There are at least 260 known monkey species in the world. These mammals are divided into New World -those found in the Americas - and Old World families - those native to Africa and Asia.
Characteristic of New and Old World monkeys are opposable thumbs. Key differences between the New and Old World families are the inclusion of prehensile tails, found in the New World, and cheek pouches for food storage and rump pads, both found in the Old World.
The smallest species, the New World pygmy marmoset, weighs just 4 ounces, while the largest is the Old World mandrill, which tips the scale at 77 pounds, according to University of Wisconsin researchers.