Old World monkeys are species belonging to the Cercopithecoidea superfamily of primates. The group has over 130 species to its credit. While most of these are arboreal (i.e., tree-dwelling), terrestrial species also exist. As far as their characteristics are concerned, Old World monkeys are typically characterized by their downward-pointing nostrils, opposable thumbs, long rostrum, and furless palms and soles. Additionally, in most species, the forelimbs are shorter than the hind limbs. Old World monkeys are medium to large in size, but larger than those found in the New World, i.e., the Americas. All Old World species do have a tail, but then, it is non-grasping or not prehensile like that of New World monkeys, so they can't use it to lift objects. It is their features, such as the shape of their nose, opposable thumbs, non-grasping tail, etc., that distinguish Old World from New World monkeys. As their name suggests, Old World monkeys are native to the Old World, i.e., Africa, Asia, and Europe. Though they are only found in Africa and Asia as of today, fossil evidence suggests that species belonging to this group also thrived in Europe in the past. These monkeys are found in a range of habitats, including tropical rainforests of Southeast Asia, African Savanna, as well as dry shrublands and even mountainous terrain with heavy snowfall. Furthermore, some of these species are even found in urban areas in the vicinity of humans. Old World monkeys are classified into two subfamilies: (i) Cercopithecinae subfamily; and (ii) Colobinae subfamily. The species belonging to the Cercopithecinae subfamily are identified by ischial callosities, i.e., a thickened piece of skin found on their buttocks and cheek pouches where they store food. Also, the species belonging to this subfamily are largely omnivores. Their diet includes leaves, fruits, insects, fish, and even meat. Baboons, for instance, are known to hunt other monkeys as well as young ones of other mammals. The Cercopithecinae subfamily comprises species like baboons (genus Papio), mangabeys (genus Lophocebus, Cercocebus, and recently discovered Rungwecebus), mandrills (genus Mandrillus), patas monkey (lone species in genus Erythrocebus), macaques (genus Macaca), guenons (genus Cercopithecus), talapoins (genus Miopithecus), etc. The species belonging to the Colobinae subfamily are characterized by their sacculated stomachs and absence of cheek pouches. They are predominantly herbivores, with low-protein, fibrous leaves making a chunk of their diet. The bacteria and enzymes present in the sack-like compartments of their sacculated stomach help them to break down plant cellulose more effectively, and thus, provide them more usable calories. The Colobinae subfamily comprises langurs or leaf monkeys (Genus Trachypithecus, Presbytis, and Semnopithecus), colobus monkeys (genus Colobus), proboscis monkey (lone species in genus Nasalis), snub-nosed monkeys (genus Rhinopithecus), etc. As with other primates, even Old World monkeys are threatened by loss of habitat and habitat fragmentation resulting from human activities. As a result, at least ten species, including the Dryas monkey (Cercopithecus dryas), Delacour's langur (Trachypithecus delacouri), and Celebes crested macaque (Macaca nigra), have been enlisted as critically endangered species by the International Union for Conservation of Nature (IUCN), while several others are categorized as endangered. Old World monkeys are more closely related to anthropoid apes, like gorilla and chimpanzees, which isn't surprising, as both are Old World Catarrhines. However, there do exist differences between these two groups. While Old World monkeys have a tail, apes don't. Also, apes are predominantly ground-dwelling and generally, larger of the two.