Whether it's with a sympathetic tilt of the head or the excited sweep of a tail, dogs seem to be saying they can sense exactly what we're feeling.

Science is still undecided on the matter, although evidence in favor of the idea is stacking up.

Now, a new study has found that dogs are able to tell the difference between happy and angry human facial expressions. (Related: "Animal Minds" in National Geographic magazine.)

Biologist Corsin Müller of the University of Veterinary Medicine in Vienna, Austria, and colleagues tested 11 dogs—including border collies, a fox terrier, a golden retriever, a German shepherd, and some mutts—using a touchscreen. The scientists trained the dogs to touch either a happy face or an angry face for a treat.

They presented dogs with either the top half or the bottom half of the faces to ensure the animals weren't just responding to a smile or the baring of teeth. Emotions show on all parts of a human face, not just the mouth, says Müller, whose study was published February 12 in the journal Current Biology.

"If you're angry, a wrinkle between the eyes shows up," he explains. The shape of the eyes can change too.

So if the dogs were truly able to spot an emotion, they should be able to do so regardless of which part of the face they looked at. (See "Can Dogs Feel Our Emotions? Yawn Study Suggests Yes.")

Face Time

Once Müller and colleagues trained the dogs, they ran them through choice trials, in which the animals had to pick between strange faces with either happy or angry expressions. The researchers presented the pooches with either the top, bottom, or left half of a face. The scientists chose the left half because previous studies found that dogs prefer to look at the left side of a face.

The pets trained to pick out happy expressions could do so when presented with different halves of a face, as well as when presented with faces the animals hadn't seen before. (Watch a video on working dogs.)

The dogs trained to respond to angry faces were also able to pick out angry expressions among the choices they were asked to make. However, it took them longer to learn their task than the dogs trained on happy faces.

Müller thinks the lag could be because the dogs had negative associations with angry faces. Perhaps angry faces meant a dog wouldn't receive any pats, while happy faces meant a belly rub, he suggests. (See "Dogs Get Jealous, Too.")

Man's Best Friend

Researchers don't yet know whether the dogs' ability to discriminate between the two expressions is because of past experiences or the result of the domestication process.

Whatever the reason, it's not so surprising that dogs can tell facial expressions apart, Müller says. "Because they spend so much time with humans, they have a lot of opportunities to see human expressions." (See "5 Amazing Stories of Devoted Dogs.")

Marc Bekoff, a behavioral ecologist who specializes in canines at the University of Colorado, Boulder, agrees. People and dogs have forged an incredibly close connection over thousands of years together, says Bekoff, who wasn't involved in the study. Along the way, dogs have been bred for certain traits, and "one of the traits would be the ability to read us." (Read "How to Build a Dog" in National Geographic magazine.)

The question now is whether dogs that spend a lot of time with people would be as good at picking up our expressions as dogs without a lot of people experience, Bekoff adds.

Müller plans to pursue that question, and to look at whether domestication played a role in the ability to read human expressions. For that study, the biologist plans to test species such as cats, pigs, and hand-raised wolves.