Monkey — A monkey is any member of either the New World monkeys or Old World monkeys, two of the three groupings of simian primates, the third group being the apes. There are 264 known extant species of monkey.
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.
Because of their similarity to monkeys, apes such as chimpanzees and gibbons are often called “monkeys” in informal usage, though they are not monkeys. Conversely, due to its size (up to 1 m) 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 14-16 cm (5-6 inch) long (plus tail) and 120-140 g (4-5 oz) in weight, to the male Mandrill, almost 1 metre (3 ft) long and weighing 35 kg (75 lb). Some are arboreal (living in trees), some live on the savannah; diets differ among the various species but may contain any of the following: fruit, leaves, seeds, nuts, flowers, insects, spiders, eggs and small animals.
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 colour 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. In order to understand the monkeys, it is necessary to study the characteristics of the different groups individually.