For the past two and a half decades, I have had the privilege of working in a tropical biodiversity hotspot that is home to hundreds of species of birds and mammals, thousands of plants, and an untold diversity of insects. The Yasuní Biosphere Reserve – located in the Amazon rainforest of lowland Ecuador – hosts a community of 10 species of nonhuman primates as well as robust populations of predators and competitors. Working in collaboration with numerous students, volunteer research assistants, and colleagues, I have spent much of my time conducting a long-term comparative study of the societies of two little-known ateline primates: lowland woolly monkeys (Lagothrix lagotricha poeppigii) and white-bellied spider monkeys (Ateles belzebuth). Both species show notable convergences with the great apes – particularly with chimpanzees and bonobos – in various aspects of their behavioral biology and natural history. Still, our team’s long-term observations, coupled with the results of molecular genetic studies, have highlighted a number of interesting and enigmatic aspects of these species’ social systems. This talk will review some of the insights gleaned from our studies and highlight important questions that remain. I will also share some of the challenges and rewards associated with doing fieldwork in the remote Amazon, the importance of collaborative and comparative work, and the special pleasure of working with platyrrhines.