Why do monkeys rapidly smack their lips when approached by human or another monkey?

Monkeys rapidly smack their lips as a form of social communication, often signaling excitement, submission, or a non-threatening stance. This behavior can indicate that they are not aggressive and are approaching in a friendly manner. Lip-smacking helps to reinforce social bonds among group members and can also serve as a way to appease or signal to potential threats, like humans or other monkeys. Overall, it plays a crucial role in their social interactions and dynamics.