The Smallest Monkey in the World

Native to the rain-forests of northern South America, the pygmy marmoset tops out at an adult weight of approximately 4 ounces! They typically reside in social groups of about 15 members, which consist of a mating couple and all their offspring.
They defend their home area of less than an acre through calling, chasing, and exposing their distinct white genitals


source: http://yourmindblown.com/post/1058516296/the-smallest-monkey-in-the-world-is-only-about-15-cm