Word to the wise: Don't play poker with your dog. He can read your face like a book. A new study shows that man's best friend is remarkably good at discerning happy expressions from angry ones – even when he's only looking at half of a person's face.
The authors say the findings, published in the journal Current Biology, are the first to provide solid evidence that an animal can read the expressions of a totally different species (that is, those of humans).
"This new work continues to build the case for just how sensitive dogs are to our subtle behaviors," Duke University researcher Brian Hare, who was not involved in the study, wrote in an email. "This is the strongest evidence yet that dogs may even read our facial expressions."
When it comes to reading other animals' emotional states, much gets lost in translation. Take one of the most basic human expressions: a smile. While baring your teeth might be friendly among Homo sapiens, it can be a deadly threat among a host of other animals, from chimpanzees to wolves.
Humans seem to have the same problem as any other animal, said lead author Corsin Müller, a cognitive biologist at the Messerli Research Institute, University of Veterinary Medicine Vienna.
"We're mostly interested in what's going on in the heads of animals," Müller said, adding that such insights could help us understand how to care for the animals we live and work with. "Most of this is about how we keep them and how we treat them. What sort of conditions do the animals need to live happy lives?"
Being able to read other species' faces seems like a logical skill, the study authors point out – after all, it would probably pay for a predator to be able to read fear and anger in its potential prey's face. It could also be useful for species that work together, such as dogs and humans.
Dogs, believed to have been domesticated from wolves some 11,000 to 16,000 years ago, have developed in close connection with their human companions. Thus, it's possible that they've become very good at reading human faces.
And of course, such research gets at a deeper issue – does your dog know what you're feeling?
Before answering that question, scientists need to know whether a dog can differentiate between human expressions. Without that ability, it would be difficult for a dog to read emotions, Müller pointed out. Previous work had tried to parse this ability, but had not ended up with completely convincing results.
So, for this study, 11 dogs (mostly border collies, plus a sheltie and a fox terrier) stood in front of a little kiosk with a computer screen showing them two images of the same person making an angry face or a happy face. However, they only showed them half of each face – either the tops (dominated by the eyes), or the bottoms (dominated by the mouths). Some of the dogs were trained to pick the angry half-face; some were trained to pick the happy half-face. When they touched the correct face on screen with their noses, they received a treat.
That was the training. For the test rounds, the dogs again had to pick the angry or happy half-face, but the researchers threw in four different twists. Sometimes they were shown the opposite half of the same person's face that was used in training (if they'd seen the upper half, they would now see the lower half, and vice versa). In other cases, they were shown the same half of a different person's face. They were also shown the opposite half of a different person's face; and finally, they were asked to judge the left half of the same person's face used in training. Each dog saw each twist 10 times, for a total of 40 rounds.
Depending on the twist, the dogs got it right around 70% to 80% of the time, Müller said – much better than chance.
"I was impressed by the dogs' performance. We expected this was a difficult task, and we weren't sure they would be able to solve it," Müller said.
The fact that they were able to do this task without ever seeing the whole face was the clincher, Hare said.
"This means when being trained to categorize the original set of faces as either happy or unhappy the dogs did not rely on features specific to the photos they were being trained on," Hare said. "They must have used a higher-level rule like 'happy face' or 'unhappy face' that also allowed them to succeed with the new faces. That is a surprising finding."
But just because dogs can tell one facial expression looks different from another doesn't necessarily mean that they know what those expressions mean. Does a dog know a happy face is, well, happy?
This study can't say either way, but there was a tantalizing hint: The scientists found that the dogs that were rewarded for picking out the angry face were not as good at learning the task, possibly because "the dogs had to overcome their natural tendency to move away from aversive (or threatening) stimuli," the study authors wrote. An angry face is one best avoided, it would seem.
So how much of this skill is learned, and how much is innate? Müller said he and his colleagues might try to get at that question by testing "hand-raised" wolves at the Wolf Science Center in Austria. If these feral canids are just as good at learning the task as their domesticated brethren are, it would seem that learning, not breeding, is responsible for this handy people-reading skill.