Monkeys communicate through visual gestures and movements, sounds and smells. The most common form of communication amongst monkeys is the call, because the thick forest can make it difficult to see or smell other monkeys in the area.
Monkeys will call out to one another when they need to let another monkey know of an incoming threat, or if they need help locating another monkey. They will also call out when they want to assert dominance and let other monkeys know that they are in control of a particular area. The monkeys can change their message by doing what humans do and altering the sound of the call through "pitch, loudness, and duration," writes PBS.