some New World monkeys, such as spider monkeys, howlers, and woolly monkeys, and it is missing from every Old World monkey and every ape, so a monkey seen hanging by its tail is always an American species
Monkey is not a scientific group: In the primate family tree the Old World monkeys are more closely related to apes than they are to the New World monkeys, so the everyday category monkey, which lumps both together while excluding apes, is not a natural group, and strictly speaking apes and humans are nested inside the monkey lineage
A huge size range: Monkeys span from the pygmy marmoset of the western Amazon, at around 100 g and small enough to sit on a finger, to the mandrill of central African forests, in which a big male can weigh 35 kg and shows some of the most vivid facial and rump colours of any mammal
Cheek pouches and sitting pads: Many Old World monkeys, including macaques and baboons, have large cheek pouches for stuffing food to eat later in safety, and hardened, often brightly coloured pads of skin on the buttocks called ischial callosities that let them sleep sitting upright on branches, neither of which occurs in New World monkeys
Complex societies: Most monkeys live in groups, from small family units in marmosets and titis to troops of over a hundred in baboons and some macaques, with dominance hierarchies, grooming networks, alliances, vocal and gestural communication, and, in several species, local traditions such as the Japanese macaques that wash sweet potatoes and bathe in hot springs
Range: The tropics and subtropics of the Americas, Africa, and Asia, with New World monkeys from southern Mexico through Central and South America to northern Argentina, and Old World monkeys across sub-Saharan Africa, a small population in Gibraltar, and southern and eastern Asia as far north as the snow country of Japan and the Tibetan plateau, in rainforest, montane and dry forest, savanna woodland, mangroves, grassland, rocky hills, and, increasingly, towns and cities.
Conservation status: Not Evaluated ↓ ≈ Monkeys are not assessed as a single group, and their situations range from abundant and expanding, as in the rhesus macaque and some baboons that thrive around people, to critically endangered. Overall the trend is downward, and primates are among the most threatened groups of mammals, with well over half of all monkey species declining. The main pressures are the clearing and fragmentation of tropical forest for agriculture, logging, and mining, hunting for bushmeat, and capture for the pet trade and biomedical research, and several monkeys, including some langurs, the various lion tamarins, and a number of African red colobus, are on the list of the world's most endangered primates in the wild