Squirrel Monkey: these primates are only 1-3 pounds! But if you think they would make cute pets, think again. They live in large groups of up to 200 monkeys, and are bold - if a predator approaches, the entire group fights back. They also exhibit a behavior called "scent marking," leaving tracks by rubbing their urine on their hands. Squirrel monkeys are from the northern regions of South America, but are also bred in the U.S. for the pet industry and their extensive use in research.