This expression is often mistaken for a happy smile, which it isn't, at least not when shown by a macaque. It is an expression of fear and submission, usually given by a lower-ranking monkey to a dominant one, here by a long-tailed macaque in a protected forest area in Lombok, Indonesia. The macaque grin is also an attempt at appeasement, and this is where it gets closer to the human smile, which is thought to derive from this general primate expression.