Lucas Bustamante / Spix's night monkeys (Aotus vociferans) in a tree hole, Yasuni National Park, Orellana, Ecuador

Lucas Bustamante / Three Spix's night monkeys (Aotus vociferans) looking out of a hole in a tree trunk, Cuyabeno National Park, Sucumbios, Ecuador.

Murray Cooper / Spix's Night Monkey (Aotus vociferans) group looking out of tree hole, Amazon, Ecuador

Cyril Ruoso / Spix's Night Monkey (Aotus vociferans), Pacaya Samiria National Park, Peru

Murray Cooper / Spix's Night Monkey (Aotus vociferans) group emerging from tree cavity, Amazon, Ecuador

Karine Aigner / Spix's night monkeys (Aotus vociferans) in a tree hole, Yasuni National Park, Orellana, Ecuador