Occasionally referred to as the "pocket monkey" because they can easily fit into your breast pocket, these adorable, curious animals  native to the Amazon rain forest are the world's smallest monkeys.

 

It's hard to imagine a monkey getting any smaller; pygmy marmosets rarely exhibit a length greater than about 15 or 16 centimeters, and adults typically weigh under 140 grams. They also live a long time given their size, occasionally reaching ages in excess of 20 years in captivity.