A monkey is any cercopithecoid (Old World monkey) or platyrrhine (New World monkey) primate. All primates that are not prosimians (lemurs and tarsiers) or apes are monkeys. The 264 known extant monkey species represent two of the three groupings of simian primates (the third group being the 21 species of apes). Monkeys are usually smaller and/or longer-tailed than apes.
The New World monkeys are classified within the parvorder Platyrrhini, whereas the Old World monkeys (superfamily Cercopithecoidea) form part of the parvorder Catarrhini, which also includes the apes. Thus, scientifically speaking, monkeys are paraphyletic (not a single coherent group), and Old World monkeys are actually more closely related to the apes than they are to the New World monkeys.
Due to its size (up to 1 m/3 ft) the Mandrill is often thought to be an ape, but it is actually an Old World monkey. Also, a few monkey species have the word "ape" in their common name.
Monkeys range in size from the Pygmy Marmoset, at 140 to 160 millimetres (5-6 in) long (plus tail) and 120 to 140 grams (4-5 oz) in weight, to the male Mandrill, almost 1 metre (3.3 ft) long and weighing 35 kilograms (77 lb). Some are arboreal (living in trees) while others live on the savanna; diets differ among the various species but may contain any of the following: fruit, leaves, seeds, nuts, flowers, eggs, and small animals (including insects and spiders).
Some characteristics are shared among the groups; most New World monkeys have prehensile tails while Old World monkeys have non-prehensile tails or no visible tail at all. Some have trichromatic color vision like that of humans, others are dichromats or monochromats. Although both the New and Old World monkeys, like the apes, have forward facing eyes, the faces of Old World and New World monkeys look very different, though again, each group shares some features such as the types of noses, cheeks, and rumps.
According to the Oxford English Dictionary, the word "monkey" may originate in a German version of the Reynard the Fox fable, published circa 1580. In this version of the fable, a character named Moneke is the son of Martin the Ape. The word Moneke may have been derived from the Italian monna, which means "a female ape". The name Moneke likely persisted over time due to the popularity of Reynard the Fox.
A group of monkeys may be referred to as a mission or a tribe.
The following list shows where the various monkey families (bolded) are placed in the Primate classification.
* ORDER PRIMATES o Suborder Strepsirrhini: non-tarsier prosimians o Suborder Haplorrhini: tarsiers, monkeys and apes + Infraorder Tarsiiformes # Family Tarsiidae: tarsiers + Infraorder Simiiformes: simians # Parvorder Platyrrhini: New World monkeys * Family Cebidae: marmosets, tamarins, capuchins, and squirrel monkeys (56 species) * Family Aotidae: night monkeys, owl monkeys, douroucoulis (8 species) * Family Pitheciidae: titis, sakis and uakaris (41 species) * Family Atelidae: howler, spider and woolly monkeys (24 species) # Parvorder Catarrhini * Superfamily Cercopithecoidea o Family Cercopithecidae: Old World monkeys (135 species) * Superfamily Hominoidea: apes o Family Hylobatidae: gibbons ("lesser apes") (13 species) o Family Hominidae: great apes including humans (7 species)
Note that the smallest grouping that contains them all is the Simiiformes, the simians, which also contains the apes. Calling apes "monkeys" is considered scientifically incorrect as apes are distinctly defined as different from monkeys. Apes were included in earlier use of the term, predating modern classifications. Including some or all apes (other than humans) remained the common usage in the early 20th century and is still in colloquial use. Calling either a simian is correct.