They are small; I would even say tiny, in fact. They moved quickly, and it was tricky to see them in the dark and even harder to photograph them at the time. The Western pygmy marmosets were also very cute and adorable to watch. They are primarily found in the northwestern Amazon rainforests of Brazil, Columbia, Ecuador, and Peru. Known as the world's smallest monkey. They frequently have fraternal twins, and the females are known to be able to prevent reproduction in their population through hormonal suppression. When a single dominant female marmoset does not wish her subordinates to reproduce, she can produce pheromonal signals that suppress hormones.