Do Only Humans Smile?

Apparently smiling at a Chimpanzee is a mistake. Chimpanzees are thought to interpret this as a sign of aggression. Since Chimpanzees are our nearest living relatives this means that smiling (in the sense that this is a friendly sign) might be a distinctly human characteristic. While in Tamil Nadu, I was fortunate enough to have witnessed several instances of Bonnet Macaques baring their teeth at each other or while alone. Bonnet Macaques are Old World Monkeys and their ancestors are thought to have diverged from our lineage some 25 million years ago. The main clip below lasts some 2 minutes and includes several incidents of teeth baring obviously associated with aggressive confrontation between an alpha male and those jockeying for higher positions within the troupe.

There are a few interesting points to observe. Thus in the photogaph below we can see that the alpha male is baring his teeth and the other macaque is observing in a state of readiness.

This same macaque then slowly lowers into what I interpret as a position of submission. Thus in this instance the baring of teeth (supported with vocalisations) is associated with a submission response in the other macaque without resorting to physical conflict (albeit for a brief period). What is also interesting although difficult to be certain about is that the macaque combines the teeth baring with a downward movement of his eyes at one point.

The Macaque’s eyes quickly return to the upward position and this is somewhat reminiscent of human behaviour when one person might ‘look down their nose’ at the other person to signal that the other person has less perceived social value. In the case of the Macaques there is a clear struggle for social position. However the sceptic would be right to suggest that the above inference may be erroneous based as it is on a single observation. I had not stayed long enough to observe a repetition of this behaviour in the same context.

In the next clip, there is an even more interesting occurrence. This time a Macaque can be observed sitting on a tree baring its teeth. While this may also be an isolated occurrence, the first association that came to mind was that of an actor rehearsing their facial expressions in front of a mirror.

Perhaps the Macaque was recalling an earlier confrontation and re-enacting the scene or else preparing for a future confrontation. In the first clip the reader might agree that there is an element of ‘performance’ to these confrontations and that the analogy with actors may hold scope for future exploration.

So what does all of this tell us?

Firstly a macaque can bare its teeth to reduce the need for physical conflict. This might result from classical conditioning. If a Macaque lower in the hierarchy has lost several encounters with the Alpha male and these conflicts have been associated with biting, then baring of the teeth becomes associated with ‘punishment’ and the submission response may follow. This may be avoided if the context is different. In other words if such conditioning does occur then the Macaque might be able to over-ride this when it cooperates with another Macaque to attack the alpha-male.

Secondly although humans show their teeth when smiling this particular behaviour of baring teeth in the Macaques is associated with hostility and confrontation. Thus the emotional correlates appear to be quite distinct.

Thirdly there is possible observational learning in the Macaques. Towards the end of the clip, the Alpha male bares its teeth at two other Macaques and is quickly joined by an ‘ally’ who follows this behaviour. Perhaps these facial expressions can be used to forge alliances.

So the last question is why do people smile as a friendly signal instead of as a sign of hostility? I’m sure the reader might have some good ideas about this. My thought is that Chimpanzees and Macaques use their teeth when fighting each other whereas this is not typical in humans. So the teeth have lost their association with hostility and become more associated with activities such as laughing and sharing food. People such as Professor Robin Dunbar have suggested that laughing in humans has taken the place of grooming and so the association of smiling with laughter would reasonably make it a more friendly activity.

Perhaps a close examination of Bonobos (a close relative of the Chimpanzee who have interesting social traits) might give us some insights into how people gained their smiles.

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7 thoughts on “Do Only Humans Smile?

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  6. S. Hayes-Light

    In my expereince (born and raised in East Africa) apes do smile to indicate pleasure, A smile is when the lips are closed (altough pursing the lips can be a signal of an ape preparing to attack). Baring the teeth is not a ‘smile’ in ape communication as it may be in humans, but an expresson of fear and/or hostility.

  7. Dr Justin Marley Post author

    Many thanks for your sharing your interesting observations. If that holds then it would suggest that only humans smile by baring teeth. Regards Justin

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