Monkey Facts & Species, Monkey Species
What Is a Monkey? Definition, Traits and How to Tell One Apart
A monkey is a primate belonging to the simian group Simiiformes that is not an ape. In practice that means a tailed, mostly tree-dwelling mammal with grasping hands, forward-facing eyes and a highly social lifestyle. Around 260 to 270 species fit that description, spread across Africa, Asia, Central America and South America.
Key takeaways
- Monkeys are simian primates split into Old World monkeys (Africa, Asia) and New World monkeys (the Americas).
- Nearly all monkeys have a tail; apes never do.
- They walk along branches on all fours rather than swinging arm-over-arm like gibbons.
- Size ranges from the 100 g pygmy marmoset to the 35 kg mandrill.
- “Monkey” is a practical everyday label, not a strict evolutionary group, because apes descend from monkey-like ancestors.
The scientific definition
Primates divide into two suborders. Strepsirrhines include lemurs, lorises and galagos. Haplorhines include tarsiers plus the simians. Simians then split again into Catarrhini — Old World monkeys and apes — and Platyrrhini, the New World monkeys. Monkeys are therefore every simian except the apes.
Because apes evolved from inside the catarrhine branch, biologists call “monkey” a paraphyletic group. That technicality does not stop the word being useful: in the field, the difference between a vervet and a chimpanzee is obvious in three seconds. The details are laid out in Monkeys vs Apes.
Physical traits that define a monkey
| Trait | What to look for |
|---|---|
| Tail | Present in almost every species; prehensile in a few New World genera |
| Chest | Narrow and deep, unlike the broad barrel chest of apes |
| Locomotion | Quadrupedal running along branches; leaping rather than brachiating |
| Hands | Five digits with nails, opposable or pseudo-opposable thumb |
| Eyes | Forward-facing, giving stereoscopic depth perception for canopy travel |
| Brain | Large relative to body size, with heavy investment in vision and social processing |
| Teeth | Bilophodont molars in Old World species, adapted for shearing leaves and fruit |
More on how those features evolved: Monkey Adaptations and Monkey Evolution.
What monkeys are not
Not apes
Gibbons, orangutans, gorillas, chimpanzees, bonobos and humans are apes. No tail, broader shoulders, bigger brains, and a shoulder joint built for hanging.
Not lemurs or bushbabies
Lemurs, lorises and galagos are strepsirrhine primates with a wet nose, a reflective eye layer for night vision and a grooming claw. They are primates, but not monkeys.
Not tarsiers
Tarsiers sit on their own branch — enormous eyes, elongated ankle bones, entirely carnivorous.
A simple field checklist
- Is there a tail? Yes suggests monkey; no suggests ape.
- How does it move? Running along the top of branches means monkey. Hanging and swinging beneath them means ape.
- Which continent? Africa or Asia points to Old World; the Americas point to New World.
- Look at the nose. Narrow downward nostrils are Old World; wide side-facing nostrils are New World.
- Check for cheek pouches or sitting pads. Both are Old World features.
Work through Old World vs New World Monkeys to sharpen steps three to five.
Where monkeys fit in the primate family
Monkeys are not a footnote to the primate story — they are the bulk of it. Of roughly 500 primate species, over half are monkeys. They occupy niches from mangrove swamps to snowy Japanese valleys, from the top of the Amazon canopy to Cape Town’s suburbs. Follow that spread in Where Do Monkeys Live? and How Many Monkey Species Exist?
Frequently asked questions
Is a baboon a monkey?
Yes. Baboons are large Old World monkeys in the genus Papio, and they keep their tails throughout life.
Is a chimpanzee a monkey?
No. Chimpanzees are great apes and our closest living relatives.
Do all monkeys live in trees?
No. Baboons, geladas, patas monkeys and many macaques spend most of the day on the ground, though most still sleep off the ground for safety.
Are marmosets and tamarins monkeys?
Yes — they are New World monkeys in the family Callitrichidae, and among the smallest primates alive. See the tamarin guide.
What is the closest relative of monkeys?
Old World monkeys are more closely related to apes than they are to New World monkeys, which is why the term “monkey” is paraphyletic.
Continue with the Ultimate Guide to Monkeys for the full picture.
