Monkeys

Monkeys are tree-dwelling mammals that, along with prosimians, apes, and humans, make up the order Primata of the primates. The primate suborder Anthropoidea includes two different infraorders: the Platyrrhini, comprised of New World monkeys, marmosets and tamarins; and the Catarrhini, the Old World monkeys, apes, and humans.

The major division between New World and Old World monkeys, in addition to their distribution, is that the New World monkeys have three premolars and the Old World only two. Platyrrhini, literally translated from the Greek, means "broad flat nose." New World monkeys have rounded nostrils set fairly far apart and face outward. The Old World monkeys, in contrast, have narrow nostrils with only a thin membrane between them, and they tend to face downward. Catarrhini, literally means "downward nose." Note that the term "New World monkey" includes the marmoset/tamarin group as well as species in family Cebidae. New World monkeys often have a prehensile, or grasping, tail while Old World monkeys lack a prehensile tail. Old World monkeys typically sleep sitting upright on narrow branches, and this group usually has ischial callosities, which are hard, hairless pads on their posterior. New World monkeys, on the other hand, tend to sleep stretched out on a branch, and lack these callosities. The Old World monkeys have a fully opposable thumb. This wide separation of the thumb from the rest of the fingers allows them to pick up small objects and to grasp tree branches firmly. The thumbs of the smaller New World monkeys are not fully opposable.

Primates such as gibbons, that swing by their arms (brachiate) to move among trees, have arms that are longer than their legs. Primates that leap from tree to tree, such as prosimians, have legs that are longer than their arms. Most monkeys fit in a third category of quadrupedal (four-footed) ground runners that have arms and legs of equal length, which allows them to run along branches on all-fours.

Old World monkeys (family Ceropithecidae) live in sub-Saharan Africa and in India and Southeast Asia, and include langurs, macaques, guenons, baboons, mandrills, colobus and leaf monkeys, and mangabeys. Most monkeys are forest dwellers but a few species, such as baboons, live in open savanna or rocky highlands.

The only European monkey is the Babary "ape," which is actually a macaque monkey (Macaca sylvanus). Originally from the Atlas Mountains of North Africa, it has taken up residence on the peninsula of Gibraltar, off southern Spain. Another macaque (M. fuscata) lives on the two main islands of Japan, at higher latitudes than any other monkey.

New World monkeys are found from southern Mexico to southern Brazil, and are grouped into two separate families. Marmosets and tamarins are in the family Callitrichidae, while the capuchins, titis, night monkeys (or douroucoulis), sakis, howlers, wooly monkeys, and spider monkeys are included in the family Cebidae. The cebid monkeys are sometimes referred to as "capuchin-like monkeys" for lack of a better non-scientific term to separate them from the marmosets and tamarins.

Monkeys are social creatures that usually depend on their group to help educate infants, protect the members, and find mates. A group may be as small as three (a male, a female, and a single offspring) or as large as 200 individuals in some species. Such large groups contain several males, many females, and many young.

Male monkeys tend to be somewhat larger than females, and the males of many species have greatly enlarged canine teeth. Some species of monkeys exhibit distinct sexual dimorphism (different appearance for male and female), while others are similar.

It was long thought that Old World and New World monkeys had evolved separately and that their similarities were the result of their occupying similar habitats, a process called convergent evolution. However, more recent genetic studies have revealed that Old World and New World monkeys evolved from a common Old World ancestor in Africa, about 65 million years ago. The ancestors of the New World monkeys probably reached South America from Africa during the late Eocene, when the two continents were closer together than they are today. It has been postulated that the monkeys were carried on floating islands of fallen trees and debris across the ocean to South America, where they established themselves in the rainforest and subsequently evolved into many different species.

Many species of monkeys are now endangered as a result of the loss of their habitat into agricultural and other land-uses by humans. They are also widely overhunted as a source of food. Many of these species will become extinct if their habitat is not effectively conserved, and the hunting is not stopped.