Emily Pond, of Manawatu College, asks :-

Why do humans use smiles in a friendly, placatory, way whereas other primates use them as threats?

Rachael Stratton, an animal behaviourist Massey University's Institute of Veterinary, Animal, and Biomedical Sciences, responded.

Perhaps there is confusion over grimace/snarl and smile? They can look similar – lips parted and drawn back, teeth showing. Primates display a play face and make vocalisations that resemble laughter in humans. Analysis of the noises that young apes make when being tickled has shown genetic similarity to human laughter. However a display such as a fear grimace (teeth barred) can be a signal of submission or placation in macaques and rhesus monkeys. Perhaps the ‘tone’ or intent of the ‘smile’ is best inferred from the other behaviour that is being displayed. For example, if a teeth barred display is accompanied by withdrawal then it is probably fearful whereas if the animal is attacking (chest puffed up and body weight forward) then it is aggressive.

The ‘smile’ is one piece of the jigsaw puzzle that requires other behaviours/expressions to see the whole picture.