Discovering a new mammal is a relatively rare occurrence; discovering a new monkey is rarer still. In 2010, a team of scientists ventured into the Amazon jungle in southern Colombia and documented, for the first time, the Caqueta titi monkey, an adorable, tiny primate notable for being one of the few monogamous monkeys and for purring like a cat. Primatologist Thomas Defler led his team to the Colombia's Caqueta province, which was too dangerous (because of gang activity) to visit just a few years ago. Sadly, just as we're discovering the Caqueta titi monkey, we're considering listing it as an endangered species. It's estimated that there are fewer than 250 living in the wild.