A monkey is a long-tailed, medium-sized member of the order of Primates. The primate order also includes macaques, baboons, guenons, capuchins, marmosets, and tamarins.
Monkeys today are a member of two of the three groups of simian primates, the New World monkeys and the Old World monkeys, of which there are 264 known species. Apes and chimpanzees are not scientifically classified as monkeys, a common misconception due to their physical similarities.
Some distinguishing features between New World and Old World monkeys include the tail. Most New World monkeys have prehensile tails while Old World monkeys do not. The facial features of each group of monkeys also differ substantially; however, there are a number of shared features as well.
Monkeys are a very diverse family of species, ranging in size from the 5-6 inch Pygmy Marmoset, to the adult male Mandrill, which can be 3 feet tall. Some monkeys spend the majority if their lives in treetops, while others call savannas and grasslands home. Most monkeys survive of a diet of fruit, leaves, nuts, berries, eggs, insects and they occasionally hunt other smaller animals.
To train two rhesus macaques, a control system navigated the wheelchair system (passive navigation) to a food reward. Both monkeys successfully acquired the ability to navigate themselves to the reward after they were conditioned.
The fossil of a small primate with "goggle" eyes that strode atop tree branches, snagging snacks of fruit, suggests the last common ancestor of all apes might have been less like humans' closest living relatives than often thought, researchers say.