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Dogs Understand Human Smiles, Scowls

With a brief glance, your dog can figure out your mood.

Feeling happy or mad? A dog can tell your mood just by looking at the expression on your face.

New research in the journal Current Biology represents the first solid evidence that an animal other than humans can discriminate between emotional expressions in another species.

As any dog owner knows, canines are skilled at figuring us out, but previously more attention was paid by scientists to how dogs read us using their other senses, such as smell and hearing, and by observing our behaviors. The latest study strongly suggests that the sight alone of a smile, frown, scowl and more conveys our moods to dogs.

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"We think the dogs in our study could have solved the task only by applying their knowledge of emotional expressions in humans to the unfamiliar pictures we presented to them," co-author Corsin Müller of the University of Veterinary Medicine Vienna said in a press release.

For the study, Müller and his team took photos of the same person making either a happy or an angry face. The researchers then showed the photos to dogs, which were trained to discriminate between the happy and angry expressions. The researchers then showed only the upper or lower halves of the images to the dogs, which lost none of their ability to discriminate.

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The researchers next presented the dogs with photos of different people making their own happy and angry expressions. Müller and his colleagues mixed up the images, sometimes showing the dogs one person or the other, or showing the upper or lower halves of the pictures.

The dogs were able to select the angry or happy face more often than would be expected by random chance in every case.

"Our study demonstrates that dogs can distinguish angry and happy expressions in humans," lead author Ludwig Huber of the University of Veterinary Medicine Vienna's Messerli Research Institute said. He added that "they can tell that these two expressions have different meanings, and they can do this not only for people they know well, but even for faces they have never seen before."

He continued that "it appears likely to us that the dogs associate a smiling face with a positive meaning and an angry facial expression with a negative meaning."

Supporting this is the fact that dogs have a hard time learning to associate an angry human face with a reward, suggesting that they already know -- based on prior experience -- that mad people aren't likely to offer treats and head rubs.

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More studies are in the works to learn more about how dogs recognize human emotions. The researchers also plan to study how dogs themselves express different emotions, and how their feelings are influenced by those of their owners or other people.

"We expect to gain important insights into the extraordinary bond between humans and one of their favorite pets, and into the emotional lives of animals in general," Müller said.

It could be that dogs are so close to humans that they understand us better than any other species. But maybe other pets, such as cats, understand our expressions too.

The answer to that remains unknown to science for now, but anecdotal evidence suggests that other animals can also read our moods with a brief glance.

Photo: The experimental set-up used to test whether dogs can discriminate emotional expressions of human faces. Credit: Anjuli Barber, Messerli Research Institute

Facial expressions among social animals appear to have universal qualities, to the point where humans and other animals can discern how certain species feel just by looking at their faces. That's the suggestion in two new studies -- published in the Journal of Veterinary Behavior and PLOS ONE -- that help explain how humans can have such close, understanding relationships with animals such as dogs and horses, the subjects of the investigations. The research confirmed, through animal behavioral analysis, the underlying meaning of dog and horse facial expressions and also demonstrated that people have a natural knack for figuring out what they mean. For example, "this dog is experiencing a positive emotional state, as his owner has just come back," Emanuela Dalla Costa told Discovery News. She led both studies and is a researcher in the Department of Veterinary Science and Public Health at the Università degli Studi di Milano. She explained that the dog’s eyes are wide open, as is his mouth, yet his facial muscles are somewhat tense. Together, these features and others suggest that he is happy, eager and hopeful.

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Here is another happy dog. In this case, Dalla Costa explained, the dog's "lips are retracted, but with no exposure of the teeth." The dog is thrilled that its owner has just returned and eagerly looks to the human for guidance.

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This poor pooch "is tense, due to the departure of his owner," Dalla Costa said. Every part of the dog's face is turned in the direction of his owner's recent exit, maximizing the pup's ability to find him. The dog's eyes and tense mouth convey his worry and loneliness.

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This dog's gaze is riveted on its owner, who is holding food. "This emotional condition is considered positive, and we can assume that this dog is happy," said Dalla Costa, adding that "there is no visible tension in the facial muscles." Even though the eyes, ears and face are pointed in one direction, just as they were for the worried and lonely dog in the previous slide, this hopeful canine feels no anxiety.

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Every aspect of this dog’s face communicates concern and worry. "The mouth is opened and the dog is panting," Dalla Costa said. "The dog’s lips are partly drawn back, with no teeth exposure. The facial muscles show some degree of tension, visible through ridges that emerge on the lateral side of the face and near the eyes." His ears are up, yet not fully open, an indication that he’s attentive but also worried.

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If this dog could talk, the canine would likely be saying, "Oh, please -- feed me." The photo was snapped as the dog looked longingly at its owner, who was holding a favorite food treat. Dalla Costa explained that there is no visible tension in the dog’s face. Its expression cleverly communicates desire and gentleness, while also revealing a sense of hopeful expectation.

Horse expressions, meanwhile, share similar qualities with dogs. This horse, similar to the dogs happily looking at their owners, is attentive and awaiting direction. "The eyes are open and focused on the environment, ears are moving in the direction of sounds, and there is no muscle tension in the mouth," said Dalla Costa.

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This unfortunate horse was photographed while in pain, which, thankfully, turned out to be only temporary. In this moment, however, Dalla Costa noted that the horse's ears are in a sideways position. Its eyes are partially closed, and its "chewing muscles are strained and prominent." Even the horse’s nostrils are stiff over its tightly shut mouth.

While perhaps not as uncomfortable as the previous horse, this horse was also photographed while experiencing temporary, minor discomfort. In this case, the horse’s nostrils are wide open, while its lower lip is drawn back. Even its "ears are held passively backward," Dalla Costa said.

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This horse is completely relaxed. "There is no tension in the mouth and in the chewing muscles," said Dalla Costa. "The nostrils are relaxed." The horse was happy to be chilling out on a pleasant day in an open field.