Monkeys are primates that normally have a tail and are part of over 260 species. Monkeys are very skilled climbers and spend a lot of time in the trees where they are safe from predators. Monkeys vary in size from the mandrill measuring 3.3 feet long and weighing up to 79 pounds, to the pygmy marmoset weighing 3.5 ounces and measuring 4.6 inches.
Because of their close social behaviors and DNA structures to humans, some labs use them for scientific testing. But for the same reason, some organizations have trained capuchin monkeys to be service animals for quadriplegics or individuals with mobility impairments.