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Quiz: Can You Tell If This Monkey Wants to Bite?

A new study finds people get their facial expressions dangerously wrong.

Can You Tell a Monkey's Mood From Its Face?

As one of our closest biological relatives, some primates can appear eerily similar to humans.

But our different interpretations of facial expressions can be a misunderstanding capable of endangering both people and monkeys.

A new study from the University of Lincoln found that when humans try to guess the facial expressions of Barbary macaques, they often get them wrong—really wrong.

Laëtitia Maréchal, one of the study's authors, attributes this misunderstanding to the human tendency to anthropomorphize animals, or attribute them with human characteristics.

"[Tourists] would see the macaques do what they think is blowing a kiss and try to imitate the interaction," she said. In other words, what the macaques displayed as a warning not to come closer was often perceived as an invitation by people.

Maréchal tested how easily a macaque's facial expression could be intuited among three different groups of online participants. Photos of macaques were shown to people who had worked with the animals for at least two months, those who were shown 2D images of the monkeys' expressions, and those who had rarely or never encountered live monkeys.

They found all participants, regardless of experience, made some mistakes. Of the mistakes made in correctly guessing what a macaque was attempting to communicate, experts made just under seven percent of the mistakes, participants exposed to 2D images made 20 percent of the mistakes, and novices accounted for 40 percent of the mistakes made.

"Aggression displayed by the lips covering their teeth was the most misunderstood," said Maréchal. This was also the expression participants most assumed was the macaque blowing a kiss. She further explained that people tend to be equally bad at reading the facial cues of other primates, such as chimps and orangutans.

"If they appear to be 'smiling' that indicates distrust," Maréchal noted. "You can commonly see [chimpanzees] smiling on birthday cards, but that is a face of distress."

She and the other researchers hope their study can have practical implications for parks that offer wildlife tourism. While macaques are not known to be aggressive, they will respond if they feel provoked.

Making tour guides and park rangers more knowledgeable of the miscommunications between macaques and visitors, she hopes, will prevent distress for the monkeys or the rare monkey bite.

Agustín Fuentes, a National Geographic explorer and chair of the Department of Anthropology at the University of Notre Dame was unaffiliated with the study but has worked to understand the differences and similarities in human and monkey behavior. He emphasized that those engaging in wildlife tourism should never approach or attempt to pet a monkey, even if it doesn't display an aggressive expression.

Common human behaviors that denote friendliness, such as smiling and making eye contact, are dangerous gestures that can provoke a monkey. Fuentes noted that in addition to facial expressions, macaques communicate using their body language and how closely they stand next to another individual. If observed in the wild, two friendly macaques will stand near each other and create distance when threatened.

"The best thing to do on a wildlife tour is be mellow, hang out, and just watch them," he said.