Smiles: the good, the bad and the funny

Photo: Getty Images

Emily Pond, Manawatu College, of Foxton, asked: Why do humans use smiles in a friendly, placatory way when other primates use them as threats?

Associate Professor Kirk L. Hamilton, a physiologist from the University of Otago, responded.

"Smile though your heart is aching..." These words come from a 1938 song for which Charlie Chaplin wrote the music, and lyrics were added by others in 1954. We, humans, perceive smiling as a normal behaviour indicating happiness, friendship, showing interest, attraction, agreeing with what is being said, acknowledging positives, and/or enjoying humour, and more.

However, some people might perceive a certain type of smile as "sinister or a dastardly look". What about a smiling politician?

Humans and other primates smile or appear to be smiling but the interpretation of smiles may be vastly different. Many believe Charles Darwin was the first biologist to describe and catalogue human facial expressions, and made comparisons with other animals, including primates, as recorded in his 1872 book The Expression of Emotions in Man and Animals. Others have followed Darwin’s lead, such as Dame Jane Goodall.

What looks like a friendly smile to a human can be a "fear grin" as seen on this bonobo, sometimes used when a lower-ranked chimp is approached by a higher-ranking animal. Photos: Getty Images

As one researches this question, one quickly realises primate "smiles" can be interpreted differently by humans and even between different species of the same genus of primate. When chimpanzees, macaques and baboons display a wide open mouth with lips curled back and teeth wide apart this might appear as a large smile to humans, but actually indicates a threat of dominance, or possible attack.

Members of the individual’s troop respond to the perceived threat by displaying their teeth touching, lips curled. Also they hold their hands in front of their face to hide a "grin" in a sign of submission. Interestingly, when a human shows a wide mouth smile, including the upper and lower teeth, it is seen as a threatening expression to chimpanzees.

As tourism increases throughout the world, but even before Covid, researchers had grave concern about holiday destinations where people may interact with wild animals, monkeys in particular. People continue to not recognise the smiling behaviour of monkeys is an act of aggression. Recent studies estimate bites from monkeys are the second cause of injury, next to dogs, in South East Asia.

On a personal note, I try to make my family smile every day by, what some in our family say are, "Dad Jokes". In the end, they are smiling, so, mission accomplished! Anatomically, one uses fewer muscles to smile than to frown, in a sense ... smile and you are saving energy. So, just smile!